Author name code: kumar
ADS astronomy entries on 2022-09-14
=author:"Kumar, Pawan"
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Title: Transparency of fast radio burst waves in magnetar
magnetospheres
Authors: Qu, Yuanhong; Kumar, Pawan; Zhang, Bing
Bibcode: 2022MNRAS.515.2020Q
Altcode: 2022arXiv220410953Q; 2022MNRAS.tmp.1840Q
At least some fast radio bursts (FRBs) are produced by magnetars. Even
though mounting observational evidence points towards a magnetospheric
origin of FRB emission, the question of the location for FRB
generation continues to be debated. One argument suggested against
the magnetospheric origin of bright FRBs is that the radio waves
associated with an FRB may lose most of their energy before escaping the
magnetosphere because the cross-section for e± to scatter
large-amplitude electromagnetic waves in the presence of a strong
magnetic field is much larger than the Thompson cross-section. We have
investigated this suggestion and find that FRB radiation travelling
through the open field line region of a magnetar's magnetosphere does
not suffer much loss due to two previously ignored factors. First,
the plasma in the outer magnetosphere ($r \gtrsim 10^9$ cm), where the
losses are potentially most severe, is likely to be flowing outwards
at a high Lorentz factor γp ≥ 103. Secondly,
the angle between the wave vector and the magnetic field vector,
θB, in the outer magnetosphere is likely of the order of 0.1
radian or smaller due in part to the intense FRB pulse that tilts open
magnetic field lines so that they get aligned with the pulse propagation
direction. Both these effects reduce the interaction between the FRB
pulse and the plasma substantially. We find that a bright FRB with
an isotropic luminosity $L_{\rm frb} \gtrsim 10^{42} \, {\rm erg \
s^{-1}}$ can escape the magnetosphere unscathed for a large section
of the γp - θB parameter space, and therefore
conclude that the generation of FRBs in magnetar magnetosphere passes
this test.
Title: Propagation of Alfvén waves in the charge starvation regime
Authors: Kumar, Pawan; Gill, Ramandeep; Lu, Wenbin
Bibcode: 2022MNRAS.tmp.2283K
Altcode:
We present numerical simulation results for the propagation of Alfvén
waves in the charge starvation regime. This is the regime where the
plasma density is below the critical value required to supply the
current for the wave. We analyse a conservative scenario where Alfvén
waves pick up charges from the region where the charge density exceeds
the critical value and advect them along at a high Lorentz factor. The
system consisting of the Alfvén wave and charges being carried with
it, which we call charge-carrying Alfvén wave (CC-AW), moves through
a medium with small, but non-zero, plasma density. We find that the
interaction between CC-AW and the stationary medium has a 2-stream like
instability which leads to the emergence of a strong electric field
along the direction of the unperturbed magnetic field. The growth rate
of this instability is of order the plasma frequency of the medium
encountered by the CC-AW. Our numerical code follows the system for
hundreds of wave periods. The numerical calculations suggest that
the final strength of the electric field is of order a few per cent
of the Alfvén wave amplitude. Little radiation is produced by the
sinusoidally oscillating currents associated with the instability
during the linear growth phase. However, in the nonlinear phase,
the fluctuating current density produces strong EM radiation near the
plasma frequency and limits the growth of the instability.
Title: Propagation of Alfvén waves in the charge starvation regime
Authors: Kumar, Pawan; Gill, Ramandeep; Lu, Wenbin
Bibcode: 2022arXiv220812806K
Altcode:
We present numerical simulation results for the propagation of Alfvén
waves in the charge starvation regime. This is the regime where the
plasma density is below the critical value required to supply the
current for the wave. We analyze a conservative scenario where Alfvén
waves pick up charges from the region where the charge density exceeds
the critical value and advect them along at a high Lorentz factor. The
system consisting of the Alfvén wave and charges being carried with
it, which we call charge-carrying Alfvén wave (CC-AW), moves through
a medium with small, but non-zero, plasma density. We find that the
interaction between CC-AW and the stationary medium has a 2-stream like
instability which leads to the emergence of a strong electric field
along the direction of the unperturbed magnetic field. The growth rate
of this instability is of order the plasma frequency of the medium
encountered by the CC-AW. Our numerical code follows the system for
hundreds of wave periods. The numerical calculations suggest that
the final strength of the electric field is of order a few percent
of the Alfvén wave amplitude. Little radiation is produced by the
sinusoidally oscillating currents associated with the instability
during the linear growth phase. However, in the nonlinear phase,
the fluctuating current density produces strong EM radiation near the
plasma frequency and limits the growth of the instability.
Title: Gravitational lensing in the presence of plasma scattering
with application to Fast Radio Bursts
Authors: Kumar, Pawan; Beniamini, Paz
Bibcode: 2022arXiv220803332K
Altcode:
We describe how gravitational lensing of fast radio bursts (FRBs) is
affected by a plasma screen in the vicinity of the lens or somewhere
between the source and the observer. Wave passage through a turbulent
medium affects gravitational image magnification, lensing probability
(particularly for strong magnification events), and the time delay
between images. The magnification is suppressed because of the
broadening of the angular size of the source due to scattering by
the plasma. The time delay between images is modified as the result
of different dispersion measure (DM) along photon trajectories for
different images. Each of the image lightcurve is also broadened due
to wave scattering so that the images could have distinct temporal
profiles. The first two effects are most severe for stellar and
sub-stellar mass lens, and the last one (scatter broadening)
for lenses and plasma screens at cosmological distances from the
source/observer. This could limit the use of FRBs to measure their
cosmic abundance. On the other hand, when the time delay between images
is large, such that the lightcurve of a transient source has two or
more well separated peaks, the different DMs along the wave paths of
different images can probe density fluctuations in the IGM on scales
$\lesssim 10^{-6}$ rad and explore the patchy reionization history of
the universe using lensed FRBs at high redshifts. Different rotation
measure (RM) along two image paths can convert linearly polarized
radiation from a source to partial circular polarization.
Title: Physical link of the polar field buildup with the Waldmeier
effect broadens the scope of early solar cycle prediction: Cycle 25
is likely to be slightly stronger than Cycle 24
Authors: Kumar, Pawan; Biswas, Akash; Karak, Bidya Binay
Bibcode: 2022MNRAS.513L.112K
Altcode: 2022arXiv220311494K; 2022MNRAS.tmpL..44K
Prediction of the solar cycle is challenging but essential because it
drives space weather. Several predictions with varying amplitudes of
the ongoing Cycle 25 have been made. We show that an aspect of the
Waldmeier effect (WE2), i.e. a strong positive correlation between
the rise rate and the amplitude of the cycle, has a physical link with
the buildup of the previous cycle's polar field after its reversal. We
find that the rise rate of the polar field is highly correlated with
the rise rate and the amplitude of the next solar cycle. Thus, the
prediction of the amplitude of the solar cycle can be made just a few
years after the reversal of the previous cycle's polar field, thereby
extending the scope of the solar cycle prediction to much earlier than
the usual time. Our prediction of Cycle 25 based on the rise rate of
the previous polar field is 137 ± 23, which is quite close to the
prediction 138 ± 26 based on the WE2 computed from the available 2
yr sunspot data of the ongoing cycle.
Title: The large landscape of supernova, GRB, and cocoon interactions
Authors: De Colle, Fabio; Kumar, Pawan; Hoeflich, Peter
Bibcode: 2022MNRAS.512.3627D
Altcode: 2021arXiv210509376D; 2022MNRAS.tmp..715D
Long gamma-ray bursts (LGRBs) are associated to the collapse of a
massive star and the formation of a relativistic jet. As the jet
propagates through the star, it forms an extended, hot cocoon. The
dynamical evolution of the jet/cocoon system and its interaction
with the environment has been studied extensively both analytically
and numerically. On the other hand, the role played by the supernova
(SN) explosion associated with LGRBs in determining the outcome of
the system has been barely considered. In this paper, we discuss
the large landscape of outcomes resulting from the interaction of
the SN, jet, and cocoon. We show that the outcome depends mainly on
three time-scales: the times for the cocoon and SN shock wave to break
through the surface of the progenitor star, and the time needed for the
cocoon to engulf completely the progenitor star. The delay between the
launch of the SN shock moving through the progenitor star and the jet
can be related to these three time-scales. Depending on the ordering
of these time-scales, the jet-cocoon might propagate inside the SN
ejecta or the other way around, and the outcome for the properties
of the explosion would be different. We discuss the imprint of the
complex interaction between the jet-cocoon and the SN shock on the
emergent thermal and non-thermal radiation.
Title: Faraday depolarization and induced circular polarization by
multipath propagation with application to FRBs
Authors: Beniamini, Paz; Kumar, Pawan; Narayan, Ramesh
Bibcode: 2022MNRAS.510.4654B
Altcode: 2021MNRAS.tmp.3445B; 2021arXiv211000028B
We describe how the observed polarization properties of an astronomical
object are related to its intrinsic polarization properties and
the finite temporal and spectral resolutions of the observing
device. Moreover, we discuss the effect that a scattering screen,
with non-zero magnetic field, between the source and observer has on
the observed polarization properties. We show that the polarization
properties are determined by the ratio of observing bandwidth and
coherence bandwidth of the scattering screen and the ratio of temporal
resolution of the instrument and the variability time of screen,
as long as the length over which the Faraday rotation induced by the
screen changes by ~π is smaller than the size of the screen visible
to the observer. We describe the conditions under which a source that
is 100 per cent linearly polarized intrinsically might be observed
as partially depolarized, and how the source's temporal variability
can be distinguished from the temporal variability induced by the
scattering screen. In general, linearly polarized waves passing
through a magnetized scattering screen can develop a significant
circular polarization. We apply the work to the observed polarization
properties of a few fast radio bursts (FRBs), and outline potential
applications to pulsars.
Title: Evolution of the Sun's activity and the poleward transport
of remnant magnetic flux in Cycles 21-24
Authors: Mordvinov, Alexander V.; Karak, Bidya Binay; Banerjee,
Dipankar; Golubeva, Elena M.; Khlystova, Anna I.; Zhukova, Anastasiya
V.; Kumar, Pawan
Bibcode: 2022MNRAS.510.1331M
Altcode: 2021arXiv211115585M; 2021MNRAS.tmp.3199M
Detailed study of the solar magnetic field is crucial to understand
its generation, transport, and reversals. The timing of the reversals
may have implications on space weather and thus identification of the
temporal behaviour of the critical surges that lead to the polar field
reversals is important. We analyse the evolution of solar activity and
magnetic flux transport in Cycles 21-24. We identify critical surges
of remnant flux that reach the Sun's poles and lead to the polar field
reversals. We reexamine the polar field build-up and reversals in their
causal relation to the Sun's low-latitude activity. We further identify
the major remnant flux surges and their sources in the time-latitude
aspect. We find that special characteristics of individual 11-yr
cycles are generally determined by the spatiotemporal organization
of emergent magnetic flux and its unusual properties. We find a
complicated restructuring of high-latitude magnetic fields in Cycle
21. The global rearrangements of solar magnetic fields were caused
by surges of trailing and leading polarities that occurred near the
activity maximum. The decay of non-Joy and anti-Hale active regions
resulted in the remnant flux surges that disturbed the usual order in
magnetic flux transport. We finally show that the leading-polarity
surges during cycle minima sometimes link the following cycle and a
collective effect of these surges may lead to secular changes in the
solar activity. The magnetic field from a Babcock-Leighton dynamo
model generally agrees with these observations.
Title: Implications of a rapidly varying FRB in a globular cluster
of M81
Authors: Lu, Wenbin; Beniamini, Paz; Kumar, Pawan
Bibcode: 2022MNRAS.510.1867L
Altcode: 2021arXiv210704059L; 2021MNRAS.tmp.3156L
A repeating source of fast radio bursts (FRBs) is recently discovered
from a globular cluster of M81. Association with a globular cluster
(or other old stellar systems) suggests that strongly magnetized neutron
stars, which are the most likely objects responsible for FRBs, are born
not only when young massive stars undergo core-collapse, but also by
mergers of old white dwarfs. We find that the fractional contribution
to the total FRB rate by old stellar populations is at least a few per
cent, and the precise fraction can be constrained by FRB searches in
the directions of nearby galaxies, both star-forming and elliptical
ones. Using very general arguments, we show that the activity time of
the M81-FRB source is between 104 and 106 yr,
and more likely of the order of 105 yr. The energetics of
radio outbursts put a lower limit on the magnetic field strength of
10$^{13}\,$G, and the spin period $\gtrsim 0.2\,$s, thereby ruling
out the source being a milli-second pulsar. The upper limit on the
persistent X-ray luminosity (provided by Chandra), together with the
high FRB luminosity and frequent repetitions, severely constrains
(or rules out) the possibility that the M81-FRB is a scaled-up version
of giant pulses from Galactic pulsars. Finally, the 50-ns variability
time of the FRB light curve suggests that the emission is produced in
a compact region inside the neutron star magnetosphere, as it cannot
be accounted for when the emission is at distances $\gtrsim 10^{10}\rm
\, cm$.
Title: Multiscale Photonic Emissivity Engineering for Relativistic
Lightsail Thermal Regulation
Authors: Brewer, John; Campbell, Matthew F.; Kumar, Pawan; Kulkarni,
Sachin; Jariwala, Deep; Bargatin, Igor; Raman, Aaswath P.
Bibcode: 2022NanoL..22..594B
Altcode: 2021arXiv210603558B
The Breakthrough Starshot Initiative aims to send a gram-scale probe
to Proxima Centuri B using a laser-accelerated lightsail traveling at
relativistic speeds. Thermal management is a key lightsail design
objective because of the intense laser powers required but has
generally been considered secondary to accelerative performance. Here,
we demonstrate nanophotonic photonic crystal slab reflectors composed
of 2H-phase molybdenum disulfide and crystalline silicon nitride,
highlight the inverse relationship between the thermal band extinction
coefficient and the lightsail's maximum temperature, and examine the
trade-off between the acceleration distance and setting realistic sail
thermal limits, ultimately realizing a thermally endurable acceleration
minimum distance of 16.3~Gm. We additionally demonstrate multi-scale
photonic structures featuring thermal-wavelength-scale Mie resonant
geometries, and characterize their broadband Mie resonance-driven
emissivity enhancement and acceleration distance reduction. Our
results highlight new possibilities in simultaneously controlling
optical and thermal response over broad wavelength ranges in ultralight
nanophotonic structures.
Title: On explaining prompt emission from GRB central engines with
photospheric emission model
Authors: Bhattacharya, Mukul; Kumar, Pawan
Bibcode: 2021arXiv211014792B
Altcode:
Although the observed spectra for gamma-ray burst (GRB) prompt emission
is well constrained, the underlying radiation mechanism is still not
very well understood. We explore photospheric emission in GRB jets by
modelling the Comptonization of fast cooled synchrotron photons whilst
the electrons and protons are accelerated to highly relativistic
energies by repeated energy dissipation events as well as Coulomb
collisions. In contrast to the previous simulations, we implement
realistic photon-to-particle number ratios of $N_{\gamma}/N_e \sim
10^{5}$ or higher, that are consistent with the observed radiation
efficiency of relativistic jets. Using our Monte Carlo radiation
transfer (MCRaT) code, we can successfully model the prompt emission
spectra when the electrons are momentarily accelerated to highly
relativistic energies (Lorentz factor $\sim 50-100$) after getting
powered by $\sim30-50$ episodic dissipation events in addition to their
Coulomb coupling with the jet protons, and for baryonic outflows that
originate from moderate optical depths $\sim20-30$. We also show that
the resultant shape of the photon spectrum is practically independent
of the initial photon energy distribution and the jet baryonic energy
content, and hence independent of the emission mechanism.
Title: Supercriticality of the Dynamo Limits the Memory of the Polar
Field to One Cycle
Authors: Kumar, Pawan; Karak, Bidya Binay; Vashishth, Vindya
Bibcode: 2021ApJ...913...65K
Altcode: 2021arXiv210311754K
The polar magnetic field precursor is considered to be the most
robust and physics-based method for the prediction of the next solar
cycle strength. However, to make a reliable prediction of a cycle,
is the polar field at the solar minimum of the previous cycle enough
or do we need the polar field of many previous cycles? To answer this
question, we performed several simulations using Babcock-Leighton-type
flux-transport dynamo models with a stochastically forced source for
the poloidal field (α term). We show that when the dynamo is operating
near the critical dynamo transition or only weakly supercritical, the
polar field of cycle n determines the amplitude of the next several
cycles (at least three). However, when the dynamo is substantially
supercritical, this correlation of the polar field is reduced to one
cycle. This change in the memory of the polar field from multiple to
one cycle with the increase of the supercriticality of the dynamo is
independent of the importance of various turbulent transport processes
in the model. Our this conclusion contradicts the existing idea. We
further show that when the dynamo operates near the critical transition,
it produces frequent extended episodes of weaker activity, resembling
the solar grand minima. The occurrence of grand minima is accompanied
by the multicycle correlation of the polar field. The frequency of grand
minima decreases with the increase of supercriticality of the dynamo.
Title: Fast radio burst dispersion measure distribution as a probe
of helium reionization
Authors: Bhattacharya, Mukul; Kumar, Pawan; Linder, Eric V.
Bibcode: 2021PhRvD.103j3526B
Altcode: 2020arXiv201014530B
Fast radio burst (FRB) discoveries are occurring rapidly, with thousands
expected from upcoming surveys. The dispersion measures (DM) observed
for FRB include important information on cosmological distances and the
ionization state of the universe from the redshift of emission until
today. Rather than considering the DM-redshift relation, we investigate
the statistical ensemble of the distribution of dispersion measures. We
explore the use of this abundance information, with and without
redshift information, to probe helium reionization through simulated
data to redshift z =6 . Carrying out Monte Carlo simulations of FRB
survey samples, we examine the effect of different source redshift
distributions, host galaxy models, sudden vs gradual reionization,
and covariance with cosmological parameters on determination of helium
reionization properties. We find that a fluence limited survey with
104 FRBs can discriminate different helium reionization
histories at ∼6 σ using the DM-distribution of bursts, without
redshift information (and ∼10 σ with redshifts).
Title: Exploring the epoch of hydrogen reionization using FRBs
Authors: Beniamini, Paz; Kumar, Pawan; Ma, Xiangcheng; Quataert, Eliot
Bibcode: 2021MNRAS.502.5134B
Altcode: 2021MNRAS.tmp..339B; 2020arXiv201111643B
We describe three different methods for exploring the hydrogen
reionization epoch using fast radio bursts (FRBs) and provide arguments
for the existence of FRBs at high redshift (z). The simplest way,
observationally, is to determine the maximum dispersion measure
(DMmax) of FRBs for an ensemble that includes bursts during
the reionization. The DMmax provides information regarding
reionization much like the optical depth of the cosmic microwave
background to Thomson scattering does, and it has the potential to be
more accurate than constraints from Planck, if DMmax can
be measured to a precision better than 500 pccm-3. Another
method is to measure redshifts of about 40 FRBs between z of 6 and
10 with ${\sim}10{{\ \rm per\ cent}}$ accuracy to obtain the average
electron density in four different z-bins with ${\sim}4{{\ \rm per\
cent}}$ accuracy. These two methods do not require knowledge of the
FRB luminosity function and its possible redshift evolution. Finally,
we show that the reionization history is reflected in the number of
FRBs per unit DM, given a fluence limited survey of FRBs that includes
bursts during the reionization epoch; we show using FIRE simulations
that the contribution to DM from the FRB host galaxy and circumgalactic
medium during the reionization era is a small fraction of the observed
DM. This third method requires no redshift information but does require
knowledge of the FRB luminosity function.
Title: The Polar Precursor Method for Solar Cycle Prediction:
Comparison of Predictors and Their Temporal Range
Authors: Kumar, Pawan; Nagy, Melinda; Lemerle, Alexandre; Karak,
Bidya Binay; Petrovay, Kristof
Bibcode: 2021ApJ...909...87K
Altcode: 2021arXiv210105013K
The polar precursor method is widely considered to be the most robust
physically motivated method to predict the amplitude of an upcoming
solar cycle. It uses indicators of the magnetic field concentrated near
the poles around the sunspot minimum. Here, we present an extensive
analysis of the performance of various such predictors, based on both
observational data (Wilcox Solar Observatory (WSO) magnetograms, Mount
Wilson Observatory polar faculae counts, and Pulkovo A(t) index) and
outputs (polar cap magnetic flux and global dipole moment) of various
existing flux transport dynamo models. We calculate Pearson correlation
coefficients (r) of the predictors with the next cycle amplitude
as a function of time measured from several solar cycle landmarks:
setting r = 0.8 as a lower limit for acceptable predictions, we find
that observations and models alike indicate that the earliest time
when the polar predictor can be safely used is 4 yr after the polar
field reversal. This is typically 2-3 yr before the solar minimum
and about 7 yr before the predicted maximum, considerably extending
the usual temporal scope of the polar precursor method. Reevaluating
the predictors another 3 yr later, at the time of the solar minimum,
further increases the correlation level to r ≳ 0.9. As an illustration
of the result, we determine the predicted amplitude of Cycle 25 based
on the value of the WSO polar field at the now official minimum date
of 2019 December as 126 ± 3. A forecast based on the value in early
2017, 4 yr after the polar reversal would have only differed from this
final prediction by 3.1 ± 14.7%.
Title: The LIGO HET Response (LIGHETR) Project to Discover and
Spectroscopically Follow Optical Transients Associated with Neutron
Star Mergers
Authors: Bustamante Rosell, María José; Wheeler, Craig; Gebhardt,
Karl; Zimmerman, Aaron; Matzner, Richard; Zeimann, Greg; Shetrone,
Matthew; Janowiecki, Steven; Kumar, Pawan; Pooley, David; Thomas,
Benjamin P.; Hanna, Chad; Radice, David; Wang, Lifan; Chen, Sijie;
Vinkó, Jozsef; Sand, David; Fryer, Chris; Korobkin, Oleg; Wollaeger,
Ryan; Hessman, Frederic V.; McQuinn, Kristen B.
Bibcode: 2021APS..APRB09006B
Altcode:
The LIGO HET Response (LIGHETR) project is a group of several
institutions performing spectroscopic followup of gravitational wave
sources discovered by the LIGO/Virgo collaboration (LVC). LIGHETR uses
two integrated field unit spectrographs (IFUs) with deep coverage in the
blue, VIRUS and LRS2, both mounted on the 11 m Hobby Ebberly Telescope
(HET). Our strategy is to target the most probable galaxies within the
LVC sky-map, with the aim to acquire the earliest, rapidly varying,
blue spectra of the electromagnetic counterparts. Alternatively, we
also perform follow-up on transient candidates identified by other
observatories. The unique challenges of the observations (fixed zenith
angle, IFUs) necessitate custom pipelines for rapid observation planning
and data reductions using novel techniques which will be presented here.
Title: What does FRB light-curve variability tell us about the
emission mechanism?
Authors: Beniamini, Paz; Kumar, Pawan
Bibcode: 2020MNRAS.498..651B
Altcode: 2020arXiv200707265B
A few fast radio bursts' (FRBs) light curves have exhibited large
intrinsic modulations of their flux on extremely short ($t_{\rm r}\sim
10\, \mu$s) time-scales, compared to pulse durations (tFRB ~
1 ms). Light-curve variability time-scales, the small ratio of rise time
of the flux to pulse duration, and the spectro-temporal correlations
in the data constrain the compactness of the source and the mechanism
responsible for the powerful radio emission. The constraints are
strongest when radiation is produced far (≳1010 cm)
from the compact object. We describe different physical set-ups
that can account for the observed tr/tFRB
≪ 1 despite having large emission radii. The result is either
a significant reduction in the radio production efficiency
or distinct light-curve features that could be searched for in
observed data. For the same class of models, we also show that due
to high-latitude emission, if a flux f1(ν1)
is observed at t1 then at a lower frequency
ν2 < ν1 the flux should be at least
(ν2/ν1)2f1 at a slightly
later time (t2 = t1ν1/ν2)
independent of the duration and spectrum of the emission in the comoving
frame. These features can be tested, once light-curve modulations
due to scintillation are accounted for. We provide the time-scales
and coherence bandwidths of the latter for a range of possibilities
regarding the physical screens and the scintillation regime. Finally,
if future highly resolved FRB light curves are shown to have intrinsic
variability extending down to ${\sim}\mu$s time-scales, this will
provide strong evidence in favour of magnetospheric models.
Title: A unified picture of Galactic and cosmological fast radio
bursts
Authors: Lu, Wenbin; Kumar, Pawan; Zhang, Bing
Bibcode: 2020MNRAS.498.1397L
Altcode: 2020arXiv200506736L
The discovery of a fast radio burst (FRB) in our Galaxy associated with
a magnetar (neutron star with strong magnetic field) has provided
a critical piece of information to help us finally understand
these enigmatic transients. We show that the volumetric rate of
Galactic-FRB like events is consistent with the faint end of the
cosmological FRB rate, and hence they most likely belong to the
same class of transients. The Galactic FRB had an accompanying
X-ray burst, but many X-ray bursts from the same object had no radio
counterpart. Their relative rates suggest that for every FRB there
are roughly 102-103 X-ray bursts. The radio
light curve of the Galactic FRB had two spikes, separated by 30 ms
in the 400-800 MHz frequency band. This is an important clue and
highly constraining of the class of models where the radio emission
is produced outside the light cylinder of the magnetar. We suggest
that magnetic disturbances close to the magnetar surface propagate to
a distance of a few tens of neutron star radii where they damp and
produce radio emission. The coincident hard X-ray spikes associated
with the two FRB pulses seen in this burst and the flux ratio between
the two frequency bands can be understood in this scenario. This model
provides a unified picture for faint bursts like the Galactic FRB as
well as the bright events seen at cosmological distances.
Title: Population Modeling of Fast Radio Bursts from Source Properties
Authors: Bhattacharya, Mukul; Kumar, Pawan
Bibcode: 2020ApJ...899..124B
Altcode: 2019arXiv190210225B
We present a method to estimate the source properties of Fast Radio
Bursts (FRBs) from observations by assuming a fixed dispersion measure
contribution from a Milky Way-like host galaxy, pulse temporal
broadening models for turbulent plasma, and a flat FRB energy
spectrum. Then we perform Monte Carlo simulations to constrain the
properties of the FRB source, its host galaxy, and scattering in
the intervening plasma from the observational data of FRBs detected
with Parkes. The typical scatter broadening of the intrinsic pulse
is found to be considerably small, ≲ 10-2 - 1 ms, from
physical models, with the interstellar medium contribution suppressed
significantly relative to that of the intergalactic medium. The
intrinsic width for nonrepeating FRBs is broadened by a factor of ∼2-3
on average, primarily due to dispersive smearing. From the simulations,
we find that the host galaxy dispersion measure contribution is
likely to be comparable to the Galactic contribution and the FRB
energy decreases significantly at high frequencies with a negative
spectral index. The FRB spatial density is found to increase up to
redshift ∼2.0 and then drops significantly at larger distances. We
obtain the energy distribution for FRB 121102 with repetition rate
∼0.1-0.3 hr-1 and exponential energy cutoff that is
significantly smaller compared to typical FRB energies. We find that
the probability of observing none of the other FRBs to be repeating
at Parkes is ∼0.8-0.9 with the current follow-up data insufficient
to suggest more than one class of FRB progenitors.
Title: FRB coherent emission from decay of Alfvén waves
Authors: Kumar, Pawan; Bošnjak, Željka
Bibcode: 2020MNRAS.494.2385K
Altcode: 2020MNRAS.tmp..716K; 2020arXiv200400644K
We present a model for fast radio bursts (FRBs) where a large-amplitude
Alfvén wave packet is launched by a disturbance near the surface of a
magnetar, and a substantial fraction of the wave energy is converted
to coherent radio waves at a distance of a few tens of neutron star
radii. The wave amplitude at the magnetar surface should be about
1011 G in order to produce an FRB of isotropic luminosity
1044 erg s-1. An electric current along the static
magnetic field is required by Alfvén waves with non-zero component of
transverse wave vector. The current is supplied by counter-streaming
electron-positron pairs, which have to move at nearly the speed of light
at larger radii as the plasma density decreases with distance from the
magnetar surface. The counter-streaming pairs are subject to two-stream
instability, which leads to formation of particle bunches of size
of the order of c/ωp, where ωp is the plasma
frequency. A strong electric field develops along the static magnetic
field when the wave packet arrives at a radius where electron-positron
density is insufficient to supply the current required by the wave. The
electric field accelerates particle bunches along the curved magnetic
field lines, and that produces the coherent FRB radiation. We provide
a number of predictions of this model.
Title: Radiation forces constrain the FRB mechanism
Authors: Kumar, Pawan; Lu, Wenbin
Bibcode: 2020MNRAS.494.1217K
Altcode: 2020arXiv200400645K; 2020MNRAS.tmp..730K
We provide constraints on fast radio burst (FRB) models by careful
considerations of radiation forces associated with these powerful
transients. We find that the induced Compton scatterings of the
coherent radiation by electrons/positrons accelerate particles
to very large Lorentz factors (LFs) in and around the source of
this radiation. This severely restricts those models for FRBs that
invoke relativistic shocks and maser-type instabilities at distances
less than about 1013 cm of the neutron star. Radiation
travelling upstream, in these models, forces particles to move away
from the shock with an LF larger than the LF of the shock front. This
suspends the photon generation process after it has been operating
for less than ∼0.1 ms (observer frame duration). We show that masers
operating in shocks at distances larger than 1013 cm cannot
simultaneously account for the burst duration of 1 ms or more and the
observed ∼GHz frequencies of FRBs without requiring an excessive
energy budget (>1046 erg); the energy is not calculated
by imposing any efficiency consideration, or other details, for the
maser mechanism, but is entirely the result of ensuring that particle
acceleration by induced Compton forces upstream of the shock front
does not choke off the maser process. For the source to operate more
or less continuously for a few ms, it should be embedded in a strong
magnetic field - cyclotron frequency ≫ wave frequency - so that
radiation forces do not disperse the plasma and shut off the engine.
Title: Explaining GRB prompt emission with sub-photospheric
dissipation and Comptonization
Authors: Bhattacharya, Mukul; Kumar, Pawan
Bibcode: 2020MNRAS.491.4656B
Altcode: 2019MNRAS.tmp.2762B; 2019arXiv190907398B
Even though the observed spectra for GRB prompt emission is well
constrained, no single radiation mechanism can robustly explain
its distinct non-thermal nature. Here, we explore the radiation
mechanism with the photospheric emission model using our Monte Carlo
Radiative Transfer code. We study the sub-photospheric Comptonization
of fast cooled synchrotron photons while the Maxwellian electrons and
mono-energetic protons are accelerated to relativistic energies by
repeated dissipation events. Unlike previous simulations, we implement
a realistic photon to electron number ratio Nγ/Ne
∼ 105 consistent with the observed radiative efficiency of
a few per cent. We show that it is necessary to have a critical number
of episodic energy injection events Nrh,cr ∼ few tens to
hundreds in the jet in addition to the electron-proton Coulomb coupling
in order to inject sufficient energy Einj,cr ∼ 2500-4000
mec2 per electron and produce an output photon
spectrum consistent with observations. The observed GRB spectrum can
be generated when the electrons are repeatedly accelerated to highly
relativistic energies γe,in ∼ few tens to hundreds in a
jet with bulk Lorentz factor Γ ∼ 30-100, starting out from moderate
optical depths τin ∼ 20-40. The shape of the photon
spectrum is independent of the initial photon energy distribution
and baryonic energy content of the jet and hence independent of the
emission mechanism, as expected for photospheric emission.
Title: Linear polarization in gamma-ray burst prompt emission
Authors: Gill, Ramandeep; Granot, Jonathan; Kumar, Pawan
Bibcode: 2020MNRAS.491.3343G
Altcode: 2019MNRAS.tmp.2582G; 2018arXiv181111555G
Despite being hard to measure, GRB prompt gamma-ray emission
polarization is a valuable probe of the dominant emission mechanism
and the GRB outflow's composition and angular structure. During the
prompt emission the GRB outflow is ultrarelativistic with Lorentz
factors Γ ≫ 1. We describe in detail the linear polarization
properties of various emission mechanisms: synchrotron radiation from
different magnetic field structures (ordered: toroidal Btor
or radial B∥, and random: normal to the radial direction
B⊥), Compton drag, and photospheric emission. We calculate
the polarization for different GRB jet angular structures (e.g. top-hat,
Gaussian, power law) and viewing angles θobs. Synchrotron
with B⊥ can produce large polarizations, up to 25 per
cent≲ Π ≲ 45 per cent, for a top-hat jet but only for lines
of sight just outside (θobs - θj ∼ 1/Γ)
the jet's sharp edge at θ = θj. The same also holds for
Compton drag, albeit with a slightly higher overall Π. Moreover, we
demonstrate how Γ-variations during the GRB or smoother jet edges
(on angular scales ≳ 0.5/Γ) would significantly reduce Π. We
construct a semi-analytic model for non-dissipative photospheric
emission from structured jets. Such emission can produce up to Π ≲
15 per cent with reasonably high fluences, but this requires steep
gradients in Γ(θ). A polarization of 50 per cent≲ Π ≲ 65 per
cent can robustly be produced only by synchrotron emission from a
transverse magnetic field ordered on angles ≳ 1/Γ around our line
of sight (like a global toroidal field, Btor, for 1/Γ <
θobs < θj). Therefore, such a model would
be strongly favoured even by a single secure measurement within this
range. We find that such a model would also be favoured if Π ≳ 20 per
cent is measured in most GRBs within a large enough sample, by deriving
the polarization distribution for our different emission and jet models.
Title: Use of fast radio burst dispersion measures as distance
measures
Authors: Kumar, Pawan; Linder, Eric V.
Bibcode: 2019PhRvD.100h3533K
Altcode: 2019arXiv190308175K
Fast radio bursts appear to be cosmological signals whose frequency-time
structure provides a dispersion measure. The dispersion measure is a
convolution of the cosmic distance element and the electron density,
and contains the possibility of using these events as new cosmological
distance measures. We explore the challenges of extracting the distance
in a robust manner, and give quantitative estimates for the systematics
control needed for fast radio bursts to become a competitive distance
probe. The methodology can also be applied to assessing their use for
mapping electron density fluctuations or helium reionization.
Title: The emission mechanism of gamma-ray bursts: Identification
via optical-IR slope measurements
Authors: Grossan, Bruce; Kumar, Pawan; Smoot, George F.
Bibcode: 2019JHEAp..23...14G
Altcode: 2019arXiv190902152G
There is no consensus on the emission mechanism of γ-ray bursts
(GRBs). A synchrotron model can produce γ-ray spectra with the
empirical Band function form (Band et al., 1993), from a piece-wise
two-power-law electron energy distribution (2EPLS). This synchrotron
model predicts that for the same γ-ray spectrum, optical emission
can be very different in fν log slope, and in flux
relative to γ-rays, depending on model parameter values. This
prediction is consistent with the huge range of optical/γ flux ratios
observed. The model only allows a small set of fν log
slopes in the optical-thereby allowing a clear path to verification
or falsification. Measurements of prompt γ-ray burst emission in
the optical thus far give no useful information about the spectral
shape within the band, and therefore cannot be used to evaluate such
predictions. We describe an experiment that responds to GRB
position alerts with a fast-slewing telescope outfitted with three or
more simultaneously recording, high-time resolution cameras, to measure
the spectral shape of the prompt optical-IR (OIR) emission. Three
channels measure two independent spectral slopes in the OIR region,
the minimum information required to evaluate the model, assuming a
single dominant component. We propose cross-correlation of γ and
OIR light curves to verify that a given GRB is single-component
dominated, or to model and quantify the contributions from other
components. Previous CCD measurements have limited-time resolution due
to read noise, limiting cross-correlation analysis. Electron-multiplied
CCDS (EMCCDs) can be used to greatly reduce read noise, and allow
exposure times of a few hundred ms. Our collaboration has begun a
pathfinder experiment, the Nazarbayev University Transient Telescope
at Assy-Turgen Astrophysical Observatory (NUTTelA-TAO), with a 70 cm
aperture telescope that can point anywhere above the local horizon
in ≤ 8s, with three simultaneous optical channels. The NUTTelA-TAO
is expected to measure the optical slopes of 3-8 GRB/yr, and should
provide a clear verification/refutation of the 2EPLS model after a few
single-component dominated, sufficiently bright GRBs are detected during
prompt emission. A space-based platform would more easily extend the
spectral coverage down to near-IR wavelengths, for greater precision
in measuring spectral slopes, and increased chance of measuring the
self-absorption frequency, which carries valuable information on
physical conditions within the GRB jet. Additional science includes
detection of dust evaporation due to the UV flash from the burst,
which can be used to study dust around a single star at high redshift,
independent of host galaxy dust.
Title: Mergers of black hole-neutron star binaries and rates of
associated electromagnetic counterparts
Authors: Bhattacharya, Mukul; Kumar, Pawan; Smoot, George
Bibcode: 2019MNRAS.486.5289B
Altcode: 2018arXiv180900006B; 2019MNRAS.tmp.1106B
Black hole-neutron star (BHNS) binaries are amongst promising
candidates for the joint detection of electromagnetic (EM) signals
with gravitational waves (GWs) and are expected to be detected in the
near future. Here we study the effect of the BHNS binary parameters on
the merger ejecta properties and associated EM signals. We estimate
the remnant disc and unbound ejecta masses for BH mass and spin
distributions motivated from the observations of transient low-mass
X-ray binaries and a specific NS equation of state (EoS). The amount
of r-process elements synthesized in BHNS mergers is estimated to
be a factor of ∼102-104 smaller than BNS
mergers, due to the smaller dynamical ejecta and merger rates for
the former. We compute the EM luminosities and light curves for
the early- and late-time emissions from the ultrarelativistic jet,
sub-relativistic dynamical ejecta and wind, and the mildly relativistic
cocoon for typical ejecta parameters. We then evaluate the low-latency
EM follow-up rates of the GW triggers in terms of the GW detection
rate \dot{N}_{ GW} for current telescope sensitivities and typical
BHNS binary parameters to find that most of the EM counterparts are
detectable for high BH spin, small BH mass, and a stiffer NS EoS when
NS disruption is significant. Based on the relative detection rates for
given binary parameters, we find the ease of EM follow-up to be: ejecta
afterglow > cocoon afterglow ≳ jet prompt > ejecta macronova
> cocoon prompt > jet afterglow >> wind macronova >>
wind afterglow.
Title: Catching Element Formation In The Act ; The Case for a New
MeV Gamma-Ray Mission: Radionuclide Astronomy in the 2020s
Authors: Timmes, Frank; Fryer, Chris; Timmes, Frank; Hungerford, Aimee
L.; Couture, Aaron; Adams, Fred; Aoki, Wako; Arcones, Almudena; Arnett,
David; Auchettl, Katie; Avila, Melina; Badenes, Carles; Baron, Eddie;
Bauswein, Andreas; Beacom, John; Blackmon, Jeff; Blondin, Stéphane;
Bloser, Peter; Boggs, Steve; Boss, Alan; Brandt, Terri; Bravo, Eduardo;
Brown, Ed; Brown, Peter; Bruenn, Steve; Budtz-Jørgensen, Carl; Burns,
Eric; Calder, Alan; Caputo, Regina; Champagne, Art; Chevalier, Roger;
Chieffi, Alessandro; Chipps, Kelly; Cinabro, David; Clarkson, Ondrea;
Clayton, Don; Coc, Alain; Connolly, Devin; Conroy, Charlie; Côté,
Benoit; Couch, Sean; Dauphas, Nicolas; deBoer, Richard James; Deibel,
Catherine; Denisenkov, Pavel; Desch, Steve; Dessart, Luc; Diehl,
Roland; Doherty, Carolyn; Domínguez, Inma; Dong, Subo; Dwarkadas,
Vikram; Fan, Doreen; Fields, Brian; Fields, Carl; Filippenko, Alex;
Fisher, Robert; Foucart, Francois; Fransson, Claes; Fröhlich, Carla;
Fuller, George; Gibson, Brad; Giryanskaya, Viktoriya; Görres, Joachim;
Goriely, Stéphane; Grebenev, Sergei; Grefenstette, Brian; Grohs,
Evan; Guillochon, James; Harpole, Alice; Harris, Chelsea; Harris,
J. Austin; Harrison, Fiona; Hartmann, Dieter; Hashimoto, Masa-aki;
Heger, Alexander; Hernanz, Margarita; Herwig, Falk; Hirschi, Raphael;
Hix, Raphael William; Höflich, Peter; Hoffman, Robert; Holcomb, Cole;
Hsiao, Eric; Iliadis, Christian; Janiuk, Agnieszka; Janka, Thomas;
Jerkstrand, Anders; Johns, Lucas; Jones, Samuel; José, Jordi; Kajino,
Toshitaka; Karakas, Amanda; Karpov, Platon; Kasen, Dan; Kierans,
Carolyn; Kippen, Marc; Korobkin, Oleg; Kobayashi, Chiaki; Kozma,
Cecilia; Krot, Saha; Kumar, Pawan; Kuvvetli, Irfan; Laird, Alison;
Laming, (John) Martin; Larsson, Josefin; Lattanzio, John; Lattimer,
James; Leising, Mark; Lennarz, Annika; Lentz, Eric; Limongi, Marco;
Lippuner, Jonas; Livne, Eli; Lloyd-Ronning, Nicole; Longland, Richard;
Lopez, Laura A.; Lugaro, Maria; Lutovinov, Alexander; Madsen, Kristin;
Malone, Chris; Matteucci, Francesca; McEnery, Julie; Meisel, Zach;
Messer, Bronson; Metzger, Brian; Meyer, Bradley; Meynet, Georges;
Mezzacappa, Anthony; Miller, Jonah; Miller, Richard; Milne, Peter;
Misch, Wendell; Mitchell, Lee; Mösta, Philipp; Motizuki, Yuko;
Müller, Bernhard; Mumpower, Matthew; Murphy, Jeremiah; Nagataki,
Shigehiro; Nakar, Ehud; Nomoto, Ken'ichi; Nugent, Peter; Nunes,
Filomena; O'Shea, Brian; Oberlack, Uwe; Pain, Steven; Parker, Lucas;
Perego, Albino; Pignatari, Marco; Pinedo, Gabriel Martínez; Plewa,
Tomasz; Poznanski, Dovi; Priedhorsky, William; Pritychenko, Boris;
Radice, David; Ramirez-Ruiz, Enrico; Rauscher, Thomas; Reddy, Sanjay;
Rehm, Ernst; Reifarth, Rene; Richman, Debra; Ricker, Paul; Rijal,
Nabin; Roberts, Luke; Röpke, Friedrich; Rosswog, Stephan; Ruiter,
Ashley J.; Ruiz, Chris; Savin, Daniel Wolf; Schatz, Hendrik; Schneider,
Dieter; Schwab, Josiah; Seitenzahl, Ivo; Shen, Ken; Siegert, Thomas;
Sim, Stuart; Smith, David; Smith, Karl; Smith, Michael; Sollerman,
Jesper; Sprouse, Trevor; Spyrou, Artemis; Starrfield, Sumner; Steiner,
Andrew; Strong, Andrew W.; Sukhbold, Tuguldur; Suntzeff, Nick; Surman,
Rebecca; Tanimori, Toru; The, Lih-Sin; Thielemann, Friedrich-Karl;
Tolstov, Alexey; Tominaga, Nozomu; Tomsick, John; Townsley, Dean;
Tsintari, Pelagia; Tsygankov, Sergey; Vartanyan, David; Venters, Tonia;
Vestrand, Tom; Vink, Jacco; Waldman, Roni; Wang, Lifang; Wang, Xilu;
Warren, MacKenzie; West, Christopher; Wheeler, J. Craig; Wiescher,
Michael; Winkler, Christoph; Winter, Lisa; Wolf, Bill; Woolf, Richard;
Woosley, Stan; Wu, Jin; Wrede, Chris; Yamada, Shoichi; Young, Patrick;
Zegers, Remco; Zingale, Michael; Portegies Zwart, Simon
Bibcode: 2019BAAS...51c...2T
Altcode: 2019astro2020T...2T; 2019arXiv190202915F
Gamma-ray astronomy explores the most energetic photons in
nature to address some of the most pressing puzzles in contemporary
astrophysics. The unique information provided by MeV gamma-ray astronomy
to help address frontiers makes now a compelling time for the community
to advocate for a new mission to be operational in the 2020s and beyond.
Title: Measurement of the Optical-IR Spectral Shape of Prompt
Gamma-Ray Burst Emission: A Timely Call to Action for Gamma-Ray
Burst Science
Authors: Grossan, Bruce; Kumar, Pawan; Hurley, Kevin; Zhang, Bing
Bibcode: 2019BAAS...51c..47G
Altcode: 2019astro2020T..47G
Gamma-ray burst (GRB) spectra appear the same in gamma bands for
different emission mechanisms. The optical-IR (OIR) spectra are
different, however, allowing a clear test of synchrotron mechanisms. We
argue that multi-channel OIR instruments should therefore be added to
GRB-capable missions to do this important, undervalued science.
Title: Fast radio burst source properties from polarization
measurements
Authors: Lu, Wenbin; Kumar, Pawan; Narayan, Ramesh
Bibcode: 2019MNRAS.483..359L
Altcode: 2018arXiv181009459L; 2018MNRAS.tmp.2702L
Recent polarization measurements of fast radio bursts (FRBs)
provide new insights on these enigmatic sources. We show that the
nearly 100 per cent linear polarization and small variation of the
polarization position angles (PAs) of multiple bursts from the same
source suggest that the radiation is produced near the surface
of a strongly magnetized neutron star. As the emitted radiation
travels through the magnetosphere, the electric vector of the X-mode
wave adiabatically rotates and stays perpendicular to the local
magnetic field direction. The PA freezes at a radius where the plasma
density becomes too small to be able to turn the electric vector. At
the freeze-out radius, the electric field is perpendicular to the
magnetic dipole moment of the neutron star projected in the plane of
the sky, independent of the radiation mechanism or the orientation
of the magnetic field in the emission region. We discuss a number
of predictions of the model. The variation of PAs from repeating
FRBs should follow the rotational period of the underlying neutron
star (but the burst occurrence may not be periodic). Measuring this
period will provide crucial support for the neutron star nature of the
progenitors of FRBs. For FRB 121102, the small range of PA variation
means that the magnetic inclination angle is less than about 20° and
that the observer's line of sight is outside the magnetic inclination
cone. Other repeating FRBs may have a different range of PA variation
from that of FRB 121102, depending on the magnetic inclination and
the observer's viewing angle.
Title: The maximum luminosity of fast radio bursts
Authors: Lu, Wenbin; Kumar, Pawan
Bibcode: 2019MNRAS.483L..93L
Altcode: 2018MNRAS.tmpL.198L; 2018arXiv181011501L
Under the assumption that fast radio bursts (FRBs) are from coherent
curvature emission powered by the dissipation of magnetic energy in
the magnetosphere of neutron stars, we show that there is a maximum
isotropic equivalent luminosity L_max∼ (2× 10^{47} erg s^{-1})
min(ρ _6^2, B_{16}ρ _6^{4/3}ν _9^{-2/3}), where ρ _6=ρ /10 km
is the curvature radius of the magnetic field lines near the source
region, B_{16} = B/10^{16} G is the local magnetic field strength,
and ν9 = ν/GHz is the FRB wave frequency. This is
because the electric field responsible for accelerating the emitting
particles becomes close to the quantum critical strength and is then
quickly shielded by Schwinger pairs within a nano-second. Future
observations should be able to measure this cut-off luminosity and
hence provide a unique probe of the source location and magnetic field
strength. We discuss the effects of Lmax on the observed
flux distributions for repeating bursts from the same object and for
the entire FRB population.
Title: On the Missing Energy Puzzle of Tidal Disruption Events
Authors: Lu, Wenbin; Kumar, Pawan
Bibcode: 2018ApJ...865..128L
Altcode: 2018arXiv180202151L
For the majority of tidal disruption event (TDE) candidates, the
observed energy in the optical/near-UV bands is of order 1051
erg. We show that this observed energy is smaller than the minimum
bolometric energy for the radiative inefficient accretion flow model
by a factor of 10-100. We argue that this discrepancy is because the
majority of the energy released is in the extreme-UV (EUV) band and/or
in the form of relativistic jets beamed away from the Earth. The
EUV scenario is supported by existing mid-infrared data and should
be further tested by future dust reverberation observations. The jet
scenario is disfavored by radio observations of ASASSN-14li but may
still be viable for other TDE candidates. We also provide evidence
that, at least for some TDEs, most of the missing energy (in the
EUV and/or in the form of jets) is released within a few times the
orbital period of the most tightly bound material P min,
which means (1) the circularization of the fallback stream may occur
rapidly and (2) the luminosity of the accretion flow or the jet power
may not be capped near the Eddington level when the fallback rate is
super-Eddington. For most other TDEs, this energy-release timescale
is currently not strongly constrained.
Title: Radio Emission from the Cocoon of a GRB Jet: Implications
for Relativistic Supernovae and Off-axis GRB Emission
Authors: De Colle, Fabio; Kumar, Pawan; Aguilera-Dena, David R.
Bibcode: 2018ApJ...863...32D
Altcode: 2018arXiv180300602D
Relativistic supernovae constitute a subclass of Type Ic supernovae
(SNe). Their nonthermal, radio emission differs notably from that of
regular Type Ic supernovae as they have a fast expansion speed (with
velocities ∼0.6-0.8 c) which cannot be explained by a “standard”
spherical SN explosion, but advocates for a quickly evolving, mildly
relativistic ejecta associated with the SN. In this paper, we compute
the synchrotron radiation emitted by the cocoon of a long gamma-ray
burst jet (GRB). We show that the energy and velocity of the expanding
cocoon, and the radio nonthermal light curves and spectra are consistent
with those observed in relativistic SNe. Thus, the radio emission from
this events is not coming from the SN shock front, but from the mildly
relativistic cocoon produced by the passage of a GRB jet through the
progenitor star. We also show that the cocoon radio emission dominates
the GRB emission at early times for GRBs seen off-axis, and the flux
can be larger at late times compared with on-axis GRBs if the cocoon
energy is at least comparable with respect to the GRB energy.
Title: Thermal and non-thermal emission from the cocoon of a gamma-ray
burst jet
Authors: De Colle, Fabio; Lu, Wenbin; Kumar, Pawan; Ramirez-Ruiz,
Enrico; Smoot, George
Bibcode: 2018MNRAS.478.4553D
Altcode: 2017arXiv170105198D; 2018MNRAS.tmp.1249D
We present hydrodynamic simulations of the hot cocoon produced when a
relativistic jet passes through the gamma-ray burst (GRB) progenitor
star and its environment, and we compute the light curve and spectrum
of the radiation emitted by the cocoon. The radiation from the cocoon
has a nearly thermal spectrum with a peak in the X-ray band, and it
lasts for a few minutes in the observer frame; the cocoon radiation
starts at roughly the same time as when γ-rays from a burst trigger
detectors aboard GRB satellites. The isotropic cocoon luminosity
(∼1047 erg s-1) is a few times smaller than
the X-ray luminosity of a typical long-GRB afterglow during the
plateau phase. This radiation should be identifiable in the Swift
data because of its nearly thermal spectrum that is distinct from the
somewhat brighter power-law component. The detection of this thermal
component would provide information regarding the size and density
stratification of the GRB progenitor star. Photons from the cocoon are
also inverse-Compton (IC) scattered by electrons in a delayed jet. We
present the IC light curve and spectrum by post-processing the results
of the numerical simulations. The IC spectrum lies in 10 keV-MeV band
for typical GRB parameters. The detection of this IC component would
provide an independent measurement of GRB jet Lorentz factor, and it
would also help to determine the jet magnetization parameter.
Title: GW170817 Most Likely Made a Black Hole
Authors: Pooley, David; Kumar, Pawan; Wheeler, J. Craig; Grossan, Bruce
Bibcode: 2018ApJ...859L..23P
Altcode: 2017arXiv171203240P
There are two outstanding issues regarding the neutron-star merger
event GW170817: the nature of the compact remnant and the interstellar
shock. The mass of the remnant of GW170817, ∼2.7 {M}⊙ ,
implies that the remnant could be either a massive rotating neutron
star, or a black hole. We report Chandra Director’s Discretionary
Time observations made in 2017 December and 2018 January, and we
reanalyze earlier observations from 2017 August and 2017 September,
in order to address these unresolved issues. We estimate the X-ray
flux from a neutron star remnant and compare that to the measured
X-ray flux. If we assume that the spin-down luminosity of any putative
neutron star is converted to pulsar wind nebula X-ray emission in the
0.5-8 keV band with an efficiency of 10-3, for a dipole
magnetic field with 3 × 1011 G < B < 1014
G, a rising X-ray signal would result and would be brighter than that
observed by day 107; we therefore conclude that the remnant of GW170817
is most likely a black hole. Independent of any assumptions of X-ray
efficiency, however, if the remnant is a rapidly rotating magnetized
neutron star, the total energy in the external shock should rise by a
factor ∼102 (to ∼1052 erg) after a few years;
therefore, Chandra observations over the next year or two that do not
show substantial brightening will rule out such a remnant. The same
observations can distinguish between two different models for the
relativistic outflow, either an angular or radially varying structure.
Title: Seeking observational evidence for the formation of trapping
horizons in astrophysical black holes
Authors: Carballo-Rubio, Raúl; Kumar, Pawan; Lu, Wenbin
Bibcode: 2018PhRvD..97l3012C
Altcode: 2018arXiv180400663C
Black holes in general relativity are characterized by their trapping
horizon, a one-way membrane that can be crossed only inwards. The
existence of trapping horizons in astrophysical black holes can
be tested observationally using a reductio ad absurdum argument,
replacing black holes by horizonless configurations with a physical
surface and looking for inconsistencies with electromagnetic and
gravitational wave observations. In this approach, the radius of the
horizonless object is always larger than but arbitrarily close to the
position where the horizon of a black hole of the same mass would be
located. Upper bounds on the radius of these alternatives have been
provided using electromagnetic observations (in the optical/IR band)
of astronomical sources at the center of galaxies, but lower bounds
were lacking, leaving unconstrained huge regions of parameter space. We
show here that lower bounds on the radius of horizonless objects that
do not develop trapping horizons can be placed using observations of
accreting systems. This result is model independent and relies only
on the local notion of causality dictated by the spacetime geometry
around the horizonless object. These observational bounds reduce
considerably the previously allowed parameter space, boosting the
prospects of establishing the existence of trapping horizons using
electromagnetic observations.
Title: On the radiation mechanism of repeating fast radio bursts
Authors: Lu, Wenbin; Kumar, Pawan
Bibcode: 2018MNRAS.477.2470L
Altcode: 2018MNRAS.tmp..703L; 2017arXiv171010270L
Recent observations show that fast radio bursts (FRBs) are energetic
but probably non-catastrophic events occurring at cosmological
distances. The properties of their progenitors are largely unknown
in spite of many attempts to determine them using the event rate,
duration, and energetics. Understanding the radiation mechanism for
FRBs should provide the missing insights regarding their progenitors,
which is investigated in this paper. The high brightness temperatures
(≳1035 K) of FRBs mean that the emission process must be
coherent. Two general classes of coherent radiation mechanisms are
considered - maser and the antenna mechanism. We use the observed
properties of the repeater FRB 121102 to constrain the plasma
conditions needed for these two mechanisms. We have looked into a
wide variety of maser mechanisms operating in either vacuum or plasma
and find that none of them can explain the high luminosity of FRBs
without invoking unrealistic or fine-tuned plasma conditions. The
most favourable mechanism is antenna curvature emission by coherent
charge bunches where the burst is powered by magnetic reconnection
near the surface of a magnetar (B ≳ 1014 G). We show that
the plasma in the twisted magnetosphere of a magnetar may be clumpy
due to two-stream instability. When magnetic reconnection occurs,
the pre-existing density clumps may provide charge bunches for the
antenna mechanism to operate. This model should be applicable to all
FRBs that have multiple outbursts like FRB 121102.
Title: Monte Carlo Simulations of Photospheric Emission in
Relativistic Outflows
Authors: Bhattacharya, Mukul; Lu, Wenbin; Kumar, Pawan; Santana,
Rodolfo
Bibcode: 2018ApJ...852...24B
Altcode: 2016arXiv161106209B
We study the spectra of photospheric emission from highly relativistic
gamma-ray burst outflows using a Monte Carlo code. We consider the
Comptonization of photons with a fast-cooled synchrotron spectrum
in a relativistic jet with a realistic photon-to-electron number
ratio {N}γ /{N}{{e}}={10}5, using
mono-energetic protons that interact with thermalized electrons through
Coulomb interaction. The photons, electrons, and protons are cooled
adiabatically as the jet expands outward. We find that the initial
energy distributions of the protons and electrons do not have any
appreciable effect on the photon peak energy {E}γ ,{peak}
and the power-law spectrum above {E}γ ,{peak}. The
Coulomb interaction between the electrons and the protons does not
affect the output photon spectrum significantly as the energy of
the electrons is elevated only marginally. {E}γ ,{peak}
and the spectral indices for the low- and high-energy power-law
tails of the photon spectrum remain practically unchanged even with
electron-proton coupling. Increasing the initial optical depth {τ
}{in} results in a slightly shallower photon spectrum
below {E}γ ,{peak} and fewer photons at the high-energy
tail, although {f}ν \propto {ν }-0.5 above
{E}γ ,{peak} and up to ∼1 MeV, independent of {τ
}{in}. We find that {E}γ ,{peak} determines the
peak energy and the shape of the output photon spectrum. Finally,
we find that our simulation results are quite sensitive to
{N}γ /{N}{{e}}, for {N}{{e}}=3×
{10}3. For almost all our simulations, we obtain an output
photon spectrum with a power-law tail above {E}γ ,{peak}
extending up to ∼1 MeV.
Title: Radiative interaction between the relativistic jet and
optically thick envelope in tidal disruption events
Authors: Lu, Wenbin; Krolik, Julian; Crumley, Patrick; Kumar, Pawan
Bibcode: 2017MNRAS.471.1141L
Altcode: 2017arXiv170609414L
Reverberation observations yielding a lag spectrum have uncovered an
Fe K α fluorescence line in the tidal disruption event (TDE) Swift
J1644+57. The discovery paper used the lag spectrum to argue that the
source of the X-ray continuum was located very close to the black hole
(∼30 gravitational radii) and moved subrelativistically. We reanalyse
the lag spectrum, pointing out that dilution effects cause it to
indicate a geometric scale an order of magnitude larger than inferred
by Kara et al. If the X-ray continuum is produced by a relativistic
jet, as suggested by the rapid variability, high luminosity and hard
spectrum, this larger scale predicts an Fe ionization state consistent
with efficient K α photon production. Moreover, the momentum of the jet
X-rays impinging on the surrounding accretion flow on this large scale
accelerates a layer of gas to speeds ∼0.1-0.2c, consistent with the
blueshifted line profile. Implications of our results on the global
picture of jetted TDEs are discussed. A power-law γ/X-ray spectrum
may be produced by external ultraviolet (UV)-optical photons being
repetitively inverse-Compton scattered by cold electrons in the jet,
although our model for the K α reverberation does not depend on the jet
radiation mechanism (magnetic reconnection in a Poynting jet is still
a viable mechanism). The non-relativistic wind driven by jet radiation
may explain the late-time radio rebrightening in Swift J1644+57. This
energy injection may also cause the thermal UV-optical emission from
jetted TDEs to be systematically brighter than in non-jetted ones.
Title: Fast radio burst source properties and curvature radiation
model
Authors: Kumar, Pawan; Lu, Wenbin; Bhattacharya, Mukul
Bibcode: 2017MNRAS.468.2726K
Altcode: 2017arXiv170306139K
We use the observed properties of fast radio bursts (FRBs) and a number
of general physical considerations to provide a broad-brush model for
the physical properties of FRB sources and the radiation mechanism. We
show that the magnetic field in the source region should be at least
1014 G. This strong field is required to ensure that the
electrons have sufficiently high ground state Landau energy so that
particle collisions, instabilities and strong electromagnetic fields
associated with the FRB radiation do not perturb electrons' motion
in the direction transverse to the magnetic field and destroy their
coherent motion; coherence is required by the high observed brightness
temperature of FRB radiation. The electric field in the source region
required to sustain particle motion for a wave period is estimated to
be of the order of 1011 esu. These requirements suggest
that FRBs are produced near the surface of magnetars perhaps via
forced reconnection of magnetic fields to produce episodic, repeated,
outbursts. The beaming-corrected energy release in these bursts is
estimated to be about 1036 erg, whereas the total energy
in the magnetic field is at least ∼1045 erg. We provide
a number of predictions for this model which can be tested by future
observations. One of which is that short duration FRB-like bursts
should exist at much higher frequencies, possibly up to optical.
Title: Stellar disruption events support the existence of the black
hole event horizon
Authors: Lu, Wenbin; Kumar, Pawan; Narayan, Ramesh
Bibcode: 2017MNRAS.468..910L
Altcode: 2017arXiv170300023L
Many black hole (BH) candidates have been discovered in X-ray binaries
and in the nuclei of galaxies. The prediction of Einstein's general
relativity is that BHs have an event horizon - a one-way membrane
through which particles fall into the BH but cannot exit. However,
except for the very few nearby supermassive BH candidates, our
telescopes are unable to resolve and provide a direct proof of the
event horizon. Here, we propose a novel observation that supports
the existence of event horizons around supermassive BH candidates
heavier than 107.5 M⊙. Instead of an event
horizon, if the BH candidate has a hard surface, when a star falls
on to the surface, the shocked baryonic gas will form a radiation
pressure-supported envelope that shines at the Eddington luminosity
for an extended period of time from months to years. We show that such
emission has already been ruled out by the Pan-STARRS1 3π survey
if supermassive BH candidates have a hard surface at radius larger
than (1 + 10-4.4) times the Schwarzschild radius. Future
observations by Large Synoptic Survey Telescope should be able to
improve the limit to 1 + 10-6.
Title: A universal EDF for repeating fast radio bursts?
Authors: Lu, Wenbin; Kumar, Pawan
Bibcode: 2016MNRAS.461L.122L
Altcode: 2016arXiv160504605L
We make three assumptions: fast radio bursts (FRBs) are produced
by neutron stars at cosmological distances; FRB rate tracks the
core-collapse supernova rate; and all FRBs repeat with a universal
energy distribution function (EDF) ddot{N}/dE ∝ E^{-β } with a cutoff
at burst energy Emax. We then find that observations so far
are consistent with a universal EDF with 1.5 ≲ β ≲ 2.2, high-end
cutoff Emax/E0 ≳ 30 and normalization dot{N}_0
≲ 2 d^{-1}; where dot{N}_0 is the integrated rate above the reference
energy E_0 ∼eq 1.2× 10^{39} f_r^{-1} erg (fr is the
radio emission efficiency). Implications of such an EDF are discussed.
Title: Dynamic Impact of Fluoride Dust on Industrial Workers in
Thermal Power Plant and its Feasibility Study on Troposphere
Authors: Katiyar, Swati; Kumar, Pawan
Bibcode: 2016cosp...41E.955K
Altcode:
Fluorine is a common element that does not occur in the elemental state
in nature because of its high reactivity. It accounts for about 0.3
g kg-1 of the Earth's crust and exists in the form of fluorides in a
number of minerals, of which fluorspar, cryolite and fluorapatite are
the most common. This paper focuses on the analysis of flouride on the
industrial workers in various working conditions on troposphere. To
check the impact of flouride on workers various samples were taken from
different conditions of aluminum plant industries like pot room workers
and non-pot room workers as fluoride has both beneficial and detrimental
effects on human health. 50 workers in pot room and 10 workers in non
pot room were chosen for taking urine and serum samples. 0.09 to 3.77
mg Kg -1 and 0.39 to 1.15 mg Kg-1 (of ash weight) was recorded in the
nails of pot room and non pot room workers respectively. The average
flouride content was recorded as 1.10 mg Kg -1 and 0.65 mg Kg -1 in
pot room and non pot room workers respectively. The outcome results
clearly indicated the ill effect and dangerous for the dental health
as well as physical health of the workers. A preventive measure or
precaution should be taken by the management or persons to avoid the
impact of flouride on the body. The clinical significance lie in the
maintaining hygienic condition while preventing the any possible effect
of flouride on the workers of the industries, as this will affect the
production as well as the human value in term of physical capabilities
and social aspects in providing medical facilities. Keywords: Fluoride,
Ecosystem, Dynamic impact, Air pollutant, detrimental effects.
Title: Adverse Impact of Electromagnetic Radiation on Urban
Environment and Natural Resources using Optical Sensors
Authors: Kumar, Pawan; Katiyar, Swati; Rani, Meenu
Bibcode: 2016cosp...41E1084K
Altcode:
We are living in the age of a rapidly growing population and changing
environmental conditions with an advance technical capacity.This has
resulted in wide spread land cover change. One of the main causes
for increasing urban heat is that more than half of the world's
population lives in a rapidly growing urbanized environment. Satellite
data can be highly useful to map change in land cover and other
environmental phenomena with the passage of time. Among several
human-induced environmental and urban thermal problems are reported
to be negatively affecting urban residents in many ways. The built-up
structures in urbanized areas considerably alter land cover thereby
affecting thermal energy flow which leads to development of elevated
surface and air temperature. The phenomenon Urban Heat Island implies
'island' of high temperature in cities, surrounded by relatively lower
temperature in rural areas. The UHI for the temporal period is estimated
using geospatial techniques which are then utilized for the impact
assessment on climate of the surrounding regions and how it reduce
the sustainability of the natural resources like air, vegetation. The
present paper describes the methodology and resolution dynamic urban
heat island change on climate using the geospatial approach. NDVI
were generated using day time LANDSAT ETM+ image of 1990, 2000 and
2013. Temperature of various land use and land cover categories was
estimated. Keywords: NDVI, Surface temperature, Dynamic changes.
Title: India's mission to Mars cost less than the movie Gravity:
Multidimensional View in Engineering Education
Authors: Rani, Meenu; Kumar, Pawan; Vandana, Vandana
Bibcode: 2016cosp...41E1623R
Altcode:
Over the years, Mars has been the centre of attraction for science
fiction writers, Hollywood movie makers, astrologers, astronomers
and the scientific community. For scientists and technologists, Mars
continues to be an enigma. This is essentially because even tough humans
have dreamt for long about human colonisation of Mars. Indian space
programme had a very humble beginning during the early 1960s. India
launched its first satellite in 1975 with assistance from the erstwhile
USSR. India achieved the status of space-faring nation2 by 1980,
and by the end of 2014 has launched around 75 satellites. India has
become the first nation to reach Mars on its maiden attempt after its
Mars Orbiter Mission completed its 10-month journey and successfully
entered the Red Planet's orbit. The Mars Orbiter Mission, a low-cost 74
million project, blasted off from Earth on November 5, 2013, aboard an
Indian Polar Satellite Launch Vehicle. At its initial stage, the rocket
booster placed the probe into Earth's orbit before the craft fired
the engines to break free of Earth's gravity en route to Mars. This
is India's first mission into such deep space to search for evidence
of life on the Red Planet. But the mission's primary objective is
technological-if successful, the country will be joining an elite club
of nations: the United States, Russia and Europe. India is becoming
known for low-cost innovation in diverse fields such as healthcare
and education. The technological capability being demonstrated and
the knowledge gained from the operations of the mission will be
invaluable in future developments and also in the training of the
flight operations and mission control staff. All of this capability can
be carried forward to future launches and operations. The sustained
presence of methane observed by previous missions suggests that an
active production mechanism is at work, most likely tectonic in nature,
although there are some suggestions that it may point to a biological
origin. The MOM observations will help increase our knowledge of the
methane plumes and possibly provide some clues as to their origin.
Title: Infrared emission from tidal disruption events - probing the
pc-scale dust content around galactic nuclei
Authors: Lu, Wenbin; Kumar, Pawan; Evans, Neal J.
Bibcode: 2016MNRAS.458..575L
Altcode: 2015arXiv151200020L; 2016MNRAS.tmp...97L
Recent UV-optical surveys have been successful in finding tidal
disruption events (TDEs), in which a star is tidally disrupted by
a supermassive black hole (BH). These TDEs release a huge amount of
radiation energy Erad ∼ 1051-1052
erg into the circum-nuclear medium. If the medium is dusty, most of the
radiation energy will be absorbed by dust grains within ∼1 pc from
the BH and re-radiated in the infrared. We calculate the dust emission
light curve from a 1D radiative transfer model, taking into account the
time-dependent heating, cooling and sublimation of dust grains. We show
that the dust emission peaks at 3-10 μm and has typical luminosities
between 1042 and 1043 erg s-1 (with
sky covering factor of dusty clouds ranging from 0.1 to 1). This is
detectable by current generation of telescopes. In the near future,
James Webb Space Telescope will be able to perform photometric and
spectroscopic measurements, in which silicate or polycyclic aromatic
hydrocarbon features may be found. Dust grains are non-spherical and
may be aligned with the magnetic field, so the dust emission may be
significantly polarized. Observations at rest-frame wavelength ≥ 2
μm have only been reported from two TDE candidates, SDSS J0952+2143
and SwiftJ1644+57. Although consistent with the dust emission from
TDEs, the mid-infrared fluxes of the two events may be from other
sources. Long-term monitoring is needed to draw a firm conclusion. We
also point out two nearby TDE candidates (ASASSN-14ae and -14li)
where the dust emission may be currently detectable. Detection of
dust infrared emission from TDEs would provide information regarding
the dust content and its distribution in the central pc of non-active
galactic nuclei, which is hard to probe otherwise.
Title: External inverse-Compton emission from jetted tidal disruption
events
Authors: Lu, Wenbin; Kumar, Pawan
Bibcode: 2016MNRAS.458.1071L
Altcode: 2016arXiv160201468L
The recent discoveries of Sw J1644+57 and Sw J2058+05 show that tidal
disruption events (TDEs) can launch relativistic jets. Super-Eddington
accretion produces a strong radiation field of order Eddington
luminosity. In a jetted TDE, electrons in the jet will inverse-Compton
scatter the photons from the accretion disc and wind (external radiation
field). Motivated by observations of thermal optical-UV spectra in
Sw J2058+05 and several other TDEs, we assume the spectrum of the
external radiation field intercepted by the relativistic jet to be
blackbody. Hot electrons in the jet scatter this thermal radiation
and produce luminosities 1045-1048 erg s-
1 in the X/γ-ray band. This model of thermal plus inverse-Compton
radiation is applied to Sw J2058+05. First, we show that the blackbody
component in the optical-UV spectrum most likely has its origin in
the super-Eddington wind from the disc. Then, using the observed
blackbody component as the external radiation field, we show that the
X-ray luminosity and spectrum are consistent with the inverse-Compton
emission, under the following conditions: (1) the jet Lorentz factor
is Γ ≃ 5-10; (2) electrons in the jet have a power-law distribution
dN_e/dγ _e ∝ γ _e^{-p} with γmin ∼ 1 and p = 2.4;
(3) the wind is mildly relativistic (Lorentz factor ≳ 1.5) and has
isotropic-equivalent mass-loss rate ∼ 5 M⊙ yr-
1. We describe the implications for jet composition and the
radius where jet energy is converted to radiation.
Title: Erratum: Inverse-Compton drag on a highly magnetized GRB jet
in stellar envelope
Authors: Ceccobello, Chiara; Kumar, Pawan
Bibcode: 2016MNRAS.458.1374C
Altcode: 2016MNRAS.tmp..216C
No abstract at ADS
Title: X-ray flares in GRBs: general considerations and photospheric
origin
Authors: Beniamini, Paz; Kumar, Pawan
Bibcode: 2016MNRAS.457L.108B
Altcode: 2015arXiv151003873B
Observations of X-ray flares from Gamma Ray Bursts imply strong
constraints on possible physical models. We provide a general discussion
of these. In particular, we show that in order to account for the
relatively flat and weak optical flux during the X-ray flares, the
size of the emitting region should be ≲3 × 1014cm. The
bolometric luminosity of flares also strongly constrain the energy
budget, and are inconsistent with late time activity of a central
engine powered by the spin-down of a magnetar. We provide a simple toy
model according to which flares are produced by an outflow of modest
Lorentz factor (a few tens instead of hundreds) that is launched more
or less simultaneously with the highly relativistic jet which produced
the prompt gamma-ray emission. The `slower moving outflow produces
the flare as it reaches its photosphere. If the X-ray flare jets are
structured, the existence of such a component may naturally resolve
the observational challenges imposed by flares, outlined in this work.
Title: Monte Carlo simulations of the photospheric process
Authors: Santana, Rodolfo; Crumley, Patrick; Hernández, Roberto A.;
Kumar, Pawan
Bibcode: 2016MNRAS.456.1049S
Altcode: 2015arXiv151200463S
We present a Monte Carlo (MC) code we wrote to simulate the
photospheric process and to study the photospheric spectrum above the
peak energy. Our simulations were performed with a photon-to-electron
ratio Nγ/Ne = 105, as determined
by observations of the Gamma-ray Burst prompt emission. We searched
an exhaustive parameter space to determine if the photospheric
process can match the observed high-energy spectrum of the prompt
emission. If we do not consider electron re-heating, we determined that
the best conditions to produce the observed high-energy spectrum are
low photon temperatures and high optical depths. However, for these
simulations, the spectrum peaks at an energy below 300 keV by a factor
of ∼10. For the cases we consider with higher photon temperatures
and lower optical depths, we demonstrate that additional energy in
the electrons is required to produce a power-law spectrum above the
peak energy. By considering electron re-heating near the photosphere,
the spectra for these simulations have a peak energy ∼300 keV and a
power-law spectrum extending to at least 10 MeV with a spectral index
consistent with the prompt emission observations. We also performed
simulations for different values of Nγ/Ne
and determined that the simulation results are very sensitive to
Nγ/Ne. Lastly, in addition to Comptonizing
a blackbody spectrum, we also simulate the Comptonization of
a fν ∝ ν-1/2 fast cooled synchrotron
spectrum. The spectrum for these simulations peaks at ∼104
keV, with a flat spectrum fν ∝ ν0 below the
peak energy.
Title: Probing massive stars around gamma-ray burst progenitors
Authors: Lu, Wenbin; Kumar, Pawan; Smoot, George F.
Bibcode: 2015MNRAS.453.1458L
Altcode: 2015arXiv150107606L
Long gamma-ray bursts (GRBs) are produced by ultra-relativistic
jets launched from core collapse of massive stars. Most massive
stars form in binaries and/or in star clusters, which means that
there may be a significant external photon field (EPF) around the
GRB progenitor. We calculate the inverse-Compton scattering of EPF
by the hot electrons in the GRB jet. Three possible cases of EPF
are considered: the progenitor is (I) in a massive binary system,
(II) surrounded by a Wolf-Rayet-star wind and (III) in a dense star
cluster. Typical luminosities of 1046-1050 erg
s-1 in the 1-100 GeV band are expected, depending on the
stellar luminosity, binary separation (I), wind mass-loss rate (II),
stellar number density (III), etc. We calculate the light curve and
spectrum in each case, taking fully into account the equal-arrival
time surfaces and possible pair-production absorption with the prompt
γ-rays. Observations can put constraints on the existence of such EPFs
(and hence on the nature of GRB progenitors) and on the radius where
the jet internal dissipation process accelerates electrons.
Title: How Bad or Good Are the External Forward Shock Afterglow
Models of Gamma-Ray Bursts?
Authors: Wang, Xiang-Gao; Zhang, Bing; Liang, En-Wei; Gao, He;
Li, Liang; Deng, Can-Min; Qin, Song-Mei; Tang, Qing-Wen; Kann,
D. Alexander; Ryde, Felix; Kumar, Pawan
Bibcode: 2015ApJS..219....9W
Altcode: 2015arXiv150303193W
The external forward shock models have been the standard paradigm to
interpret the broadband afterglow data of gamma-ray bursts (GRBs). One
prediction of the models is that some afterglow temporal breaks
at different energy bands should be achromatic; that is, the break
times should be the same in different frequencies. Multiwavelength
observations in the Swift era have revealed chromatic afterglow
behaviors at least in some GRBs, casting doubts on the external
forward shock origin of GRB afterglows. In this paper, using a large
sample of GRBs with both X-ray and optical afterglow data, we perform
a systematic study to address the question: how bad or good are the
external forward shock models? Our sample includes 85 GRBs up to
2014 March with well-monitored X-ray and optical light curves. Based
on how well the data abide by the external forward shock models,
we categorize them into five grades and three samples. The first two
grades (Grade I and II) include 45 of 85 GRBs. They show evidence
of, or are consistent with having, an achromatic break. The temporal
and spectral behaviors in each afterglow segment are consistent with
the predictions (the “closure relations”) of the forward shock
models. These GRBs are included in the Gold sample. The next two grades
(Grade III and IV) include 37 of 85 GRBs. They are also consistent with
having an achromatic break, even though one or more afterglow segments
do not comply with the closure relations. These GRBs are included in
the Silver sample. Finally, Grade V (3/85) shows direct evidence of
chromatic behaviors, suggesting that the external shock models are
inconsistent with the data. These are included in the Bad sample. We
further perform statistical analyses of various observational properties
(temporal index α, spectral index β, break time tb)
and model parameters (energy injection index q, electron spectral
index p, jet opening angle {θ }j, radiative efficiency
ηγ, and so on) of the GRBs in the Gold sample, and
derive constraints on the magnetization parameter ɛB in
the forward shock. Overall, we conclude that the simplest external
forward shock models can account for the multiwavelength afterglow
data of at least half of the GRBs. When more advanced modeling (e.g.,
long-lasting reverse shock, structured jets, arbitrary circumburst
medium density profile) is invoked, up to >90% of the afterglows
may be interpreted within the framework of the external shock models.
Title: Inverse-Compton drag on a highly magnetized GRB jet in
stellar envelope
Authors: Ceccobello, Chiara; Kumar, Pawan
Bibcode: 2015MNRAS.449.2566C
Altcode: 2015arXiv150305935C
The collimation and evolution of relativistic outflows in γ-ray bursts
are determined by their interaction with the stellar envelope through
which they travel before reaching the much larger distance where the
energy is dissipated and γ-rays are produced. We consider the case
of a Poynting-flux-dominated relativistic outflow and show that it
suffers strong inverse-Compton (IC) scattering drag near the stellar
surface and the jet is slowed down to sub-relativistic speed if its
initial magnetization parameter (σ0) is larger than about
105. If the temperature of the cocoon surrounding the jet
were to be larger than about 10 keV, then an optically thick layer of
electrons and positrons forms at the interface of the cocoon and the
jet, and one might expect this pair screen to protect the interior
of the jet from IC drag. However, the pair screen turns out to be
ephemeral, and instead of shielding the jet it speeds up the IC drag on
it. Although a high σ0 jet might not survive its passage
through the star, a fraction of its energy is converted to 1-100 MeV
radiation that escapes the star and appears as a bright flash lasting
for about 10 s.
Title: A Novel Paradigm for Short Gamma-Ray Bursts With Extended
X-Ray Emission
Authors: Rezzolla, Luciano; Kumar, Pawan
Bibcode: 2015ApJ...802...95R
Altcode: 2014arXiv1410.8560R
The merger of a binary of neutron stars provides natural explanations
for many of the features of short gamma-ray bursts (SGRBs), such as
the generation of a hot torus orbiting a rapidly rotating black hole,
which can then build a magnetic jet and provide the energy reservoir
to launch a relativistic outflow. However, this scenario has problems
explaining the recently discovered long-term and sustained X-ray
emission associated with the afterglows of a subclass of SGRBs. We
propose a new model that explains how an X-ray afterglow can be
sustained by the product of the merger and how the X-ray emission is
produced before the corresponding emission in the gamma-band, though
it is observed to follow it. Overall, our paradigm combines in a novel
manner a number of well-established features of the emission in SGRBs
and results from simulations. Because it involves the propagation of an
ultra-relativistic outflow and its interaction with a confining medium,
the paradigm also highlights a unifying phenomenology between short
and long GRBs.
Title: The physics of gamma-ray bursts & relativistic jets
Authors: Kumar, Pawan; Zhang, Bing
Bibcode: 2015PhR...561....1K
Altcode: 2014arXiv1410.0679K
We provide a comprehensive review of major developments in our
understanding of gamma-ray bursts, with particular focus on the
discoveries made within the last fifteen years when their true nature
was uncovered. We describe the observational properties of photons
from the radio to 100s GeV bands, both in the prompt emission and the
afterglow phases. Mechanisms for the generation of these photons in
GRBs are discussed and confronted with observations to shed light on
the physical properties of these explosions, their progenitor stars and
the surrounding medium. After presenting observational evidence that
a powerful, collimated, jet moving at close to the speed of light is
produced in these explosions, we describe our current understanding
regarding the generation, acceleration, and dissipation of the
jet. We discuss mounting observational evidence that long duration
GRBs are produced when massive stars die, and that at least some short
duration bursts are associated with old, roughly solar mass, compact
stars. The question of whether a black-hole or a strongly magnetized,
rapidly rotating neutron star is produced in these explosions is also
discussed. We provide a brief summary of what we have learned about
relativistic collisionless shocks and particle acceleration from GRB
afterglow studies, and discuss the current understanding of radiation
mechanism during the prompt emission phase. We discuss theoretical
predictions of possible high-energy neutrino emission from GRBs and
the current observational constraints. Finally, we discuss how these
explosions may be used to study cosmology, e.g. star formation, metal
enrichment, reionization history, as well as the formation of first
stars and galaxies in the universe.
Title: Some implications of inverse-Compton scattering of hot cocoon
radiation by relativistic jets in gamma-ray bursts
Authors: Kumar, Pawan; Smoot, George F.
Bibcode: 2014MNRAS.445..528K
Altcode: 2014arXiv1402.2656K
Long gamma-ray bursts (GRBs) relativistic jets are surrounded by hot
cocoons which confine jets during their punch out from the progenitor
star. These cocoons are copious sources of X-ray photons that can
be and are inverse-Compton (IC) scattered to MeV-GeV energies by
electrons in the relativistic jet. We provide detailed estimates for
IC flux resulting from various interactions between X-ray photons and
the relativistic jet, and describe what we can learn about GRBs jets
and progenitor stars from the detection (or an upper limit) of these
IC scattered photons.
Title: Magnetic Fields in Relativistic Collisionless Shocks
Authors: Santana, Rodolfo; Barniol Duran, Rodolfo; Kumar, Pawan
Bibcode: 2014ApJ...785...29S
Altcode: 2013arXiv1309.3277S
We present a systematic study on magnetic fields in gamma-ray burst
(GRB) external forward shocks (FSs). There are 60 (35) GRBs in our
X-ray (optical) sample, mostly from Swift. We use two methods to study
epsilon B (fraction of energy in magnetic field in the FS):
(1) for the X-ray sample, we use the constraint that the observed
flux at the end of the steep decline is >= X-ray FS flux; (2) for
the optical sample, we use the condition that the observed flux arises
from the FS (optical sample light curves decline as ~t -1, as
expected for the FS). Making a reasonable assumption on E (jet isotropic
equivalent kinetic energy), we converted these conditions into an upper
limit (measurement) on epsilon B n 2/(p + 1)
for our X-ray (optical) sample, where n is the circumburst density and
p is the electron index. Taking n = 1 cm-3, the distribution
of epsilon B measurements (upper limits) for our optical
(X-ray) sample has a range of ~10-8-10-3
(~10-6-10-3) and median of ~few × 10-5
(~few × 10-5). To characterize how much amplification is
needed, beyond shock compression of a seed magnetic field ~10 μG,
we expressed our results in terms of an amplification factor, AF,
which is very weakly dependent on n (AFvpropn 0.21). The
range of AF measurements (upper limits) for our optical (X-ray) sample
is ~1-1000 (~10-300) with a median of ~50 (~50). These results suggest
that some amplification, in addition to shock compression, is needed
to explain the afterglow observations.
Title: Radio Emission from the Bow Shock of G2
Authors: Crumley, Patrick; Kumar, Pawan
Bibcode: 2013arXiv1312.3901C
Altcode:
The radio flux from the synchrotron emission of electrons accelerated in
the forward bow shock of G2 is expected to have peaked when the forward
shock passes close to the pericenter from the Galactic Center, around
autumn of 2013. This radio flux is model dependent. We find that if G2
were to be a momentum-supported bow shock of a faint star with a strong
wind, the radio synchrotron flux from the forward-shock heated ISM is
well below the quiescent radio flux of Sgr A*. By contrast, if G2 is a
diffuse cloud, the radio flux is predicted to be much larger than the
quiescent radio flux and therefore should have already been detected
or will be detected shortly. No such radiation has been observed to
date. Radio measurements can reveal the nature of G2 well before G2
completes its periapsis passage.
Title: Model-Dependent High-Energy Neutrino Flux from Gamma-Ray Bursts
Authors: Zhang, Bing; Kumar, Pawan
Bibcode: 2013PhRvL.110l1101Z
Altcode: 2012arXiv1210.0647Z
The IceCube Collaboration recently reported a stringent upper limit
on the high energy neutrino flux from gamma-ray bursts (GRBs), which
provides a meaningful constraint on the standard internal shock
model. Recent broadband electromagnetic observations of GRBs also
challenge the internal shock paradigm for GRBs, and some competing
models for γ-ray prompt emission have been proposed. We describe
a general scheme for calculating the GRB neutrino flux, and compare
the predicted neutrino flux levels for different models. We point out
that the current neutrino flux upper limit starts to constrain the
standard internal shock model. The dissipative photosphere models are
also challenged if the cosmic ray luminosity from GRBs is at least 10
times larger than the γ-ray luminosity. If the neutrino flux upper
limit continues to go down in the next few years, then it would suggest
the following possibilities: (i) the photon-to-proton luminosity ratio
in GRBs is anomalously high for shocks, which may be achieved in some
dissipative photosphere models and magnetic dissipation models; or (ii)
the GRB emission site is at a larger radius than the internal shock
radius, as expected in some magnetic dissipation models such as the
internal collision-induced magnetic reconnection and turbulence model.
Title: Simulations of Accretion Powered Supernovae in the Progenitors
of Gamma-Ray Bursts
Authors: Lindner, Christopher C.; Milosavljević, Miloš; Shen,
Rongfeng; Kumar, Pawan
Bibcode: 2012ApJ...750..163L
Altcode: 2011arXiv1108.1415L
Observational evidence suggests a link between long-duration gamma-ray
bursts (LGRBs) and Type Ic supernovae. Here, we propose a potential
mechanism for Type Ic supernovae in LGRB progenitors powered solely
by accretion energy. We present spherically symmetric hydrodynamic
simulations of the long-term accretion of a rotating gamma-ray burst
progenitor star, a "collapsar," onto the central compact object,
which we take to be a black hole. The simulations were carried out
with the adaptive mesh refinement code FLASH in one spatial dimension
and with rotation, an explicit shear viscosity, and convection in the
mixing length theory approximation. Once the accretion flow becomes
rotationally supported outside of the black hole, an accretion shock
forms and traverses the stellar envelope. Energy is carried from the
central geometrically thick accretion disk to the stellar envelope
by convection. Energy losses through neutrino emission and nuclear
photodisintegration are calculated but do not seem important following
the rapid early drop of the accretion rate following circularization. We
find that the shock velocity, energy, and unbound mass are sensitive to
convective efficiency, effective viscosity, and initial stellar angular
momentum. Our simulations show that given the appropriate combinations
of stellar and physical parameters, explosions with energies ~5 ×
1050 erg, velocities ~3000 km s-1, and unbound
material masses >~ 6 M ⊙ are possible in a rapidly
rotating 16 M ⊙ main-sequence progenitor star. Further
work is needed to constrain the values of these parameters, to identify
the likely outcomes in more plausible and massive LRGB progenitors,
and to explore nucleosynthetic implications.
Title: Supernovae Powered by Collapsar Accretion in Gamma-Ray
Burst Sources
Authors: Milosavljević, Miloš; Lindner, Christopher C.; Shen,
Rongfeng; Kumar, Pawan
Bibcode: 2012ApJ...744..103M
Altcode: 2010arXiv1007.0763M
The association of long-duration gamma-ray bursts (LGRBs) with
Type Ic supernovae presents a challenge to supernova explosion
models. In the collapsar model for LGRBs, gamma rays are produced
in an ultrarelativistic jet launching from the magnetosphere of the
black hole that forms in the aftermath of the collapse of a rotating
progenitor star. The jet is collimated along the star's rotation axis,
but the concomitant luminous supernova should be relatively—though
certainly not entirely—spherical, and should synthesize a substantial
mass of 56Ni. Our goal is to provide a qualitative assessment
of the possibility that accretion of the progenitor envelope onto the
black hole, which powers the LGRB, could also deposit sufficient energy
and nickel mass in the envelope to produce a luminous supernova. For
this, the energy dissipated near the black hole during accretion must
be transported outward, where it can drive a supernova-like shock
wave. Here we suggest that the energy is transported by convection
and develop an analytical toy model, relying on global mass and energy
conservation, for the dynamics of stellar collapse. The model suggests
that a ~10 000 km s-1 shock can be driven into the envelope
and that ~1051 erg explosions are possible. The efficiency
with which the accretion energy is being transferred to the envelope
is governed by the competition of advection and convection at distances
~100-1000 km from the black hole and is sensitive to the values of the
convective mixing length, the magnitude of the effective viscous stress,
and the specific angular momentum of the infalling envelope. Substantial
masses of 56Ni may be synthesized in the convective accretion
flow over the course of tens of seconds from the initial circularization
of the infalling envelope around the black hole. The synthesized nickel
is convectively mixed with a much larger mass of unburned ejecta.
Title: Erratum: The late jet in gamma-ray bursts and its interactions
with a supernova ejecta and a cocoon
Authors: Shen, Rongfeng; Kumar, Pawan; Piran, Tsvi
Bibcode: 2011MNRAS.418.2106S
Altcode: 2011MNRAS.tmp.1906S
No abstract at ADS
Title: Constraints on cold magnetized shocks in gamma-ray bursts
Authors: Narayan, Ramesh; Kumar, Pawan; Tchekhovskoy, Alexander
Bibcode: 2011MNRAS.416.2193N
Altcode: 2011MNRAS.tmp.1215N; 2011arXiv1105.0003N
We consider a model in which the ultrarelativistic jet in a gamma-ray
burst (GRB) is cold and magnetically accelerated. We assume that the
energy flux in the outflowing material is partially thermalized via
internal shocks or a reverse shock, and we estimate the maximum amount
of radiation that could be produced in such magnetized shocks. We
compare this estimate with the available observational data on prompt
γ-ray emission in GRBs. We find that, even with highly optimistic
assumptions, the magnetized jet model is radiatively too inefficient
to be consistent with observations. One way out is to assume that
much of the magnetic energy in the post-shock, or even pre-shock, jet
material is converted to particle thermal energy by some unspecified
process, and then radiated. This can increase the radiative efficiency
sufficiently to fit observations. Alternatively, jet acceleration may be
driven by thermal pressure rather than magnetic fields. In this case,
which corresponds to the traditional fireball model, sufficient prompt
GRB emission could be produced either from shocks at a large radius
or from the jet photosphere closer to the centre.
Title: Cosmic Explosions in Three Dimensions
Authors: Höflich, Peter; Kumar, Pawan; Wheeler, J. Craig
Bibcode: 2011cetd.book.....H
Altcode:
Introduction: 3-D Explosions: a meditation on rotation (and magnetic
fields) J. C. Wheeler; Part I. Supernovae: Observations Today:
1. Supernova explosions: lessons from spectropolarimetry L. Wang;
2. Spectropolarimetric observations of Supernovae A. Filippenko
and D. C. Leonard; 3. Observed and physical properties of type II
plateau supernovae M. Hamuy; 4. SN1997B and the different types
of Type Ic Supernovae A. Clocchiatti, B. Leibundgut, J. Spyromilio,
S. Benetti, E. Cappelaro, M. Turatto and M. Phillips; 5. Near-infrared
spectroscopy of stripped-envelope Supernovae C. L. Gerardy, R. A. Fesen,
G. H. Marion, P. Hoeflich and J. C. Wheeler; 6. Morphology of
Supernovae remnants R. Fesen; 7. The evolution of Supernova remnants
in the winds of massive stars V. Dwarkadas; 8. Types for the galactic
Supernovae B. E. Schaefer; Part II. Theory of Thermonuclear Supernovae:
9. Semi-steady burning evolutionary sequences for CAL 83 and CAL 87:
supersoft X-ray binaries are Supernovae Ia progenitors S. Starrfield,
F. X. Timmes, W. R. Hix, E. M. Sion, W. M. Sparks and S. Dwyer; 10. Type
Ia Supernovae progenitors: effects of the spin-up of the white dwarfs
S.-C. Yoon and N. Langer; 11. Terrestrial combustion: feedback to the
stars E. S. Oran; 12. Non-spherical delayed detonations E. Livne;
13. Numerical simulations of Type Ia Supernovae: deflagrations and
detonations V. N. Gamezo, A. M. Khokhlov and E. S. Oran; 14. Type Ia
Supernovae: spectroscopic surprises D. Branch; 15. Aspherity effects
in Supernovae P. Hoeflich, C. Gerardy and R. Quimby; 16. Broad light
curve SneIa: asphericity or something else? A. Howell and P. Nugent;
17. Synthetic spectrum methods for 3-D SN models R. Thomas; 18. A hole
in Ia' spectroscopic and polarimetric signatures of SN Ia asymmetry
due to a companion star D. Kasen; 19. Hunting for the signatures
of 3-D explosions with 1-D synthetic spectra E. Lentz, E. Baron and
P. H. Hauschildt; 20. On the variation of the peak luminosity of Type
Ia J. W. Truran, E. X. Timmes and E. F. Brown; Part III. Theory of
Core Collapse Supernovae: 21. Rotation of core collapse progenitors:
single and binary stars N. Langer; 22. Large scale convection and the
convective Supernova mechanism S. Colgate and M. E. Herant; 23. Topics
in core-collapse Supernova A. Burrows, C. D. Ott and C. Meakin;
24. MHD Supernova jets: the missing link D. Meier and M. Nakamura;
25. Effects of super strong magnetic fields in core collapse Supernovae
I. S. Akiyama; 26. Non radial instability of stalled accretion shocks
advective-acoustic cycle T. Foglizzo and P. Galletti; 27. Asymmetry
effects in Hypernovae K. Maeda, K. Nomoto, J. Deng and P.A. Mazzali;
28. Turbulent MHD jet collimation and thermal driving P. T. Williams;
Part IV. Magnetars, N-Stars, Pulsars: 29. Supernova remnants and pulsar
wind nebulae R. Chevalier; 30. X-Ray signatures of Supernovae D. Swartz;
31. Asymmetric Supernovae and Neutron Star Kicks D. Lai and D. Q. Lamb;
32. Triggers of magnetar outbursts R. Duncan; 33. Turbulent MHD Jet
Collimation and Thermal Driving P. Williams; 34. The interplay between
nuclear electron capture and fluid dynamics in core collapse Supernovae
W. R. Hix, O. E. B. Messer and A. Mezzacappa; Part V. Gamma-Ray
Bursts: 35. GRB 021004 and Gamma-ray burst distances B. E. Schaefer;
36. Gamma-ray bursts as a laboratory for the study of Type Ic Supernovae
D. Q. Lamb, T. Q. Donaghy and C. Graziani; 37. The diversity of cosmic
explosions: Gamma-ray bursts and Type Ib/c Supernovae E. Berger;
38. A GRB simulation using 3D relativistic hydrodynamics J. Cannizo,
N. Gehrels and E. T. Vishniac; 39. The first direct link in the
Supernova/GRB connection: GRB 030329 and SN 2003dh T. Matheson; Part
VI. Summary: 40. Three-dimensional explosions C. Wheeler.
Title: Supernovae-induced accretion and star formation in the inner
kiloparsec of a gaseous disc
Authors: Kumar, Pawan; Johnson, Jarrett L.
Bibcode: 2010MNRAS.404.2170K
Altcode: 2010arXiv1002.0590K; 2010MNRAS.tmp..364K
We consider the effects of supernovae (SNe) on accretion and
star formation in a massive gaseous disc in a large primeval
galaxy. The gaseous disc we envisage, roughly 1 kpc in size with
>~108Msolar of gas, could have formed as
a result of galaxy mergers where tidal interactions removed angular
momentum from gas at larger radius and thereby concentrated it within
the central ~1 kpc region. We find that SNe lead to accretion in the
disc at a rate of roughly 0.1-1Msolar yr-1
and induce star formation at a rate of ~10-100Msolar
per year which contributes to the formation of a bulge; a part of
the stellar velocity dispersion is due to SN shell speed from which
stars are formed and a part due to the repeated action of stochastic
gravitational field of SNe remnant network on stars. The rate of
SN in the inner kpc is shown to be self-regulating, and it cycles
through phases of low and high activity. The SN-assisted accretion
transports gas from about 1 kpc to within a few pc of the centre. If
this accretion were to continue down to the central black hole then
the resulting ratio of black hole mass to the stellar mass in the
bulge would be of the order of ~ 10-2-10-3,
in line with the observed Magorrian relation.
Title: Collapsar Accretion and the Gamma-Ray Burst X-Ray Light Curve
Authors: Lindner, Christopher C.; Milosavljević, Miloš; Couch,
Sean M.; Kumar, Pawan
Bibcode: 2010ApJ...713..800L
Altcode: 2009arXiv0910.4989L
We present axisymmetric hydrodynamical simulations of the long-term
accretion of a rotating gamma-ray burst (GRB) progenitor star,
a "collapsar," onto the central compact object, which we take
to be a black hole. The simulations were carried out with the
adaptive-mesh-refinement code FLASH in two spatial dimensions and with
an explicit shear viscosity. The evolution of the central accretion
rate exhibits phases reminiscent of the long GRB γ-ray and X-ray
light curve, which lends support to the proposal by Kumar et al. that
the luminosity is modulated by the central accretion rate. In the
first "prompt" phase, the black hole acquires most of its final mass
through supersonic quasiradial accretion occurring at a steady rate
of ~0.2 M sun s-1. After a few tens of seconds,
an accretion shock sweeps outward through the star. The formation and
outward expansion of the accretion shock is accompanied with a sudden
and rapid power-law decline in the central accretion rate \dot{M}∝
t^{-2.8}, which resembles the L X vprop t -3
decline observed in the X-ray light curves. The collapsed, shock-heated
stellar envelope settles into a thick, low-mass equatorial disk embedded
within a massive, pressure-supported atmosphere. After a few hundred
seconds, the inflow of low angular momentum material in the axial
funnel reverses into an outflow from the thick disk. Meanwhile, the
rapid decline of the accretion rate slows down, which is potentially
suggestive of the "plateau" phase in the X-ray light curve. We
complement our adiabatic simulations with an analytical model that
takes into account the cooling by neutrino emission and estimate that
the duration of the prompt phase can be ~20 s. The model suggests
that the steep decline in GRB X-ray light curves is triggered by
the circularization of the infalling stellar envelope at radii where
the virial temperature is below 1010 K, such that neutrino
cooling is inefficient and an outward expansion of the accretion shock
becomes imminent; GRBs with longer prompt γ-ray emission should have
more slowly rotating envelopes.
Title: The late jet in gamma-ray bursts and its interactions with
a supernova ejecta and a cocoon
Authors: Shen, Rongfeng; Kumar, Pawan; Piran, Tsvi
Bibcode: 2010MNRAS.403..229S
Altcode: 2009arXiv0910.5727S; 2010MNRAS.tmp..104S
Late X-ray flares observed in X-ray afterglows of gamma-ray bursts
(GRBs) suggest late central engine activities at a few minutes
to hours after the burst. A few unambiguously confirmed cases of
supernova associations with nearby long GRBs imply that an accompanying
supernova-like component might be a common feature in all long GRB
events. These motivate us to study the interactions of a late jet,
responsible for an X-ray flare, with various components in a stellar
explosion, responsible for a GRB. These components include a supernova
shell-like ejecta and a cocoon that was produced when the main jet
producing the GRB itself was propagating through the progenitor star. We
find that the interaction between the late jet and the supernova ejecta
may produce a luminous (up to 1049ergs-1) thermal
X-ray transient lasting for ~10s. The interaction between the late jet
and the cocoon produces synchrotron self-absorbed non-thermal emission,
with the observed peak X-ray flux density from 0.001μJy to 1mJy at
1 keV and a peak optical flux density from 0.01μJy to 0.1Jy (for a
redshift z = 2). The light curve due to the late-jet-cocoon interaction
has a very small pulse-width-to-time ratio, Δt/t ~ 0.01-0.5, where
t is the pulse peak time since the burst trigger. Identifying these
features in current and future observations would open a new frontier
in the study of GRB progenitor stars.
Title: GRB 080319B: evidence for relativistic turbulence, not
internal shocks
Authors: Kumar, Pawan; Narayan, Ramesh
Bibcode: 2009MNRAS.395..472K
Altcode: 2008arXiv0812.0021K; 2009MNRAS.tmp..374K
We show that the excellent optical and gamma-ray data available for GRB
080319B rule out the internal shock model for the prompt emission. The
data instead point to a model in which the observed radiation was
produced close to the deceleration radius (~1017cm) by a
turbulent source with random Lorentz factors of ~10 in the comoving
frame. The optical radiation was produced by synchrotron emission from
relativistic electrons, and the gamma-rays by inverse-Compton scattering
of the synchrotron photons. The gamma-ray emission originated both in
eddies and in an inter-eddy medium, whereas the optical radiation was
mostly from the latter. Therefore, the gamma-ray emission was highly
variable whereas the optical was much less variable. The model explains
all the observed features in the prompt optical and gamma-ray data of
GRB 080319B. We are unable to determine with confidence whether the
energy of the explosion was carried outwards primarily by particles
(kinetic energy) or magnetic fields. Consequently, we cannot tell
whether the turbulent medium was located in the reverse shock (we can
rule out the forward shock) or in a Poynting-dominated jet.
Title: A turbulent model of gamma-ray burst variability
Authors: Narayan, Ramesh; Kumar, Pawan
Bibcode: 2009MNRAS.394L.117N
Altcode: 2009MNRAS.tmpL.193N; 2008arXiv0812.0018N
A popular paradigm to explain the rapid temporal variability observed
in gamma-ray burst (GRB) light curves is the internal shock model. We
propose an alternative model in which the radiating fluid in the
GRB shell is relativistically turbulent with a typical eddy Lorentz
factor γt. In this model, all pulses in the gamma-ray
light curve are produced at roughly the same distance R from the
centre of the explosion. The burst duration is ~R/cΓ2,
where Γ is the bulk Lorentz factor of the expanding shell,
and the duration of individual pulses in the light curve is
~R/cΓ2γ2t. The model naturally
produces highly variable light curves with ~γ2t
individual pulses. Even though the model assumes highly inhomogeneous
conditions, nevertheless the efficiency for converting jet energy to
radiation is high.
Title: Mass fall-back and accretion in the central engine of
gamma-ray bursts
Authors: Kumar, Pawan; Narayan, Ramesh; Johnson, Jarrett L.
Bibcode: 2008MNRAS.388.1729K
Altcode: 2008MNRAS.tmp..750K; 2008arXiv0807.0441K
We calculate the rate of in-fall of stellar matter on an accretion disc
during the collapse of a rapidly rotating massive star and estimate
the luminosity of the relativistic jet that results from accretion
on to the central black hole. We find that the jet luminosity remains
high for about 102 s, at a level comparable to the typical
luminosity observed in gamma-ray bursts (GRBs). The luminosity then
decreases rapidly with time for about ~103 s, roughly as
~t-3 the duration depends on the size and rotation speed
of the stellar core. The rapid decrease of the jet power explains the
steeply declining X-ray flux observed at the end of most long-duration
GRBs. Observations with the Swift satellite show that, following
the steep decline, many GRBs exhibit a plateau in the X-ray light
curve (XLC) that lasts for about 104 s. We suggest that this
puzzling feature is due to continued accretion in the central engine. A
plateau in the jet luminosity can arise when the viscosity parameter
α is small, ~10-2 or less. A plateau is also produced
by continued fall-back of matter - either from an extended stellar
envelope or from material that failed to escape with the supernova
ejecta. In a few GRBs, the XLC is observed to drop suddenly at the end
of the plateau phase, while in others the XLC declines more slowly as
~ t-1 - t-2. These features arise naturally in
the accretion model depending on the radius and mean specific angular
momentum of the stellar envelope. The total energy in the disc-wind
accompanying accretion is found to be about 1052 erg. This
is comparable to the energy observed in supernovae associated with
GRBs, suggesting that the wind might be the primary agent responsible
for the explosion. The accretion model thus provides a coherent
explanation for the diverse and puzzling features observed in the early
XLC of GRBs. It might be possible to use this model to invert gamma-ray
and X-ray observations of GRBs and thereby infer basic properties of
the core and envelope of the GRB progenitor star.
Title: Properties of Gamma-Ray Burst Progenitor Stars
Authors: Kumar, Pawan; Narayan, Ramesh; Johnson, Jarrett L.
Bibcode: 2008Sci...321..376K
Altcode: 2008arXiv0807.0445K
We determined some basic properties of stars that produce spectacular
gamma-ray bursts at the end of their lives. We assumed that accretion
of the outer portion of the stellar core by a central black hole
fuels the prompt emission and that fall-back and accretion of the
stellar envelope later produce the plateau in the x-ray light curve
seen in some bursts. Using x-ray data for three bursts, we estimated
the radius of the stellar core to be ~(1 - 3) × 1010 cm
and that of the stellar envelope to be ~(1 - 2) × 1011
cm. The density profile in the envelope is fairly shallow, with ρ ~
r-2 (where ρ is density and r is distance from the center
of the explosion). The rotation speeds of the core and envelope are
~0.05 and ~0.2 of the local Keplerian speed, respectively.
Title: A general scheme for modelling γ-ray burst prompt emission
Authors: Kumar, Pawan; McMahon, Erin
Bibcode: 2008MNRAS.384...33K
Altcode: 2008MNRAS.tmp....2K; 2008arXiv0802.2704K
We describe a general method for modelling γ-ray burst (GRB) prompt
emission, and determine the range of magnetic field strength, electron
energy, Lorentz factor of the source and the distance of the source from
the central explosion that is needed to account for the prompt γ-ray
emission of a typical long-duration burst. We find that for the burst
to be produced via the synchrotron process unphysical conditions are
required - the distance of the source from the centre of the explosion
(Rγ) must be larger than ~1017cm and the source
Lorentz factor >~103 for such a high Lorentz factor
the deceleration radius (Rd) is less than Rγ
even if the number density of particles in the surrounding medium
is as small as ~0.1cm-3. The result, Rγ >
Rd, is in contradiction with the early X-ray and optical
afterglow data that show that γ-rays precede the afterglow flux
that is produced by a decelerating forward shock. This problem for
the synchrotron process applies to all long GRBs other than those
that have the low-energy spectrum precisely ν-1/2. In
order for the synchrotron process to be a viable mechanism for long
bursts, the energy of electrons radiating in the γ-ray band needs
to be continuously replenished by some acceleration mechanism during
much of the observed spike in GRB light curve - this is not possible
if GRB-prompt radiation is produced in shocks (at least the kind that
has been usually considered for GRBs) where particles are accelerated
at the shock front and not as they travel downstream and emit γ-rays,
but might work in some different scenarios such as magnetic outflows. The synchrotron-self-Compton (SSC) process fares much better. There is
a large solution space for a typical GRB-prompt emission to be produced
via the SSC process. The prompt optical emission accompanying the burst
is found to be very bright (<~14 mag; for z ~ 2) in the SSC model,
which exceeds the observed flux (or upper limit) for most GRBs. The
prompt optical is predicted to be even brighter for the subclass of
bursts that have the spectrum fν~να with α ~
1 below the peak of νfν. Surprisingly, there are no SSC
solutions for bursts that have α ~ 1/3 these bursts might require
continuous or repeated acceleration of electrons or some physics
beyond the simplified, although generic, SSC model considered in this
work. Continuous acceleration of electrons can also significantly
reduce the optical flux that would otherwise accompany γ-rays in the
SSC model.
Title: GRB 060313: A New Paradigm for Short-Hard Bursts?
Authors: Roming, Peter W. A.; Vanden Berk, Daniel; Pal'shin, Valentin;
Pagani, Claudio; Norris, Jay; Kumar, Pawan; Krimm, Hans; Holland,
Stephen T.; Gronwall, Caryl; Blustin, Alex J.; Zhang, Bing; Schady,
Patricia; Sakamoto, Takanori; Osborne, Julian P.; Nousek, John A.;
Marshall, Frank E.; Mészáros, Peter; Golenetskii, Sergey V.; Gehrels,
Neil; Frederiks, Dmitry D.; Campana, Sergio; Burrows, David N.; Boyd,
Patricia T.; Barthelmy, Scott; Aptekar, R. L.
Bibcode: 2006ApJ...651..985R
Altcode: 2006astro.ph..5005R
We report the simultaneous observations of the prompt emission in
the gamma-ray and hard X-ray bands by the Swift BAT and the Konus-Wind
instruments of the short-hard burst, GRB 060313. The observations reveal
multiple peaks in both the gamma-ray and hard X-ray bands suggesting
a highly variable outflow from the central explosion. We also describe
the early-time observations of the X-ray and UV/optical afterglows by
the Swift XRT and UVOT instruments. The combination of the X-ray and
UV/optical observations provides the most comprehensive light curves
to date of a short-hard burst at such an early epoch. The afterglows
exhibit complex structure with different decay indices and flaring. This
behavior can be explained by the combination of a structured jet,
radiative loss of energy, and decreasing microphysics parameters
occurring in a circumburst medium with densities varying by a factor
of approximately two on a length scale of 1017 cm. These
density variations are normally associated with the environment of a
massive star and inhomogeneities in its windy medium. However, the mean
density of the observed medium (n~10-4 cm3) is
much less than that expected for a massive star. Although the collapse
of a massive star as the origin of GRB 060313 is unlikely, the merger
of a compact binary also poses problems for explaining the behavior
of this burst. Two possible suggestions for explaining this scenario
are that some short bursts may arise from a mechanism that does not
invoke the conventional compact binary model, or that soft late-time
central engine activity is producing UV/optical but no X-ray flaring.
Title: No universality for the electron power-law index (p) in
gamma-ray bursts and other relativistic sources
Authors: Shen, Rongfeng; Kumar, Pawan; Robinson, Edward L.
Bibcode: 2006MNRAS.371.1441S
Altcode: 2005astro.ph.12489S; 2006MNRAS.tmp..925S
The gamma-ray burst (GRB) prompt emission is believed to be
from highly relativistic electrons accelerated in relativistic
shocks. From the GRB high-energy power-law spectral indices β
observed by the Burst and Transient Source Experiment (BATSE) Large
Area Detectors (LAD), we determine the spectral index, p, of the
electrons' energy distribution. Both the theoretical calculations and
numerical simulations of the particle acceleration in relativistic
shocks show that p has a universal value ~2.2-2.3. We show that
the observed distribution of p during GRBs is not consistent with a
δ-function distribution or a universal p value, with the width of the
distribution >=0.54. The distributions of p during X-ray afterglows
are also investigated and found to be inconsistent with a δ-function
distribution. The p distributions in blazars and pulsar wind nebulae
are also broad, inconsistent with a δ-function distribution.
Title: Distribution of gamma-ray burst ejecta energy with Lorentz
factor
Authors: Granot, Jonathan; Kumar, Pawan
Bibcode: 2006MNRAS.366L..13G
Altcode: 2005astro.ph.11049G; 2005MNRAS.tmpL.116G
The early X-ray afterglow for a significant number of gamma-ray bursts
detected by the Swift satellite is observed to have a phase of very
slow flux decline with time (Fν~t-α with 0.2
<~α<~ 0.8) for 102.5<~t<~ 104 s,
while the subsequent decline is the usual 1 <~α3<~
1.5 behaviour, which was seen in the pre-Swift era. We show that this
behaviour is a natural consequence of a small spread in the Lorentz
factor of the ejecta, by a factor of ~2-4, where the slower ejecta
gradually catch up with the shocked external medium, thus increasing
the energy of the forward shock and delaying its deceleration. The
end of the `shallow' flux decay stage marks the beginning of the
Blandford-McKee self-similar external shock evolution. This suggests
that most of the energy in the relativistic outflow is in material
with a Lorentz factor of ~30-50.
Title: GRB Environment Deduced from Afterglow Emission
Authors: Kumar, Pawan
Bibcode: 2006sgrb.confE..34K
Altcode:
I will provide an overview of our understanding of the environment
withinabout one parsec of gamma-ray bursts. Evidence for and against
the presenceof a stratified medium carved out by theprogenitor star's
wind will be described.
Title: Cosmic explosions in three dimensions : asymmetries in
supernovae and gamma-ray bursts
Authors: Höflich, Peter; Kumar, Pawan; Wheeler, J. Craig
Bibcode: 2004cetd.conf.....H
Altcode: 2004cetd.book.....H
Recent observations have demonstrated that supernovae and gamma ray
bursts are driven by strong jets of energy and other asymmetrical
effects that reveal unknown physical properties. This volume highlights
the burgeoning era of routine supernova polarimetry and the new insights
into core collapse and thermonuclear explosions. Chapters by leading
scientists summarize the status of a rapidly developing perspective
on stellar explosions in a valuable resource for graduate students
and research scientists.
Title: The Enigmatic Gamma-Ray Bursts: A Mystery Being Solved
Authors: Kumar, Pawan
Bibcode: 2004tsra.conf...32K
Altcode:
Flashes of radiation in gamma-rays are observed once or twice a day
originating from some random part of the sky (and random in time). These
events (explosions) typically last for less than a minute. During this
time the energy radiated in gamma-rays is of order 10^{51} erg or the
kinetic energy release in a typical supernova explosion but about two
orders of magnitude larger than the EM-radiation from a supernova in a
month. Multi-wavelength observations of radiation we receive following
Gamma-ray bursts have greatly advanced our understanding of these
enigmatic explosions. For instance, we now know that gamma-ray bursts
are highly beamed and relativistic explosions. In the last few years
we have seen a number of compelling lines of evidence that at least a
certain fraction of gamma-ray bursts are associated with the death of
massive stars. I will describe recent observations and discuss what we
have learned about these bursts. The nature of the underlying object
and the currently unsolved problems will also be described.
Title: A unified treatment of the gamma-ray burst 021211 and its
afterglow
Authors: Kumar, Pawan; Panaitescu, Alin
Bibcode: 2003MNRAS.346..905K
Altcode: 2003astro.ph..5446K
The gamma-ray burst (GRB) 021211 had a simple light curve, containing
only one peak and the expected Poisson fluctuations. Such a burst
may be attributed to an external shock, offering the best chance for a
unified understanding of the gamma-ray burst and afterglow emissions. We
analyse the properties of the prompt (burst) and delayed (afterglow)
emissions of GRB 021211 within the fireball model. Consistency between
the optical emission during the first 11 min (which, presumably,
comes from the reverse shock heating of the ejecta) and the later
afterglow emission (arising from the forward shock) requires that, at
the onset of deceleration (~2 s), the energy density in the magnetic
field in the ejecta, expressed as a fraction of the equipartition
value (ɛB), is larger than in the forward shock at 11 min
by a factor of approximately 103. We find that synchrotron
radiation from the forward shock can account for the gamma-ray emission
of GRB 021211; to explain the observed GRB peak flux requires that,
at 2 s, ɛB in the forward shock is larger by a factor 100
than at 11 min. These results suggest that the magnetic field in the
reverse shock and early forward shock is a frozen-in field originating
in the explosion and that most of the energy in the explosion was
initially stored in the magnetic field. We can rule out the possibility
that the ejecta from the burst for GRB 021211 contained more than 10
electron-positron pairs per proton.
Title: The Evolution of a Structured Relativistic Jet and Gamma-Ray
Burst Afterglow Light Curves
Authors: Kumar, Pawan; Granot, Jonathan
Bibcode: 2003ApJ...591.1075K
Altcode: 2003astro.ph..3174K
We carry out a numerical hydrodynamical modeling for the evolution of a
relativistic collimated outflow as it interacts with the surrounding
medium and calculate the light curve resulting from synchrotron
emission of the shocked fluid. The hydrodynamic equations are reduced
to one-dimensional by assuming axial symmetry and integrating
over the radial profile of the flow, thus considerably reducing
the computation time. We present results for a number of different
initial jet structures, including several different power laws and
a Gaussian profile for the dependence of the energy per unit solid
angle, ɛ, and the Lorentz factor, Γ, on the angle from the jet
symmetry axis. Our choice of parameters for the various calculations
is motivated by the current knowledge of relativistic outflows from
gamma-ray bursts and the observed afterglow light curves. Comparison
of the light curves for different jet profiles with gamma-ray burst
afterglow observations provides constraints on the jet structure. One
of the main results we find is that the transverse fluid velocity
in the comoving frame (vt) and the speed of sideways
expansion for smooth jet profiles is typically much smaller than the
speed of sound (cs) throughout much of the evolution of
the jet; vt approaches cs when Γ along the
jet axis becomes of order a few (for a large angular gradient of ɛ,
vt~cs while Γ is still large). This result
suggests that the dynamics of relativistic structured jets may
be reasonably described by a simple analytic model in which ɛ is
independent of time, as long as Γ along the jet axis is larger than
a few.
Title: Constraining the Structure of Gamma-Ray Burst Jets through
the Afterglow Light Curves
Authors: Granot, Jonathan; Kumar, Pawan
Bibcode: 2003ApJ...591.1086G
Altcode: 2002astro.ph.12540G
We investigate the effect that the structure of gamma-ray burst
(GRB) jets has on the afterglow light curves for observers located
at different viewing angles, θobs, from the jet symmetry
axis. The largest uncertainty in the jet dynamics is the degree of
lateral energy transfer. Thus, we use two simple models that make
opposite and extreme assumptions for this point and calculate the light
curves for an external density that is either homogeneous or decreases
as the square of the distance from the source. The Lorentz factor, Γ,
and kinetic energy per unit solid angle, ɛ, are initially taken to
be power laws of the angle θ from the jet axis: ɛ~θ-a,
Γ~θ-b. We perform a qualitative comparison between the
resulting light curves and afterglow observations. This constrains the
jet structure, and we find that a~2 and 0<~b<~1 are required to
reproduce typical afterglow light curves. Detailed fits to afterglow
data are needed to determine whether a ``universal'' jet model,
in which all GRB jets are assumed to be intrinsically identical and
differ only by our viewing angle, θobs, is consistent with
current observations.
Title: X-Ray Lines from Gamma-Ray Bursts
Authors: Kumar, Pawan; Narayan, Ramesh
Bibcode: 2003ApJ...584..895K
Altcode: 2002astro.ph..5488K
X-ray lines have been recently detected in the afterglows of a few
gamma-ray bursts. We derive general constraints on the physical
conditions in the line-emitting gas and illustrate our results
using as an example the multiple Kα lines detected by Reeves et
al. in GRB 011211. We argue that photoionization models previously
discussed in the literature require either a very extreme geometry
or too much mass in the line-emitting region. Shock-heated models
also have a serious problem since they require the emitting region
to have a large optical depth unless electrons in this region are
shock-heated multiple times. We propose a new model in which gamma
rays from the burst and hard X-rays from the early afterglow are
backscattered by an electron-positron pair screen at a distance
of about 1014-1015 cm from the source and
irradiate the expanding outer layers of the supernova ejecta, thereby
producing X-ray lines. The model suffers from fewer problems compared to
previous models. It also has the advantage of requiring only a single
explosion to produce both the gamma-ray burst (GRB) and the supernova
ejecta, in contrast to most other models for the lines that require
the supernova to go off days or weeks prior to the GRB. The model,
however, has difficulty explaining the greater than 1048
ergs of energy emitted in the X-ray lines, which requires somewhat
extreme choices of model parameters. The difficulties associated with
the various models are not particular to GRB 011211. They are likely
to pose a problem for any GRB with X-ray lines.
Title: Angular Momentum Extraction by Gravity Waves in the Sun
Authors: Talon, Suzanne; Kumar, Pawan; Zahn, Jean-Paul
Bibcode: 2002ApJ...574L.175T
Altcode: 2002astro.ph..6479T
We review the behavior of the oscillating shear layer produced by
gravity waves below the surface convection zone of the Sun. We show
that, under asymmetric filtering produced by this layer, gravity waves
of low spherical order that are stochastically excited at the base of
the convection zone of late-type stars can extract angular momentum from
their radiative interior. The timescale for this momentum extraction
in a Sun-like star is on the order of 107 yr. The process
is particularly efficient in the central region, and it could produce
there a slowly rotating core.
Title: Off-Axis Afterglow Emission from Jetted Gamma-Ray Bursts
Authors: Granot, Jonathan; Panaitescu, Alin; Kumar, Pawan; Woosley,
Stan E.
Bibcode: 2002ApJ...570L..61G
Altcode: 2002astro.ph..1322G
We calculate gamma-ray burst (GRB) afterglow light curves from a
relativistic jet as seen by observers at various viewing angles,
θobs, relative to the jet axis. We describe three
increasingly more realistic models and compare the resulting light
curves. An observer at θobs<θ0, where
θ0 is the initial jet opening angle, should see a light
curve very similar to that for an on-axis observer. An observer at
θobs>θ0 sees a rising light curve at early
times, peaking when the jet Lorentz factor is ~1/θobs,
and approaching that seen by an on-axis observer, at later times. A
strong linear polarization (<~40%) may occur near the peak in the
light curve and slowly decay with time. We show that, if GRB jets
have a universal energy, then orphan afterglows are detectable up to
a maximum offset angle that is independent of the jet initial aperture
and thus at a rate proportional to the true GRB rate. We also discuss
the implications of the proposed connection between SN 1998bw and
GRB 980425.
Title: Erratum: ``Tidal Spin-up of Stars in Dense Stellar Cusps around
Massive Black Holes'' (ApJ, 549,
948 [2001])
Authors: Alexander, Tal; Kumar, Pawan
Bibcode: 2002ApJ...564.1061A
Altcode:
There is an error in equations (10) and (15), which relate the
eccentricity to the orbital parameters. The equations should read
as follows:Eo=1/2μv2∞=1/2me-
1rp,(10)ande=2(Eo+ΔEo)/
mrp+1,a=rp/1- e,(15)where the impactor mass
m replaces the reduced mass μ. The error does not affect the final
results in any significant way, as it is effectively absorbed in the
nonlinear correction factor, CNL. With the correct expression
for the eccentricity, CNL should be 1.4, instead of 2.
Title: Accretion Models of Gamma-Ray Bursts
Authors: Narayan, Ramesh; Piran, Tsvi; Kumar, Pawan
Bibcode: 2001ApJ...557..949N
Altcode: 2001astro.ph..3360N
Many models of gamma-ray bursts (GRBs) involve accretion onto a compact
object, usually a black hole, at a mass accretion rate on the order of
a fraction of a solar mass per second. If the accretion disk is larger
than a few tens or hundreds of Schwarzschild radii, the accretion will
proceed via a convection-dominated accretion flow (CDAF) in which most
of the matter escapes to infinity rather than falling onto the black
hole. Models involving the mergers of black hole-white dwarf binaries
and black hole-helium star binaries fall in this category. These models
are unlikely to produce GRBs since very little mass reaches the black
hole. If the accretion disk is smaller, then accretion will proceed
via neutrino cooling in a neutrino-dominated accretion disk (NDAF) and
most of the mass will reach the center. Models involving the mergers
of double neutron star binaries and black hole-neutron star binaries
fall in this category and are capable of producing bright GRBs. If
the viscosity parameter α in the NDAF has a standard value of ~0.1,
these mergers can explain short GRBs with durations under a second,
but they are unlikely to produce long GRBs with durations of tens or
hundred of seconds. If the accretion disk is fed by fallback of material
after a supernova explosion, as in the collapsar model, then the
timescale of the burst is determined by fallback, not accretion. Such
a model can produce long GRBs. Fallback models again require that the
accretion should proceed via an NDAF rather than a CDAF in order for
a significant amount of mass to reach the black hole. This condition
imposes an upper limit on the radius of injection of the gas.
Title: The Energy Distribution of Long Duration GRBS
Authors: Piran, Tsvi; Kumar, Pawan; Panaitescu, Alin; Piro, Luigi
Bibcode: 2001astro.ph..8033P
Altcode:
The energy release in gamma-ray bursts is one of the most interesting
clues on the nature of their "inner engines". We show here that
the total energy release in GRBs varies by less than one order of
magnitude from one burst to another while the energy emitted in
$\gamma$-ray photons varies by more than an order of magnitude. This
result indicates that the central engine of long duration GRB has
a remarkably constant energy output which provides very important
constraint on the nature of these enigmatic explosions. The broader
distribution of the observed $\gamma$-ray flux, about three orders of
magnitude in width, can be attributed, in part, to a variation in the
opening angle of the collimated explosion, and in part to the variation
of Lorentz factor across the jet as well as a variable efficiency for
converting the kinetic energy of explosion to $\gamma$-rays.
Title: Tidal Spin-up of Stars in Dense Stellar Cusps around Massive
Black Holes
Authors: Alexander, Tal; Kumar, Pawan
Bibcode: 2001ApJ...549..948A
Altcode: 2000astro.ph..4240A
We show that main-sequence stars in dense stellar cusps around massive
black holes are likely to rotate at a significant fraction of the
centrifugal breakup velocity as a result of spin-up by hyperbolic tidal
encounters. We use realistic stellar structure models to calculate
analytically the tidal spin-up in soft encounters and extend these
results to close and penetrating collisions using smoothed particle
hydrodynamics simulations. We find that the spin-up effect falls off
only slowly with distance from the black hole because the increased
tidal coupling in slower collisions at larger distances compensates
for the decrease in the stellar density. We apply our results to the
stars near the massive black hole in the Galactic center. Over their
lifetime, ~1 Msolar main-sequence stars in the inner 0.3
pc of the Galactic center are spun-up on average to ~10%-30% of the
centrifugal breakup limit. Such rotation is ~20-60 times higher than
is usual for such stars and may affect their subsequent evolution and
their observed properties.
Title: Source Depth for Solar P-Modes
Authors: Kumar, Pawan; Basu, Sarbani
Bibcode: 2000ApJ...545L..65K
Altcode: 2000astro.ph..6204K
Theoretically calculated power spectra are comparable with observed
solar p-mode velocity power spectra over a range of mode, degree,
and frequency. The depth for the sources responsible for exciting
p-modes of frequency 2.0 mHz is determined from the asymmetry of
their power spectra and found to be about 800 km below the photosphere
for quadrupole sources and 150 km if sources are dipole. The source
depth for high-frequency oscillations greater than ~6 mHz is 180 (50)
km for quadrupole (dipole) sources.
Title: Afterglow Emission from Naked Gamma-Ray Bursts
Authors: Kumar, Pawan; Panaitescu, Alin
Bibcode: 2000ApJ...541L..51K
Altcode: 2000astro.ph..6317K
We calculate the afterglow emission for gamma-ray bursts (GRBs) going
off in an extremely low density medium, referred to as naked bursts. Our
results also apply to the case where the external medium density falls
off sharply at some distance from the burst. The observed afterglow flux
in this case originates at high latitudes, i.e., where the angle between
the fluid velocity and the observer line of sight is greater than
Γ-1. The observed peak frequency of the spectrum for naked
bursts decreases with observer time as t-1, and the flux at
the peak of the spectrum falls off as t-2. The 2-10 keV X-ray
flux from a naked burst of average fluence should be observable by the
Swift satellite for time duration of about 103 longer than
the burst variability timescale. The high-latitude emission contributes
to the early X-ray afterglow flux for any GRB, not just naked bursts,
and can be separated from the shocked interstellar medium emission by
their different spectral and temporal properties. Measurements of the
high-latitude emission could be used to map the angular structure of
GRB-producing shells.
Title: Steepening of Afterglow Decay for Jets Interacting with
Stratified Media
Authors: Kumar, Pawan; Panaitescu, Alin
Bibcode: 2000ApJ...541L...9K
Altcode: 2000astro.ph..3264K
We calculate light curves for gamma-ray burst afterglows when material
ejected in the explosion is confined to a jet that propagates in a
medium with a power-law density profile. The observed light-curve
decay steepens by a factor of Γ2 when an observer sees
the edge of the jet. In a uniform density medium, the increase in
the power-law index (β) of the light curve as a result of this
edge effect is ~0.7 and is completed over one decade in observer
time. For a preejected stellar wind (ρ~r-2), β increases
by ~0.4 over two decades in time as a result of the edge effect, and
the steepening of the light curve as a result of the jet sideways
expansion takes about four decades in time. Therefore, a break in
the light curve for a jet in a wind model is unlikely to be detected
even for a very narrow opening angle of a few degrees or less, a case
where the lateral expansion occurs at early times when the afterglow
is bright. The light curve for the afterglow of GRB 990510, for which
an increase in β of approximately 1.35 was observed on a timescale
of 3 days, cannot be explained by only the sideways expansion and the
edge effects in a jet in a uniform interstellar medium-the increase in
β is too large and too rapid. However, the passage of the cooling or
synchrotron peak frequencies through the observing band at about 0.1-1
day together with jet edge effect explains the observed data. The jet
opening angle is found to be ~5°, and the energy in the explosion to
be about 1051 ergs.
Title: The patchy shells model
Authors: Piran, Tsvi; Kumar, Pawan
Bibcode: 2000AIPC..526..535P
Altcode: 2000hgrb.symp..535P
We propose that the angular inhomogeneities within the relativistic
flow cause the same GRB to look very different to different
observers. Consequently the most energetic bursts do not correspond
to an exceptional energy release but instead they correspond to
accidental exceptionally bright spots along the line of sight on
colliding shells. We describe the patchy shell model and calculate the
distribution function of the observed fluence for bursts with random
angular fluctuations of ejecta. We predict, according to this model,
that the GRB luminosity function will be much wider than the afterglow
X-ray luminosity function and only little correlation between the γ-ray
fluence and the afterglow emission. These two predictions are confirmed
by the GRB-afterglow data. We also predict that the early (minutes
to hours) afterglow would depict large temporal fluctuations whose
amplitude decreases with time. Finally we predict that there should be
many weak bursts with average afterglow luminosity in this scenario. .
Title: The Distribution of Burst Energy and Shock Parameters for
Gamma-Ray Bursts
Authors: Kumar, Pawan
Bibcode: 2000ApJ...538L.125K
Altcode: 1999astro.ph.12566K
We calculate the luminosity function for gamma-ray burst afterglows
in some fixed observed frequency band and at some fixed elapsed
time in observer frame (tobs) in two models-one in which
the explosion takes place in a uniform density medium and another
in which the density falls off as inverse square (expected for
stellar winds). For photon energies greater than about 500 eV and
tobs>~103 s, the afterglow flux is independent
of interstellar medium (ISM) density and luminosity functions for wind
and uniform ISM are identical. We deduce from the width of the observed
X-ray afterglow distribution, 5 hr after the burst, that the FWHM of
the distribution for isotropic energy in explosion and the fractional
energy in electrons (ɛe) are each less than about 1 order
of magnitude and the FWHM for the electron energy index is 0.6 or less.
Title: Energetics and Luminosity Function of Gamma-Ray Bursts
Authors: Kumar, Pawan; Piran, Tsvi
Bibcode: 2000ApJ...535..152K
Altcode: 1999astro.ph..9014K
Gamma-ray bursts (GRBs) are believed to be some catastrophic
event in which material is ejected at a relativistic velocity,
and internal collisions within this ejecta produce the observed
γ-ray flash. The angular size of a causally connected region
within a relativistic flow is of the order the angular width of the
relativistic beaming, γ-1. Thus, different observers along
different lines of sight could see drastically different fluxes from
the same burst. Specifically, we propose that the most energetic
bursts correspond to exceptionally bright spots along the line of
sight on colliding shells and do not represent much larger energy
release in the explosion. The energy budget for an average GRB in
this model is, however, same as in the uniform shell model. We
calculate the distribution function of the observed fluence for
random angular-fluctuation of ejecta. We find that the width of the
distribution function for the observed fluence is about 2 orders of
magnitude if the number of shells ejected along different lines of
sight is 10 or less. The distribution function becomes narrower if
number of shells along typical lines of sight increases. The analysis
of the γ-ray fluence and afterglow emissions for GRBs with known
redshifts provides support for our model, i.e., the large width of GRB
luminosity function is not due to a large spread in the energy release
but instead is due to large angular fluctuations in ejected material. We
outline several observational tests of this model. In particular,
for δ-function energy distribution in explosions we predict little
correlation between the γ-ray fluence and the afterglow emission as
in fact is observed. We predict that the early (minutes-to-hours)
afterglow would depict large temporal fluctuations whose amplitude
decreases with time. Finally, we predict that there should be many
weak bursts with about average afterglow luminosity in this scenario.
Title: Some Observational Consequences of Gamma-Ray Burst Shock Models
Authors: Kumar, Pawan; Piran, Tsvi
Bibcode: 2000ApJ...532..286K
Altcode: 1999astro.ph..6002K
Gamma-ray bursts (GRBs) are believed to be produced when fast-moving
ejecta from some central source collides with slower moving, but
relativistic, shells that were ejected at an earlier time. In this
so-called internal shock scenario we expect some fraction of the
energy of the burst to be carried by slow-moving shells that were
ejected at late times. These slow shells collide with faster moving
outer shells when the outer shells have slowed down as a result of
sweeping up material from the interstellar medium. This gives rise
to a forward shock that moves into the outer shell, producing a bump
in the afterglow light curve of the amplitude roughly proportional
to the ratio of the energy in the inner and the outer shells. In
addition, a reverse shock propagates in the inner shell and produces
emission at a characteristic frequency that is typically much smaller
than the peak of the emission from the outer shell by a factor of
~7γ20c(E2/E1)1.1,
and the observed flux at this frequency from the reverse shock is
larger compared to the flux from the outer shell by a factor of
~8(γ0cE2/E1)5/3 where
γ0c is the bulk Lorentz factor of the outer shell at the
time of collision, and E1 and E2 are the total
energy in the outer and the inner shells, respectively. The Lorentz
factor is related to the observer time as ~5(t/day)3/8. The
shell collision could produce initial temporal variability in the early
afterglow signal. The lack of significant deviation from a power-law
decline of the optical afterglow from half a dozen bursts suggests
that E2/E1 is small. Future multiwavelength
observations should be able to either detect bumps in the light curve
corresponding to both the forward and the reverse shocks or further
constrain the late time release of energy in ejecta with a small
Lorentz factor, which is expected generically in the internal shock
models for the GRBs.
Title: Gamma-Ray Burst Energetics
Authors: Kumar, Pawan
Bibcode: 1999ApJ...523L.113K
Altcode: 1999astro.ph..7096K
We estimate the fraction of the total energy in a gamma-ray burst (GRB)
that is radiated in photons during the main burst. Random internal
collisions among different shells limit the efficiency for converting
bulk kinetic energy to photons. About 1% of the energy of explosion
is converted to radiation, in the 10-103 keV energy band
in the observer frame, for long-duration bursts (lasting 10 s or
more); the efficiency is significantly smaller for shorter duration
bursts. Moreover, about 50% of the energy of the initial explosion
could be lost to neutrinos during the early phase of the burst if the
initial fireball temperature is ~10 MeV. If isotropic, the total energy
budget of the brightest GRBs is >~1055 ergs, a factor of
>~20 larger than previously estimated. Anisotropy of explosion, as
evidenced in two GRBs, could reduce the energy requirement by a factor
of 10-100. Putting these two effects together, we find that the energy
release in the most energetic bursts is about 1054 ergs.
Title: Angular Momentum Redistribution by Waves in the Sun
Authors: Kumar, Pawan; Talon, Suzanne; Zahn, Jean-Paul
Bibcode: 1999ApJ...520..859K
Altcode: 1999astro.ph..2309K
We calculate the angular momentum transport by gravito-inertial-Alfvén
waves and show that, so long as prograde and retrograde gravity waves
are excited to roughly the same amplitude, the sign of angular momentum
deposit in the radiative interior of the Sun is such as to lead to an
exponential growth of any existing small radial gradient of rotation
velocity just below the convection zone. This leads to formation of
a strong thin shear layer (of thickness about 0.3% Rsolar)
near the top of the radiative zone of the Sun on a timescale of order
20 yr. When the magnitude of differential rotation across this layer
reaches about 0.1 μHz, the layer becomes unstable to shear instability
and undergoes mixing, and the excess angular momentum deposited in the
layer is returned to the convection zone. The strong shear in this
layer generates a toroidal magnetic field which is also deposited
in the convection zone when the layer becomes unstable. This could
possibly start a new magnetic activity cycle seen at the surface.
Title: Line Asymmetry of Solar p-Modes: Properties of Acoustic Sources
Authors: Kumar, Pawan; Basu, Sarbani
Bibcode: 1999ApJ...519..396K
Altcode: 1998astro.ph..8143K
The observed solar p-mode velocity power spectra are compared with
theoretically calculated power spectra over a range of mode degree and
frequency. The shape of the theoretical power spectra depends on the
depth of acoustic sources responsible for the excitation of p-modes
and also on the multipole nature of the source. We vary the source
depth to obtain the best fit to the observed spectra. We find that
quadrupole acoustic sources provide a good fit to the observed spectra
provided that the sources are located between 700 and 1050 km below
the top of the convection zone. The dipole sources give a good fit for
a significantly shallower source, with a source depth of between 120
and 350 km. The main uncertainty in the determination of depth arises
because of poor knowledge of the nature of power leakages from modes
with adjacent degrees and the background in the observed spectra.
Title: Line Asymmetry of Solar p-Modes: Reversal of Asymmetry in
Intensity Power Spectra
Authors: Kumar, Pawan; Basu, Sarbani
Bibcode: 1999ApJ...519..389K
Altcode: 1998astro.ph..8144K
The sense of line asymmetry of solar p-modes in the intensity power
spectra is observed to be opposite of that seen in the velocity power
spectra. Theoretical calculations provide a good understanding and fit
to the observed velocity power spectra, whereas the reverse sense of
asymmetry in the intensity power spectrum has been poorly understood. We
show that when turbulent eddies arrive at the top of the convection
zone they give rise to an observable intensity fluctuation that is
correlated with the oscillation they generate, thereby affecting the
shape of the line in the p-mode power spectra and reversing the sense
of asymmetry (this point was recognized by Nigam et al. and Roxburgh
& Vorontsov). The addition of the correlated noise displaces the
frequencies of peaks in the power spectrum. Depending on the amplitude
of the noise source, the shift in the position of the peak can be
substantially larger than the frequency shift in the velocity power
spectra. In neither case are the peak frequencies precisely equal to
the eigenfrequencies of p-modes. We suggest two observations that can
provide a test of the model discussed here.
Title: The Structure of the Central Disk of NGC 1068: A Clumpy
Disk Model
Authors: Kumar, Pawan
Bibcode: 1999ApJ...519..599K
Altcode: 1999astro.ph..2308K
NGC 1068 is one of the best-studied Seyfert II galaxies, for which
the black hole mass has been determined from the Doppler velocities
of water maser. We show that the standard α-disk model of NGC 1068
gives disk mass between the radii of 0.65 and 1.1 pc (the region from
which water maser emission is detected) to be about 7×107
Msolar (for α=0.1), more than 4 times the black hole mass,
and a Toomre Q-parameter for the disk is ~0.001. This disk is therefore
highly self-gravitating and is subject to large-amplitude density
fluctuations. We conclude that the standard α-viscosity description
for the structure of the accretion disk is invalid for NGC 1068. In
this paper, we develop a new model for the accretion disk. The disk is
considered to be composed of gravitationally bound clumps; accretion
in this clumped disk model arises because of gravitational interaction
of clumps with each other and the dynamical frictional drag exerted on
clumps from the stars in the central region of the galaxy. The clumped
disk model provides a self-consistent description of the observations of
NGC 1068. The computed temperature and density are within the allowed
parameter range for water maser emission, and the rotational velocity
in the disk falls off as r-0.35.
Title: Dissipation of a Tide in a Differentially Rotating Star
Authors: Talon, Suzanne; Kumar, Pawan
Bibcode: 1998ApJ...503..387T
Altcode: 1997astro.ph..7309T
The orbital period of the binary pulsar PSR J0045-7319, which is
located in our neighboring galaxy, the Small Magellanic Cloud (SMC),
appears to be decreasing on a timescale of ~5 × 105
yr. This timescale is more than 2 orders of magnitude smaller than
what is expected from the standard theory of tidal dissipation. Kumar
& Quataert proposed that this rapid evolution could be understood
provided that the neutron star's companion, a main-sequence B star,
has set up significant differential rotation. The goal of this paper
is to evaluate the redistribution of angular momentum in the B star
due to meridional circulation and shear stresses and to calculate
the evolution of the rotation profile when these two processes act
in competition with the deposition of momentum by the tidal wave. We
find that although angular momentum redistribution is important, the
B star may continue to have sufficient differential rotation so that
tidal waves are entirely absorbed as they arrive at the surface. The
mechanism proposed by Kumar & Quataert to speed up the orbital
evolution of the SMC binary pulsar should therefore work as suggested.
Title: On the Orbital Decay of the PSR J0045-7319 Binary
Authors: Kumar, Pawan; Quataert, Eliot J.
Bibcode: 1998ApJ...493..412K
Altcode:
Recent observations of PSR J0045-7319, a radio pulsar in a close
eccentric orbit with a massive main-sequence B star companion, indicate
that the system's orbital period is decreasing on a timescale ~5 ×
105 yr. Timing observations of PSR J0045-7319 also indicate
that the B star is rotating rapidly, perhaps close to its breakup
rotation rate. For rapid (supersynchronous) prograde rotation of the
B star, tidal dissipation leads to an increasing orbital period for
the binary system, while for retrograde rotation of any magnitude,
the orbital period decreases with time. We show that if tidal effects
are to account for the observed orbital decay of the PSR J0045-7319
binary, the B star must have retrograde rotation. This implies
that the supernova that produced the pulsar in this binary system
likely had a dipole anisotropy. For a reasonably wide range of
retrograde rotation rates, the energy in the dynamical tide of the B
star needs to be dissipated in about one orbital period in order to
account for the observed orbital evolution time. We show, however,
that the radiative dissipation of the dynamical tide in a rigidly
rotating B star is too inefficient by a factor of ~103,
regardless of the magnitude of the rotation rate. We describe how,
when the surface of the B star is rotating nearly synchronously, the
energy in the dynamical tide is dissipated in less than an orbital
period, thus reconciling the theoretical and observed rates of orbital
evolution. Nonlinear parametric decay of the equilibrium tide,
for rigid retrograde rotation of the B star, may also be able to
explain the observed rate of orbital evolution, although the margin
of instability is too small to draw definitive conclusions about the
relevance of this process for the PSR J0045-7319 binary.
Title: Possible explanations for some unusually large velocity
dispersion molecular clouds near the Galactic Centre
Authors: Kumar, Pawan; Riffert, Harald
Bibcode: 1997MNRAS.292..871K
Altcode:
Molecular clouds in the Galactic Centre region typically have a velocity
dispersion that can be larger by almost a factor of 2 compared with
the velocity dispersion of similar mass clouds elsewhere in the
Galaxy. However, there are at least two giant molecular clouds, and
perhaps as many as half a dozen, located within a kpc of the Galactic
Centre that have internal random velocities, observed in ^12CO and Hi
emission lines, that extend from about 0 to almost 200 km s^-1. This is
larger by a factor of about 10 than normal giant molecular clouds. Two
of the most prominent clouds have a molecular mass, estimated from
the ^12CO emission, of ~10^6 Msolar, and their atomic hydrogen mass is
about 2x10^5 Msolar. We consider various possible physical mechanisms
for the large velocity dispersion of these clouds.
Title: Localized Helioseismic Constraints on Solar Structure
Authors: Bahcall, John N.; Basu, Sarbani; Kumar, Pawan
Bibcode: 1997ApJ...485L..91B
Altcode: 1997astro.ph..2075B
Localized differences between the real Sun and standard solar models
are shown to be small. The sound speeds of the real and the standard
model Suns typically differ by less than 0.3% for regions of radial
width ~=0.1 Rsolar in the solar core.
Title: Gas accretion in a clumpy disk with application to AGNs
Authors: Kumar, Pawan
Bibcode: 1997astro.ph..6063K
Altcode:
We analyze the collective gravitational interaction among gas clouds in
the inner regions of galactic disks and find that it leads to accretion
at a rate $\sim M_{mc}\Omega (M_{mc}/M_t)^2$; where $M_{mc}$ is the
molecular mass of the disk, $M_t$ is sum of the central plus any
axisymmetrically distributed mass, and $\Omega$ is the mean angular
speed of clumps. We discuss applications of this result to the
mega-maser galaxy NGC 4258, for which we have observational evidence
that the maser spots are concentrated in a thin molecular disk which
is clumpy, and find the accretion rate to be about $1.5\times 10^{-3}$
solar mass per year. If the gravitational energy release of this inward
falling gas were to be radiated away efficiently, then the resulting
luminosity would greatly exceed the observed central luminosity of
NGC 4258, indicating that most of the thermal energy of the gas is
advected with the flow into the blackhole as proposed by Lasota et
al. (1996). The gravitational interactions among molecular clouds
lying within the inner kpc of our galaxy give an accretion rate of
about $10^{-5}$ solar mass per year, which is consistent with the
value obtained by Narayan et al. (1995) by fitting the spectrum of
Sagitarrius A$^*$. We also discuss possible application of this work
to quasar evolution.
Title: Differential Rotation Enhanced Dissipation of Tides in the
PSR J0045-7319 Binary
Authors: Kumar, Pawan; Quataert, Eliot J.
Bibcode: 1997ApJ...479L..51K
Altcode: 1996astro.ph.11005K
Recent observations of PSR J0045-7319, a radio pulsar in a close
eccentric orbit with a massive B star companion, indicate that
the system's orbital period is decreasing on a timescale of ~5
× 105 yr. This is much shorter than the timescale of
~109 yr given by the standard theory of tidal dissipation
in radiative stars. Observations also suggest that the B star is
rotating rapidly, perhaps at nearly its breakup speed. We show that
the dissipation of the dynamical tide in a star rotating in the same
direction as the orbital motion of its companion (prograde rotation)
with a speed greater than the orbital angular speed of the star at
periastron (supersynchronous rotation) results in an increase in the
orbital period of the binary system with time. Thus, if the magnitude of
the rotation speed of the B star is supersynchronous, then the observed
decrease in the orbital period requires the direction of the rotation
of the B star to be retrograde. For subsynchronous prograde rotation of
the B star, the energy in the dynamical tide, even if it is dissipated
in one orbital period, is too small to account for the observed orbital
evolution, unless the rotation speed is close to zero. Slow rotation
of the B star is, however, ruled out by the observed apsidal motion
of the system (Lai et al., Kaspi et al.). Thus, in order to explain
both the observed apsidal motion and the orbital evolution of the
PSR J0045-7319 binary, the B star must have retrograde rotation. If the rotation in the interior of the B star is not synchronized,
which we show is the case, then the work of Goldreich & Nicholson
suggests that the B star should be rotating differentially, with the
rotation speed of the outer layers close to the synchronous value. We
show that the dissipation of the dynamical tide in such a differentially
rotating B star is enhanced by almost 3 orders of magnitude, leading
to an orbital evolution time for the PSR J0045-7319 binary that is
consistent with the observations.
Title: Angular Momentum Transport by Gravity Waves and Its Effect
on the Rotation of the Solar Interior
Authors: Kumar, Pawan; Quataert, Eliot J.
Bibcode: 1997ApJ...475L.143K
Altcode: 1996astro.ph.11006K
We calculate the excitation of low-frequency gravity waves by turbulent
convection in the Sun and the effect of the angular momentum carried by
these waves on the rotation profile of the Sun's radiative interior. We
find that the gravity waves generated by convection in the Sun provide a
very efficient means of coupling the rotation in the radiative interior
to that of the convection zone. In a differentially rotating star,
waves of different azimuthal number have their frequencies in the local
rest frame of the star Doppler shifted by different amounts. This
leads to a difference in their local dissipation rate and hence a
redistribution of angular momentum in the star. We find that the
timescale for establishing uniform rotation throughout much of the
radiative interior of the Sun is ~107 yr, which provides a
possible explanation for the helioseismic observations that the solar
interior is rotating as a solid body.
Title: On the orbital decay of the PSR J0045-7319 Binary
Authors: Kumar, Pawan; Quataert, Eliot J.
Bibcode: 1996astro.ph.12189K
Altcode:
Recent observations of PSR J0045-7319, a radio pulsar in a close
eccentric orbit with a massive main sequence B-star companion, indicate
that the system's orbital period is decreasing on a timescale $\sim 5 x
10^{5}$ years (Kaspi et al. 1996). Timing observations of PSR J0045-7319
also indicate that the B-star is rotating rapidly, perhaps close to its
breakup rotation rate. For rapid (super-synchronous) prograde rotation
of the B-star, tidal dissipation leads to an increasing orbital period
for the binary system, while for retrograde rotation of any magnitude,
the orbital period decreases with time. We show that if tidal effects
are to account for the observed orbital decay of the PSR J0045-7319
binary, the B-star must have retrograde rotation. This implies that
the supernova that produced the pulsar in this binary system likely
had a dipole anisotropy. For a reasonably wide range of retrograde
rotation rates, the energy in the dynamical tide of the B-star needs
to be dissipated in about one orbital period in order to account for
the observed orbital evolution time for the PSR J0045-7319 binary. We
show, however, that the radiative dissipation of the dynamical tide in
a rigidly rotating B-star is too inefficient by a factor of $\approx$
10$^3$, regardless of the magnitude of the rotation rate. We describe
how, when the surface of the B-star is rotating nearly synchronously
(which is expected from the work of Goldreich and Nicholson, 1989),
the energy in the dynamical tide is dissipated in less than an orbital
period, thus reconciling the theoretical and observed rates of orbital
evolution.
Title: Asymmetries of Solar p-Mode Line Profiles
Authors: Abrams, Douglas; Kumar, Pawan
Bibcode: 1996ApJ...472..882A
Altcode:
Recent observations indicate that solar p-mode line profiles are not
exactly Lorentzian but rather exhibit varying amounts of asymmetry about
their peaks. We analyze p-mode line asymmetry by using both a simplified
one-dimensional model and a more realistic solar model. We find that the
amount of asymmetry exhibited by a given mode depends on the location
of the sources exciting the mode, the mode frequency, and weakly on the
mode spherical harmonic degree but not on the particular mechanism or
location of the damping. We calculate the dependence of line asymmetry
on source location for solar p-modes and provide physical explanations
of our results in terms of the simplified model. A comparison of our
results to the observations of line asymmetry in velocity spectra
reported by Duvall et al. for modes of frequency ∼2.3 mHz suggests
that the sources for these modes are located more than 325 km beneath
the photo sphere. This source depth is greater than that found by Kumar
for acoustic waves of frequency ∼6 mHz. The difference may indicate
that waves of different frequencies are excited at different depths in
the convection zone. We find that line asymmetry causes the frequency
obtained from a Lorentzian fit to a peak in the power spectrum to differ
from the corresponding eigenfrequency by an amount proportional to a
dimensionless asymmetry parameter and to the mode line width.
Title: Asymmetries of Solar p-mode Line Profiles
Authors: Kumar, Pawan; Abrams, Douglas
Bibcode: 1996astro.ph.10254K
Altcode:
Recent observations indicate that solar p-mode line profiles are not
exactly Lorentzian, but rather exhibit varying amounts of asymmetry
about their respective peaks. We analyze p-mode line asymmetry using
both a simplified one-dimensional model and a more realistic solar
model. We find that the amount of asymmetry exhibited by a given mode
depends on the location of the sources exciting the mode, the mode
frequency, and weakly on the mode spherical harmonic degree, but not on
the particular mechanism or location of the damping. We calculate the
dependence of line asymmetry on source location for solar p-modes,
and provide physical explanations of our results in terms of the
simplified model. A comparison of our results to the observations of
line asymmetry in velocity spectra reported by Duvall et al. (1993)
for modes of frequency $\sim$ 2.3 mHz suggests that the sources for
these modes are located more than 325 km beneath the photosphere. This
source depth is greater than that found by Kumar (1994) for acoustic
waves of frequency $\sim$ 6 mHz. The difference may indicate that
waves of different frequencies are excited at different depths in
the convection zone. We find that line asymmetry causes the frequency
obtained from a Lorentzian fit to a peak in the power spectrum to differ
from the corresponding eigenfrequency by an amount proportional to a
dimensionless asymmetry parameter and to the mode linewidth.
Title: Nonlinear Damping of Oscillations in Tidal-Capture Binaries
Authors: Kumar, Pawan; Goodman, Jeremy
Bibcode: 1996ApJ...466..946K
Altcode: 1995astro.ph..9112K
We calculate the damping of quadrupole f- and low-order g-modes (primary
modes) by nonlinear coupling to other modes of the star. Primary modes
destabilize high-degree g-modes of half their frequency (daughter
modes) by 3-mode coupling in radiative zones. For Sun-like stars,
the growth time ≡η-1≍4E-½0,42
days, where E0,42 is the initial energy of the primary
mode in units of 1042 ergs, and the number of daughter
modes N ∼ 1010E5/40,42. The growth
rate is approximately equal to the angular frequency of the primary
mode times its dimensionless radial amplitude, δR/R*
≍ 0.002E-½0,42. Although the daughter modes
are limited by their own nonlinearities, collectively they absorb
most of the primary mode's energy after a time ∼10ɛ-1
provided E0 > 1040 ergs. This is orders of magnitude
smaller than usual radiative damping time. In fact, nonlinear mode
interaction may be the dominant damping process if E0
≥ 1037 ergs. These results have obvious application to
tidally captured main-sequence globular cluster stars of mass ≥ 0.5
Msun; the tidal energy is dissipated in the radiative core
of the star in about a month, which is less than the initial orbital
period. Nonlinear mode coupling is a less efficient damping
process for fully convective stars, which lack g-modes. In convective
stars, most of the tidal energy is in the quadrupole f-modes, which
nonresonantly excite high-order p-modes of degree 0, 2, and 4. The
resultant short-wavelength waves are more efficiently dissipated. The
nonlinear damping time for f-modes is shown to be proportional to
1/E0; this damping time is about 30 days for E0
≍ 1045 ergs expected in tidal captures. However, at such
a large energy the system is very nonlinear: 4-mode and higher order
couplings are as important as 3-mode couplings.
Title: On the Validity of the Classical Apsidal Motion Formula for
Tidal Distortion
Authors: Quataert, Eliot J.; Kumar, Pawan; Ao, Chi On
Bibcode: 1996ApJ...463..284Q
Altcode: 1995astro.ph..9152Q
We check the validity of the widely used classical apsidal motion
formula as a function of orbital parameters, stellar structure,
and stellar rotation rate by comparing dynamical calculations of the
periastron advance with the equilibrium tidal formula. We find that the
classical formula gives very accurate results when the periods of the
low-order quadrupole g, ft and p-modes are smaller than the periastron
passage time by a factor of about 10 or more. However, when this
condition is not satisfied, the difference between the classical formula
and the exact result can be quite large, and even periastron recession
can result. The largest difference arises when frequency of one of the
low-order modes of the star is nearly resonant with an integer multiple
of the orbital frequency minus twice the rotation rate of the star. The
resonance of higher order g-modes (number of radial nodes ≳
4) with the orbit is very unlikely to cause significant deviation from
the classical result because of their weak coupling to the tidal force
and thus their small contribution to the apsidal motion. Resonances
involving rotational modes of the star are also unlikely to make much
contribution to the apsidal motion because of their small overlap
with the tidal force, even though they have periods comparable to the
periastron passage time. We apply our work to two famous binary
systems (AS Cam and DI Her) that show abnormally small apsidal motion,
and conclude that dynamical effects are unimportant for these systems,
i.e., the equilibrium tide assumption is an excellent approximation.
Title: Observational Searches for Solar g-Modes: Some Theoretical
Considerations
Authors: Kumar, Pawan; Quataert, Eliot J.; Bahcall, John N.
Bibcode: 1996ApJ...458L..83K
Altcode: 1995astro.ph.12091K
We argue that the solar g-modes are unlikely to have caused the discrete
peaks in the power spectrum of the solar wind flux observed by Thomson
et al (1995). The lower limit to the energy of individual g-modes,
using the amplitudes given by Thomson et al., is estimated to be at
least 1036 ergs for low-order g-modes; the resulting surface velocity
amplitude is at least 50 cm s-1, larger than the observational upper
limit (5 cm s-1). We suggest that the most likely source for the
excitation of solar g-modes is turbulent stresses in the convection
zone. The surface velocity amplitude of low-degree and low-order
g-modes resulting from this process is estimated to be of order 10-2 cm
s-1. This amplitude is interestingly close to the detection threshold of
the SOHO satellite. The long lifetime of g-modes (~106 yr for low-order
modes) should be helpful in detecting these small-amplitude pulsations.
Title: Asymmetries of Solar p-mode Line Profiles
Authors: Abrams, Douglas; Kumar, Pawan
Bibcode: 1995AAS...18710102A
Altcode: 1995BAAS...27.1426A
We analyze p-mode line profiles using both a simplified one-dimensional
model and a realistic solar model. We find asymmetry to be a
general feature of the profiles, which depends on source location,
mode frequency, mode spherical harmonic degree, and linewidth, but
not on the particular mechanism or location of damping. Using the
realistic solar model, we calculate the dependence of line asymmetry
on source location, mode frequency, and spherical harmonic degree, and
provide physical explanations of our results in terms of the simplified
model. Our results are in qualitative agreement with the observational
results of Duvall et al. (1993) if we assume the sources are located
within the top 400 km of the solar convection zone, but more detailed
observations of line asymmetry in velocity spectra are called for to
accurately locate the sources. We find that line asymmetry in both the
simplified model and the realistic solar model causes the frequencies
obtained from Lorentzian fits to the power spectrum to differ from the
corresponding eigenfrequencies by an amount which is proportional to
the asymmetry of the peak as quantified in a model-independent way,
and to the linewidth.
Title: On the Validity of the Classical Apsidal Motion Formula for
Tidal Distortion
Authors: Quataert, Eliot; Kumar, Pawan; Ao, Chi On
Bibcode: 1995AAS...187.4319Q
Altcode: 1995BAAS...27.1343Q
We check the validity of the widely used classical apsidal motion
formula as a function of orbital parameters, stellar structure,
and stellar rotation rate by comparing dynamical calculations of the
periastron advance with the equilibrium tidal formula. We find that
the classical formula gives very accurate results when the periods
of the low order quadrupole g, f and p modes are smaller than the
periastron passage time by a factor of about 7 or more. However, when
this condition is not satisfied, the difference between the classical
formula and the exact result can be quite large, and even periastron
recession can result. The largest difference arises when one of the low
order modes of the star is nearly resonant with an integer multiple of
the orbital frequency minus twice the rotation rate of the star. The
resonance of higher order g-modes (number of radial nodes greater
than about 4) with the orbit is very unlikely to cause significant
deviation from the classical result because of their weak coupling
to the tidal force and thus their small contribution to the apsidal
motion. Resonances involving rotational modes of the star are also
unlikely to make much contribution to the apsidal motion because of
their small overlap with the tidal force, even though they have periods
comparable to the periastron passage time. We apply our work to two
famous binary systems (AS Cam and DI Her) which show abnormally small
apsidal motion, and conclude that dynamical effects are unimportant
for these systems, i.e. the equilibrium tide assumption is an excellent
approximation.
Title: On the Interaction of Convection and Rotation in Stars
Authors: Kumar, Pawan; Narayan, Ramesh; Loeb, Abraham
Bibcode: 1995ApJ...453..480K
Altcode:
We use a recent formulation of turbulent convection to study rotation
in convective media. The formalism is based on the Boltzmann equation
for the distribution function of convective blobs and represents an
extension of mixing-length theory. The details of the interactions
between blobs are introduced through a model of the collision term. We
obtain the stress tensor and other correlation functions of a rotating
convective fluid from the second moments of the Boltzmann equation. In
steady state, and in the absence of large-scale circulation, the
Reynolds stress must vanish. We use this condition to determine the
equilibrium rotation profile in the equatorial plane of convective
stars. Even in the simple case of isotropic scattering of convective
blobs, the equilibrium rotation profile is not necessarily solid
body rotation; solid body rotation arises only when the scattering
is perfectly elastic, whereas in the opposite limit of completely
inelastic scattering, the equilibrium profile consists of a uniform
distribution of specific angular momentum. If the scattering is allowed
to be anisotropic, then we find an even wider range of equilibrium
rotation profiles; in particular, we find that for certain choices
of the parameters it is possible to have a rising rotation profile,
similar to that observed in the equatorial plane of the Sun. In
phenomenological models of the solar rotation, the Reynolds stress
is written in a generalized form which includes anisotropic viscosity
and A-terms, the latter giving a nonzero shear stress even when there
is no velocity shear. Our analysis gives rise to both anisotropic
viscosity and A-terms quite naturally and shows that both effects
should be generically present. We provide analytical expressions for
these effects in terms of the parameters of the interblob scattering
function. We also consider the effect of rotation on the condition
for convective instability. According to linear analysis, convection
in the equatorial plane is suppressed if the epicyclic frequency
exceeds the imaginary part of the Brunt-Väisälä frequency. However,
when the scattering of blobs is included in the analysis, we show
that the stability condition is modified, and there can be a new
kind of instability even when linear theory predicts stability. This
new instability has a zero growth rate when the scattering frequency
goes to zero but has a growth time comparable to the convection time
when the collision frequency is comparable to the Brunt-Väisälä
frequency. Thus, this instability is a secular instability and possibly
a finite amplitude instability, which arises primarily as a result
of the scattering of eddies. We discuss the generalization of these
results for regions away from the equatorial plane. Finally, we
show that the direction of angular momentum flow can be very different
in the linear limit compared to fully developed convection. When the
convective perturbations are infinitesimally small, corresponding to
the linear limit, we show that the angular momentum flow is in such a
direction as to drive the system toward a constant angular momentum
configuration. This is in agreement with the result obtained by Ryu
& Goodman through linear mode analysis of a Keplerian accretion
disk. However, in saturated convection, when the perturbations are
fully nonlinear, the angular momentum flow is such as to drive the
system toward the equilibrium rotation profiles described above. Thus,
the angular momentum flux is outward in the case of fully developed
convection in a Keplerian disk. This means that convection is a viable
mechanism to generate viscosity in accretion disks.
Title: Tidal Excitation of Modes in Binary Systems with Applications
to Binary Pulsars
Authors: Kumar, Pawan; Ao, Chi On; Quataert, Eliot J.
Bibcode: 1995ApJ...449..294K
Altcode: 1995astro.ph..3053K
We consider the tidal excitation of modes in a binary system of
arbitrary eccentricity. For a circular orbit, the modes generally
undergo forced oscillation with a period equal to the orbital period
(T). For an eccentric orbit, the amplitude of each tidally excited
mode can be written approximately as the sum of an oscillatory term
that varies sinusoidally with the mode frequency and a "static" term
that follows the time dependence of the tidal forcing function. The
oscillatory term falls off exponentially with increasing b (defined
as the ratio of the periastron passage time to the mode period),
whereas the "static" term is independent of bα. For
small-bα modes (bα ≍ 1), the two terms are
comparable, and the magnitude of the mode amplitude is nearly constant
over the orbit. For large-bα modes (bα
≳ a few), the oscillatory term is very small compared with the
"static" term, in which case the mode amplitude, like the tidal
force, varies as the distance cubed. For main-sequence stars,
p-, f-, and low-order g-modes generally have large bα
and hence small amplitudes of oscillation. High overtone g-modes,
however, have small overlap with the tidal forcing function. Thus,
we expect an intermediate overtone g-mode with bα ∼ 1
to have the largest oscillation amplitude. In addition, we find that
the mode amplitude is independent of the dissipation rate except when
the mode frequency is very close to orbital resonance or the damping
time is less than T; both conditions are unlikely. Moreover, orbital
evolution causes a resonant mode to move off resonance with time. This
severely limits the amplitude of modes near resonance. Rotation of the
star shifts the mode frequencies but otherwise has little effect on
the mode amplitude (provided that the rotation rate is small). Hence,
tidally excited modes have amplitudes and phases that are periodic
with period T, making them readily distinguishable from oscillations
excited by other mechanisms. We apply our work to the SMC radio
pulsar PSR J0045 - 7319, which is believed to be in a highly eccentric
orbit with a 10 Msun B star. We find that the g7
mode (with period 1.07 days) of the B star has the largest oscillation
amplitude, with a flux variation of 2.3 mmag and a surface velocity
of 70 ms-1. The flux variation at periastron summed over
all modes, is about 10 mmag; in addition, we propose that the shape of
the light curve can be utilized to determine the orbital inclination
angle. The apsidal motion of this system, calculated without the usual
static approximation, is larger than that predicted by the classical
apsidal formula by about 1%. For the PSR B1259 - 63 system, the tidal
amplitude of the Be star companion is smaller by a factor of 70 because
of its larger periastron distance. To understand the dependence of
tidal excitation on stellar structure, detailed numerical calculations
of modes of a general polytropic star are also presented.
Title: Causality in Strong Shear Flows
Authors: Narayan, Ramesh; Loeb, Abraham; Kumar, Pawan
Bibcode: 1994ApJ...431..359N
Altcode: 1994astro.ph..1004N
It is well known that the standard transport equations violate
causality when gradients are large or when temporal variations are
rapid. We derive a modified set of transport equations that satisfy
causality. These equations are obtained from the underlying Boltzmann
equation. We use a simple model for particle collisions which enables
us to derive moment equations non-perturbatively, i.e. without making
the usual assumption that the distribution function deviates only
slightly from its equilibrium value. We apply the model to two problems:
particle diffusion and viscous transport. In both cases we show that
signals propagate at a finite speed and therefore that the formalism
obeys causality. When the velocity gradient is large on the scale
of a mean free path, the viscous shear stress is suppressed relative
to the prediction of the standard diffusion approximation. The shear
stress reaches a maximum at a finite value of the shear amplitude and
then decreases as the velocity gradient increases. In the case of a
steady Keplerian accretion disk with hydrodynamic turbulent viscosity,
the stress-limit translates to an upper bound on the Shakura-Sunyaev
$\alpha$-parameter, namely $\alpha<0.07$. The limit on $\alpha$
is much stronger in narrow boundary layers where the velocity shear
is larger than Keplerian.
Title: Properties of Acoustic Sources in the Sun
Authors: Kumar, Pawan
Bibcode: 1994ApJ...428..827K
Altcode:
The power spectrum of solar acoustic oscillations shows peaks extending
out to frequencies much greater than the acoustic cutoff frequency of
approximately 5.3 mHz, where waves are no longer trapped. Kumar &
Lu (1991) proposed that these peaks arise from the interference of
traveling waves which are generated by turbulent convection. According
to this model, the frequencies of the peaks in the power spectrum depend
on the static structure of the Sun as well as the radial location of
the sources. Kumar & Lu used this idea to determine the depth of
the acoustic sources. However, they ignored dissipative effects and
found that the theoretically computed power spectrum was falling off
much more rapidly than the observed spectrum. In this paper, we include
the interaction of radiation with acoustic waves in the computation
of the power spectrum. We find that the theoretically calculated power
spectra, when radiative damping is included are in excellent agreement
with the observed power spectra over the entire observed frequency
range of 5.3 to 7.5 mHz above the acoustic cutoff frequency. Moreover,
by matching the peak frequencies in the observed and theoretical spectra
we find the mean depth of acoustic sources to be 140 +/- 60 km below the
photosphere. We show that the spectrum of solar turbulence near the top
of the solar convection zone is consistent with the Kolmogorov spectrum,
and that the observed high frequency power spectrum provides strong
evidence that the acoustic sources in the Sun are quadrupolar. The
data, in fact, rules out dipole sources as significant contributors to
acoustic wave generation in the Sun. The radial extent of the sources
is poorly determined and is estimated to be less than about 550 km.
Title: Effect of Nonlinear Interactions on p-Mode Frequencies and
Line Widths
Authors: Kumar, Pawan; Goldreich, Peter; Kerswell, Richard
Bibcode: 1994ApJ...427..483K
Altcode:
We calculate the effect of nonlinear interactions among solar acoustic
modes upon the modal frequencies and energy loss rates (or line
widths). The frequency shift for a radial p-mode of frequency 3 mHz is
found to be about -0.5 microHz. The magnitude of nonlinear frequency
shift increases more rapidly with frequency than the inverse mode mass
(mode mass is defined as the ratio of energy in the mode to its surface
velocity amplitude squared). This frequency shift is primarily due to
nonresonant three-mode interactions and is dominated by high l surface
gravity waves (f-modes) and p-modes. The line width of a radial p-mode
of frequency 3 mHz, due to resonant nonlinear interactions, is about
0.3 microHz. This result is consistent with that of Kumar and Goldreich
(1989). We also find, in agreement with these authors, that the most
important nonlinear interactions of trapped p-modes involve f-modes
and high-frequency p-modes (frequency greater than about 5 mHz) which
propagate in the solar photosphere. Thus, using the arguments advanced
by Kumar & Goldreich (1989), we conclude that nonlinear couplings
cannot saturate the overstable solar p-modes at their small observed
amplitudes. Both the nonlinear frequency shifts and line widths,
at a fixed frequency, are proportional to the inverse of mode mass
which for modes of degree greater than about 100 is approximately
l0.8. Therefore, the frequency of an f-mode of l = 1000,
due to nonlinear interactions, is decreased by approximately 0.4%.
Title: Excitation of Solar p-Modes
Authors: Goldreich, Peter; Murray, Norman; Kumar, Pawan
Bibcode: 1994ApJ...424..466G
Altcode:
We investigate the rates at which energy is supplied to individual
p-modes as a function of their frequencies nu and angular degrees l. The
observationally determined rates are compared with those calculated on
the hypothesis that the modes are stochastically excited by turbulent
convection. The observationally determined excitation rate is assumed
to be equal to the product of the mode's energy E and its (radian)
line width Gamma. We obtain E from the mode's mean square surface
velocity with the aid of its velocity eigenfuction. We assume that
Gamma measures the mode's energy decay rate, even though quasi-elastic
scattering may dominate true absorption. At fixed l, E(Gamma) arises as
nu7 at low nu, reaches a peak at nu approximately equal 3.5
mHz, and then declines as nu4.4 at higher nu . At fixed nu,
E(Gamma) exhibits a slow decline with increasing l. To calculate energy
input rates, Palpha, we rely on the mixing-length model
of turbulent convection. We find entropy fluctuations to be about an
order of magnitude more effective than the Reynolds stress in exciting
p-modes . The calculated Palpha mimic the nu7
dependence of E(Gamma) at low nu and the nu-4.4 dependence
at high nu. The break of 11.4 powers in the nu-dependence of E(Gamma)
across its peak is attributed to a combination of (1) the reflection
of high-frequency acoustic waves just below the photosphere where the
scale height drops precipitously and (2) the absence of energy-bearing
eddies with short enough correlation times to excite high-frequency
modes. Two parameters associated with the eddy correlation time are
required to match the location and shape of the break. The appropriate
values of these parameters, while not unnatural, are poorly constrained
by theory. The calculated Palpha can also be made to fit
the magnitude of E(Gamma) with a reasonable value for the eddy aspect
ratio. Our resutls suggest a possible explanation for the decline of
mode energy with increasing l at fixed nu. Entropy fluctuations couple
to changes in volume associated with the oscillation mode. These
decrease with decreasing n at fixed nu, becoming almost zero for
the f-mode.
Title: G-Modes and the Solar Neutrino Problem
Authors: Bahcall, John N.; Kumar, Pawan
Bibcode: 1993ApJ...409L..73B
Altcode: 1993hep.ph....3229B
We show that low-order g-modes with large enough amplitudes to
affect significantly the solar neutrino fluxes would produce surface
velocities that are $10^4$ times larger than the observed upper
limits and hence are ruled out by existing data. We also demonstrate
that any large-amplitude, short-period oscillations that grow on a
Kelvin-Helmholtz time scale will require, to affect solar neutrino
fluxes, a large amount of energy (for g-modes, $10^9$ times the energy
in the observed $p-$ mode oscillations) and a tiny amount of dissipation
(for g-modes, $10^{-8}$ the dissipation rate of the $p$-modes).
Title: The Location of the Source of High-Frequency Solar Acoustic
Oscillations
Authors: Kumar, Pawan; Lu, Edward
Bibcode: 1991ApJ...375L..35K
Altcode:
Recently Libbrecht and Jefferies et al. have reported regular peaks
in the solar oscillation power spectrum extending well above 5.3 mHz,
the maximum frequency of trapped acoustic modes. Kumar et al. argued
that these peaks are primarily due to the interference of traveling
waves which are excited due to acoustic emission from turbulent
convection. In contrast with the standing wave P-mode frequencies
below 5.3 mHz, the positions of the high-frequency interference
peaks (HIPs) are dependent on the location of the source of the
acoustic oscillations. In the present work, Kumar et al.'s argument is
strengthened, and more importantly, use is made of the above dependence
to determine the acoustic source strength as a function of depth. It
is found that the acoustic source profile, and thus the convective
velocity, is peaked about 200 km deeper than what is expected from
standard mixing length theory.
Title: Thermal and Mechanical Damping of Solar p-Modes
Authors: Goldreich, Peter; Kumar, Pawan
Bibcode: 1991ApJ...374..366G
Altcode:
Nonadiabatic effects associated with the transfer of energy and with
turbulent stresses add small imaginary parts, omega-i(1) and omega-i(2),
to solar p-mode eigenfrequencies. Numerical calculations have shown that
these quite different processes make comparable contributions to omega-i
at frequencies well below the acoustic cutoff at omega-ac. Analytic
expressions are derived which reveal the connection between omega-i(1)
and omega-i(2). The estimates yield omega-i proportional to omega exp 8
for omega much less than omega-ac in good agreement with the numerical
calculations. However, the observed line width is proportional to
omega exp 4.2 at low frequencies. It is suspected that there is an
unmodeled component of perturbed convective energy transport or of
turbulent viscosity that makes an important contribution to omega-i
at omega much less than omega-ac.
Title: Implications of Solar p-Mode Frequency Shifts
Authors: Goldreich, Peter; Murray, Norman; Willette, Gregory; Kumar,
Pawan
Bibcode: 1991ApJ...370..752G
Altcode:
An expression is derived that relates solar p-mode frequency shifts to
changes in the entropy and magnetic field of the sun. The frequency
variations result from changes in path length and propagation
speed. Path length changes dominate for entropy perturbations, and
propagation speed changes dominate for most types of magnetic field
peturbations. The p-mode frequencies increased along with solar activity
between 1986 and 1989; these frequency shifts exhibited a rapid rise
with increasing frequency followed by a precipitous drop. The positive
component of the shifts can be accounted for by variations of the mean
square magnetic field strength in the vicinity of the photosphere. The
magnetic stress perturbation decays above the top of the convection
zone on a length scale comparable to the pressure scale height and
grows gradually with depth below. The presence of a resonance in the
chromospheric cavity means that the transition layer maintains enough
coherence to partially reflect acoustic waves even near cycle maximum.
Title: High Frequency Peaks in the Solar Oscillation Spectrum and
the Determination of the Acoustic Source Depth
Authors: Lu, Edward; Kumar, Pawan
Bibcode: 1991BAAS...23..821L
Altcode:
No abstract at ADS
Title: Oscillation mode excitation.
Authors: Cox, Arthur N.; Chitre, Shashikumar M.; Frandsen, Soren;
Kumar, Pawan
Bibcode: 1991sia..book..618C
Altcode:
The excitation of the oscillation modes in the Sun is very different
from that for the previously known variable stars. A review of the
normal pulsation mechanisms seen in many classes of variable stars shows
that they actually are operating in the Sun. However, most, but not all,
studies of the solar mode excitation predict that radiative damping,
and damping by convective processes also, overwhelm the driving to
stabilize all radial and nonradial modes. This is in accord with the
observations that frequently show measurable widths of the lines in
the p-mode oscillation spectrum. These line widths indicate mode
lifetimes of days for the p-modes. Most calculations predict that
solar g-modes are stable, leading to the question of how they then can
ever be observed. However, there is a possibility that low-degree and
low-order g-modes could be just slightly unstable. Improvements to the
predictions of lowest-order p-mode excitation by the inclusion of better
radiative intensity formulations at the top of the convection zone and
in the photosphere indicate even more mode stability. Calculations that
show how convection may drive solar p-modes are presented. Arguments
about three-mode couplings that are not strongly damping lead to the
conclusion that the solar p-modes are probably stochastically driven
by coupling with convection. Current disagreements among authors
are discussed.
Title: Wave Generation by Turbulent Convection
Authors: Goldreich, Peter; Kumar, Pawan
Bibcode: 1990ApJ...363..694G
Altcode:
Wave generation by turbulent convection in a plane parallel,
stratified atmosphere lying in a gravitational field is studied. The
turbulent spectrum is related to the convective energy flux via the
Kolmogorov scaling and the mixing length hypothesis. Efficiencies for
the conversion of the convective energy flux into both trapped and
propagating waves are estimated.
Title: Nonlinear Interactions among Solar Acoustic Modes
Authors: Kumar, Pawan; Goldreich, Peter
Bibcode: 1989ApJ...342..558K
Altcode:
The rates at which nonlinear interactions transfer energy among
the normal modes of a plane-parallel, stratified atmosphere are
evaluated. It is shown that every p-mode in the 5-minute band is
involved in many near-resonant triplets, and, as a consequence, the
energy transfer rates are independent of the mode line widths. It is
also found that nonlinear mode coupling cannot limit the growth of
overstable p-modes, which favors the hypothesis that the sun's p-modes
are stochastically excited by turbulent convection.
Title: Distribution Functions for the Time-averaged Energies of
Stochastically Excited Solar p-Modes
Authors: Kumar, Pawan; Franklin, Joel; Goldreich, Peter
Bibcode: 1988ApJ...328..879K
Altcode:
The excitation of a damped harmonic oscillator by a random force is
studied as a model for the stochastic excitation of a solar p-mode
by turbulent convection. An extended sequence of observations is
required to separate different p-modes and thus determine the energies
of individual modes. Therefore, the observations yield time-averaged
values of the energy. The theory of random differential equations is
applied to calculate distribution functions for the time-averaged
energy of the oscillator. The instantaneous energy satisfies a
Boltzmann distribution. With increasing averaging time, the distribution
function narrows and its peak shifts toward the mean energy. Numerical
integrations are performed to generate finite sequences of time-averaged
energies. These are treated as simulated data from which approximate
probability distributions for the time-averaged energy are obtained.
Title: The Interaction of Acoustic Radiation with Turbulence
Authors: Goldreich, Peter; Kumar, Pawan
Bibcode: 1988ApJ...326..462G
Altcode:
The authors derive expressions for the spectral emissivity and
absorptivity of acoustic radiation by low Mach number turbulent
fluids. They consider three types of turbulence. The first is free
turbulence, that is, turbulence which is not subject to external
forces. The second and third examples are special cases of forced
turbulence, turbulence maintained by stirring with spoons and turbulent
pseudoconvection. The resulting formulae are used to estimate the
equilibrium energies and quality factors of the acoustic modes in a box
which contains turbulent fluid. The scattering of acoustic radiation
by the turbulent velocity and pressure fluctuations is treated and
the rate at which nonlinear interactions transfer energy among the
acoustic modes is evaluated. This work is a first step in the attempt
to relate the excitation of the Sun's acoustic modes to the turbulence
in the solar convection zone.
Title: Excitation and Damping of Solar P-Modes
Authors: Kumar, Pawan
Bibcode: 1988PhDT.........3K
Altcode: 1988PhDT.......139K
I have carried out detailed analysis of the interaction of acoustic
radiation with homogeneous turbulence in order to understand the
excitation of solar p-modes by turbulent convection. The most
significant outcome of this investigation is the finding that, for
certain types of forced turbulences, the absorption of acoustic
waves is no greater than a free turbulence, whereas the emission
is always enhanced by a factor M^{-2}, where M is the Mach number
of the turbulence. Turbulent convection in the sun is an example of
this kind of turbulence. This leads to the conclusion that energies in
solar p-modes, due to their interaction with the convection, should be
approximately equal to the thermal energy in a resonant eddy. This is
found to be in good agreement with the observations. The ideas developed
in the above work have been applied to explain the recently observed
absorption of acoustic waves by sunspots as well. Work has also been
carried out to determine the probability distribution function for the
time averaged energy of stochastically excited modes. We hope to learn
about the nature of the excitation and damping processes for the solar
modes by comparing this theoretically determined distribution with
the observations. In an effort towards resolving the overstability
question of solar p-modes, I have investigated the effectiveness
of 3-mode couplings, the most plausible process for limiting the
amplitudes of overstable modes. The 3-mode coupling mechanism is also
a good candidate for exciting fundamental modes which are found to
be linearly stable, but are observed to have energies comparable to
p-modes of similar frequencies. The issue of mode stability remains
inconclusive due to the unknown energies of modes with period ~
3.5 minutes. However, we find the fundamental modes to be damped as
a result of mode couplings and hence they require excitation by a
mechanism other than the overstability.