Author name code: rybicki
ADS astronomy entries on 2022-09-14
author:"Rybicki, George"
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Title: ePESSTO Transient Classification Report for 2017-09-11
Authors: Gromadzki; Rybicki; Fraser; Callis; Yaron, O.; Knezevic, N.
Bibcode: 2017TNSCR1536....1G
Altcode:
No abstract at ADS
Title: ePESSTO Transient Classification Report for 2017-09-11
Authors: Gromadzki; Rybicki; Fraser; Callis; Yaron, O.; Knezevic, N.
Bibcode: 2017TNSCR.994....1G
Altcode:
No abstract at ADS
Title: ePESSTO Transient Classification Report for 2017-09-12
Authors: Rybicki; Gromadzki; Fraser; Yaron, O.; Knezevic, N.
Bibcode: 2017TNSCR.997....1R
Altcode:
No abstract at ADS
Title: Modeling Molecular Hyperfine Line Emission
Authors: Keto, Eric; Rybicki, George
Bibcode: 2010ApJ...716.1315K
Altcode: 2010arXiv1004.1617K
In this paper, we discuss two approximate methods previously
suggested for modeling hyperfine spectral line emission for molecules
whose collisional transition rates between hyperfine levels are
unknown. Hyperfine structure is seen in the rotational spectra of
many commonly observed molecules such as HCN, HNC, NH3,
N2H+, and C17O. The intensities
of these spectral lines can be modeled by numerical techniques such
as Λ-iteration that alternately solve the equations of statistical
equilibrium and the equation of radiative transfer. However, these
calculations require knowledge of both the radiative and collisional
rates for all transitions. For most commonly observed radio frequency
spectral lines, only the net collisional rates between rotational
levels are known. For such cases, two approximate methods have been
suggested. The first method, hyperfine statistical equilibrium,
distributes the hyperfine level populations according to their
statistical weight, but allows the population of the rotational
states to depart from local thermal equilibrium (LTE). The second
method, the proportional method, approximates the collision rates
between the hyperfine levels as fractions of the net rotational
rates apportioned according to the statistical degeneracy of the
final hyperfine levels. The second method is able to model non-LTE
hyperfine emission. We compare simulations of N2H+
hyperfine lines made with approximate and more exact rates and find
that satisfactory results are obtained.
Title: Thermal X-Rays from Millisecond Pulsars: Constraining the
Fundamental Properties of Neutron Stars
Authors: Bogdanov, Slavko; Grindlay, Jonathan E.; Rybicki, George B.
Bibcode: 2008ApJ...689..407B
Altcode: 2008arXiv0801.4030B
We model the X-ray properties of millisecond pulsars (MSPs) by
considering hot-spot emission from a weakly magnetized neutron star
(NS) covered by a hydrogen atmosphere. We investigate the limitations
of using the thermal X-ray pulse profiles of MSPs to constrain the
mass-to-radius (M/R) ratio of the NS. The accuracy is strongly dependent
on the viewing angle and magnetic inclination, but is ultimately
limited only by photon statistics. We demonstrate that valuable
information regarding NSs can be extracted, even from data of fairly
limited photon statistics through modeling of archival observations of
the nearby isolated PSRs J0030+0451 and J2124-3358. The X-ray emission
from these pulsars is consistent with the presence of an atmosphere and
a dipolar field configuration. For both MSPs, the favorable geometry
allows us to place limits on the allowed M/R of NSs. Assuming 1.4
M⊙, the stellar radius is constrained to be R > 9.4 km
and R > 7.8 km (68% confidence) for PSRs J0030+0451 and J2124-3358,
respectively. We explore the prospects of using future observatories
such as Constellation-X and XEUS to conduct X-ray-timing searches for
MSPs not detectable at radio wavelengths due to unfavorable viewing
geometry. We are also able to place strong constraints on the magnetic
field evolution model proposed by Ruderman. The pulse profiles indicate
that the magnetic field of an MSP does not have a tendency to align
itself with the spin axis or migrate toward one of the spin poles
during the low-mass X-ray binary phase.
Title: Neutron Star Structure Constraints from Low-Resolution X-ray
Spectroscopy
Authors: Heinke, Craig; Rybicki, G.; Grindlay, J.; Narayan, R.;
Wiknands, R.; Jonker, P.; Deloye, C.; Taam, R.
Bibcode: 2008xru..confE..85H
Altcode:
No abstract at ADS
Title: Constraints on Neutron Star Properties from X-Ray Observations
of Millisecond Pulsars
Authors: Bogdanov, Slavko; Rybicki, George B.; Grindlay, Jonathan E.
Bibcode: 2007ApJ...670..668B
Altcode: 2006astro.ph.12791B
We present a model of thermal X-ray emission from hot spots on the
surface of a rotating compact star with an unmagnetized light-element
atmosphere. An application to ROSAT, Chandra, and XMM-Newton X-ray
observations of the nearest known rotation-powered millisecond pulsar
(MSP) PSR J0437-4715 reveals that the thermal emission from this
pulsar is fully consistent with such a model, enabling constraints on
important properties of the underlying neutron star. We confirm that
the observed thermal X-ray pulsations from J0437-4715 are incompatible
with blackbody emission and require the presence of an optically thick,
light-element (most likely hydrogen) atmosphere on the neutron star
surface. The morphology of the X-ray pulse profile is consistent
with a global dipole configuration of the pulsar magnetic field but
suggests an off-center magnetic axis, with a displacement of 0.8-3 km
from the stellar center. For an assumed mass of 1.4 Msolar,
the model restricts the allowed stellar radii to R=6.8-13.8 km (90%
confidence) and R>6.7 km (99.9% confidence), which is consistent
with standard NS equations of state and rules out an ultracompact
star smaller than its photon sphere. Deeper spectroscopic and timing
observations of this and other nearby radio MSPs with current and
future X-ray facilities (Constellation-X and XEUS) can provide further
insight into the fundamental properties of neutron stars.
Title: Comptonization in the X-ray Spectra of Radio Millisecond
Pulsars
Authors: Bogdanov, Slavko; Grindlay, Jonathan E.; Rybicki, George B.
Bibcode: 2007whsn.conf....5B
Altcode: 2006astro.ph.12591B
The majority of X-ray-detected rotation-powered millisecond pulsars
(MSPs) appear to exhibit predominantly thermal emission, believed to
originate from the heated magnetic polar caps of the pulsar. In the
nearest MSP, J0437--4715 a faint PL is also observed at >3 keV,
usually associated with magnetospheric emission processes. However,
the hard emission in this and other similar MSPs may instead be due
to weak Comptonization of the thermal polar cap emission by energetic
electrons/positrons of small optical depth most likely in the pulsar
magnetosphere. This spectral model implies that all soft X-rays are of
purely thermal origin, which has important implications in the study
of neutron stars.
Title: X-Rays from Radio Millisecond Pulsars: Comptonized Thermal
Radiation
Authors: Bogdanov, Slavko; Grindlay, Jonathan E.; Rybicki, George B.
Bibcode: 2006ApJ...648L..55B
Altcode: 2006astro.ph..5273B
X-ray emission from many rotation-powered millisecond pulsars (MSPs)
is observed to be of predominantly thermal nature. In PSR J0437-4715,
the nearest MSP known, an additional faint power-law tail is observed
above 2.5 keV, commonly attributed to nonthermal magnetospheric
radiation. We propose that the hard emission in this and other
similar MSPs is instead due to weak Comptonization of the thermal
(blackbody or hydrogen atmosphere) polar cap emission by energetic
electrons/positrons of small optical depth presumably in the pulsar
magnetosphere and wind. This spectral model implies that all soft
X-rays are of purely thermal origin, which has profound implications
in the study of neutron star structure and fundamental pulsar physics.
Title: Improved Fokker-Planck Equation for Resonance-Line Scattering
Authors: Rybicki, George B.
Bibcode: 2006ApJ...647..709R
Altcode: 2006astro.ph..3047R
A new Fokker-Planck equation is developed for treating resonance-line
scattering, which is especially relevant to the treatment of Lyα in the
early universe. It is a ``corrected'' form of the equation of Rybicki
& Dell'Antonio that now obeys detailed balance, so the approach
to thermal equilibrium is properly described. The new equation takes
into account the energy changes due to scattering off moving particles,
the recoil term of Basko, and stimulated scattering. One result is a
surprising unification of the equation for resonance-line scattering
and the Kompaneets equation. An improved energy exchange formula due
to resonance-line scattering is derived. This formula is compared to
previous formulas of Madau and coworkers and Chen & Miralda-Escudé.
Title: A Hydrogen Atmosphere Spectral Model Applied to the Neutron
Star X7 in the Globular Cluster 47 Tucanae
Authors: Heinke, Craig O.; Rybicki, George B.; Narayan, Ramesh;
Grindlay, Jonathan E.
Bibcode: 2006ApJ...644.1090H
Altcode: 2005astro.ph..6563R; 2005astro.ph..6563H
Current X-ray missions are providing high-quality X-ray spectra from
neutron stars (NSs) in quiescent low-mass X-ray binaries (qLMXBs). This
has motivated us to calculate new hydrogen atmosphere models,
including opacity due to free-free absorption and Thomson scattering,
thermal electron conduction, and self-irradiation by photons from the
compact object. We have constructed a self-consistent grid of neutron
star models covering a wide range of surface gravities, as well as
effective temperatures, which we make available to the scientific
community. We present multiepoch Chandra X-ray observations of the qLMXB
X7 in the globular cluster 47 Tuc, which is remarkably nonvariable on
timescales from minutes to years. Its high-quality X-ray spectrum is
adequately fitted by our hydrogen atmosphere model without any hard
power-law component or narrow spectral features. If a mass of 1.4
Msolar is assumed, our spectral fits require that its radius
be in the range Rns=14.5+1.8-1.6 km
(90% confidence), which is larger than that expected from currently
preferred models of NS interiors. If its radius is assumed to be 10
km, then a mass of Mns=2.20+0.03-0.16
Msolar is required. Using models with the appropriate surface
gravity for each value of the mass and radius becomes important for
interpretation of the highest quality data.
Title: On the Lack of Thermal Emission from the Quiescent Black Hole
XTE J1118+480: Evidence for the Event Horizon
Authors: McClintock, Jeffrey E.; Narayan, Ramesh; Rybicki, George B.
Bibcode: 2004ApJ...615..402M
Altcode: 2004astro.ph..3251M
A soft component of thermal emission is very commonly observed
from the surfaces of quiescent, accreting neutron stars. We
searched with Chandra for such a surface component of emission
from the dynamical black hole candidate XTE J1118+480 (=J1118),
which has a primary mass M1~8 Msolar. None
was found, as one would expect if the compact X-ray source is a
bona fide black hole that possesses an event horizon. The spectrum
of J1118 is well fitted by a simple power-law model that implies
an unabsorbed luminosity of LX~3.5×1030
ergs s-1 (0.3-7 keV). In our search for a thermal
component, we fitted our Chandra data to a power-law model (with
slope and NH fixed) plus a series of nine hydrogen
atmosphere models with radii ranging from 9/8 to 2.8 Schwarzschild
radii. For the more compact models, we included the important effect
of self-irradiation of the atmosphere. Because of the remarkably
low column density to J1118, NH~1.2×1020
cm-2, we obtained very strong limits on a
hypothetical thermal source: kT∞<0.011 keV and
L∞,th<9.4×1030 ergs s-1 (99%
confidence level). In analogy with neutron stars, there are two possible
sources of thermal radiation from a hypothetical surface of J1118: deep
crustal heating and accretion. The former mechanism predicts a thermal
luminosity that exceeds the above luminosity limit by a factor of
>~25, which implies that either one must resort to contrived models
or, as we favor, J1118 is a true black hole with an event horizon. In
addition to neutron stars, we also consider emission from several
exotic models of compact stars that have been proposed as alternatives
to black holes. As we have shown previously, accreting black holes in
quiescent X-ray binaries are very much fainter than neutron stars. One
potential explanation for this difference is the larger and hence
cooler surface of an 8 Msolar compact object that might
be masked by the interstellar medium. However, our upper limit on the
total luminosity of J1118 of 1.3×1031 ergs s-1
is far below the luminosities observed for neutron stars. This result
strengthens our long-held position that black holes are faint relative
to neutron stars because they possess an event horizon.
Title: Radiative Transfer and Starless Cores
Authors: Keto, Eric; Rybicki, George B.; Bergin, Edwin A.; Plume, René
Bibcode: 2004ApJ...613..355K
Altcode: 2004astro.ph..7433K
We develop a method of analyzing radio-frequency spectral line
observations to derive data on the temperature, density, velocity,
and molecular abundance of the emitting gas. The method incorporates a
radiative transfer code with a new technique for handling overlapping
hyperfine emission lines within the accelerated Λ-iteration algorithm
and a heuristic search algorithm based on simulated annealing. We
apply this method to new observations of N2H+
in three Lynds clouds thought to be starless cores in the first stages
of star formation and determine their density structure. A comparison
of the gas densities derived from the molecular line emission and
the millimeter dust emission suggests that the required dust mass
opacity is about κ1.3mm=0.04 cm2 g-1,
consistent with models of dust grains that have opacities enhanced by
ice mantles and fluffy aggregrates.
Title: Bondi Accretion and the Problem of the Missing Isolated
Neutron Stars
Authors: Perna, Rosalba; Narayan, Ramesh; Rybicki, George; Stella,
Luigi; Treves, Aldo
Bibcode: 2003ApJ...594..936P
Altcode: 2003astro.ph..5421P
A large number of neutron stars (NSs), ~109, populate the
Galaxy, but only a tiny fraction of them is observable during the short
radio pulsar lifetime. The majority of these isolated NSs, too cold
to be detectable by their own thermal emission, should be visible in
X-rays as a result of accretion from the interstellar medium. The ROSAT
All-Sky Survey has, however, shown that such accreting isolated NSs
are very elusive: only a few tentative candidates have been identified,
contrary to theoretical predictions that up to several thousand should
be seen. We suggest that the fundamental reason for this discrepancy
lies in the use of the standard Bondi formula to estimate the accretion
rates. We compute the expected source counts using updated estimates of
the pulsar velocity distribution, realistic hydrogen atmosphere spectra,
and a modified expression for the Bondi accretion rate, as suggested by
recent MHD simulations and supported by direct observations in the case
of accretion around supermassive black holes in nearby galaxies and in
our own. We find that, whereas the inclusion of atmospheric spectra
partly compensates for the reduction in the counts due to the higher
mean velocities of the new distribution, the modified Bondi formula
dramatically suppresses the source counts. The new predictions are
consistent with a null detection at the ROSAT sensitivity.
Title: A New Kinetic Equation for Compton Scattering
Authors: Rybicki, George B.
Bibcode: 2003ApJ...584..528R
Altcode: 2002astro.ph..8542R
A kinetic equation for Compton scattering is given that differs from the
Kompaneets equation in several significant ways. By using an inverse
differential operator, this equation allows treatment of problems for
which the radiation field varies rapidly on the scale of the width
of the Compton kernel. This inverse operator method describes, among
other effects, the thermal Doppler broadening of spectral lines and
continuum edges and automatically incorporates the process of Compton
heating/cooling. It is well adapted for inclusion into a numerical
iterative solution of radiative transfer problems. The equivalent kernel
of the new method is shown to be a positive function and with reasonable
accuracy near the initial frequency, unlike the Kompaneets kernel,
which is singular and not wholly positive. It is shown that iterations
of the inverse operator kernel can be easily calculated numerically,
and a simple summation formula over these iterations is derived that
can be efficiently used to compute Comptonized spectra. It is shown
that the new method can be used for initial-value and other problems
with no more numerical effort than the Kompaneets equation and that
it more correctly describes the solution over times comparable to the
mean scattering time.
Title: The Structure of Self-gravitating Polytropic Systems with n
around 5
Authors: Medvedev, Mikhail V.; Rybicki, George
Bibcode: 2001ApJ...555..863M
Altcode: 2000astro.ph.10621M
We investigate the structure of self-gravitating polytropic stellar
systems. We present a method that allows us to obtain approximate
analytical solutions, ψn+ɛ(x) of the nonlinear
Poisson equation with the polytropic index n+ɛ, given the solution
ψn(x) with the polytropic index n, for any positive or
negative ɛ, such that |ɛ|<<1. A similar technique has been
developed independently by Seidov & Kuzakhmedov. Application of
this method to the spherically symmetric stellar polytropes with n~=5
yields the solutions that describe spatially bound systems, if n<5,
and the formation of a second core, if n>5. A heuristic approximate
expression for the radial profile is also presented. Because of the
duality between stellar and gas polytropes, our results are valid for
gaseous, self-gravitating, polytropic systems (e.g., molecular clouds)
with index γ~=6/5. The stability of such systems and observational
consequences for both stellar and gaseous systems are discussed.
Title: Scattered Lyα Radiation around Sources before Cosmological
Reionization
Authors: Loeb, Abraham; Rybicki, George B.
Bibcode: 1999ApJ...524..527L
Altcode: 1999astro.ph..2180L
The spectra of the first galaxies and quasars in the universe should
be strongly absorbed shortward of their rest-frame Lyα wavelength by
neutral hydrogen (H I) in the intervening intergalactic medium. However,
the Lyα line photons emitted by these sources are not eliminated but
rather scatter until they redshift out of resonance and escape owing to
the Hubble expansion of the surrounding intergalactic H I. We calculate
the resulting brightness distribution and the spectral shape of the
diffuse Lyα line emission around high-redshift sources before the
intergalactic medium was reionized. Typically, the Lyα photons emitted
by a source at zs~10 scatter over a characteristic angular
radius of ~15" around the source and compose a line that is broadened
and redshifted by ~103 km s-1 relative to the
source. The scattered photons are highly polarized. Detection of the
diffuse Lyα halos around high-redshift sources would provide a unique
tool for probing the neutral intergalactic medium before the epoch of
reionization. On sufficiently large scales, where the Hubble flow is
smooth and the gas is neutral, the Lyα brightness distribution can be
used to determine the cosmological mass densities of baryons and matter.
Title: Polarization of the Lyα Halos around Sources before
Cosmological Reionization
Authors: Rybicki, George B.; Loeb, Abraham
Bibcode: 1999ApJ...520L..79R
Altcode: 1999astro.ph..3291R
In Loeb & Rybicki (hereafter Paper I), it was shown that before
reionization, the scattering of Lyα photons from a cosmological
source might lead to a fairly compact (~15") Lyα halo around the
source. Observations of such halos could constrain the properties of the
neutral intergalactic medium and, in particular, yield the cosmological
density parameters of baryons and matter on scales where the Hubble
flow is unperturbed. Paper I did not treat the polarization of this
scattered radiation but did suggest that the degree of such polarization
might be large. In this Letter, we report on improved calculations for
these Lyα halos, now accounting for the polarization of the radiation
field. The polarization is linear and is oriented tangentially to
the projected displacement from the center of the source. The degree
of polarization is found to be 14% at the core radius, where the
intensity has fallen to half of the central value. It rises to 32%
and 45% at the radii where the intensity has fallen to one-tenth and
one-hundredth of the central intensity, respectively. At larger radii,
the degree of polarization rises further, asymptotically to 60%. Such
high values of polarization should be easily observable and provide
a clear signature of the phenomenon of Lyα halos surrounding sources
prior to reionization.
Title: Radiative Transfer
Authors: Rybicki, George B.
Bibcode: 1999fwdb.book...45R
Altcode:
Chandrasekhar's work in radiative transfer theory began in 1944 and
culminated with the publication of his influential treatise Radiative
Transfer in 1950. In this review his major contributions to radiative
transfer will be recounted and evaluated. These include his development
of the discrete ordinates method, the invariance principles, and his
formulation and solution of the transfer equation for polarized light.
Title: Magnification Ratio of the Fluctuating Light in Gravitational
Lens 0957+561
Authors: Press, William H.; Rybicki, George B.
Bibcode: 1998ApJ...507..108P
Altcode: 1998astro.ph..3193P
Radio observations establish the B/A magnification ratio of
gravitational lens 0957+561 at about 0.75. Yet, for more than 15
years, the optical magnification ratio has been between 0.9 and
1.12. The accepted explanation is microlensing of the optical
source. However, this explanation is mildly discordant with (1)
the relative constancy of the optical ratio and (2) recent data
indicating possible nonachromaticity in the ratio. To study these
issues, we develop a statistical formalism for separately measuring,
in a unified manner, the magnification ratio of the fluctuating and
constant parts of the light curve. Applying the formalism to the
recently published data of Kundić and coworkers, we find that the
magnification ratios of fluctuating parts in both the g and r colors
agrees with the magnification ratio of the constant part in g band,
and tends to disagree with the r-band value. One explanation could be
about 0.1 mag of consistently unsubtracted r light from the lensing
galaxy G1, which seems unlikely. Another could be that 0957+561 is
approaching a caustic in the microlensing pattern.
Title: Spectral line signatures of relativistic disks
Authors: Rybicki, George B.; Bromley, Benjamin C.
Bibcode: 1998AIPC..431..265R
Altcode: 1998apas.conf..265R
We consider emission line formation in a thin accretion disk around a
black hole, taking into account the differential flow of material in
the disk. If the disk is optically thick in the line, local velocity
gradients can cause the integrated intensity to have azimuthal
dependence in the emitter frame. We examine this effect with simple
parameterized models based on Sobolev theory to highlight the overall
character of the changes in the observed line profile. The shape of
the profile, which can serve as a diagnostic of the disk geometry
and the spin of the black hole, may be significantly altered by
the velocity-gradient effect. Specifically, we find that the effect
causes a decrease of flux in the blue Doppler peak, which in turn
would lead to an underestimate of the inner disk radius. If the inner
radius were used as a signature of black hole rotation, as when the
disk is not emissive within the marginally stable circular orbit,
then the inferred rotation would be overestimated in cases where the
emissivity of the disk has fairly shallow fall-off with radius. If the
disk were emissive even within the marginally stable orbit, then the
local azimuthal anisotropy of emission could produce features in the
line profile which distinguish rotating from nonrotating black holes.
Title: Emission Line Formation in a Relativistic Accretion Disk
Authors: Rybicki, George B.; Bromley, Benjamin C.
Bibcode: 1997astro.ph.11104R
Altcode:
The observed profile of spectral lines from a relativistic accretion
disk can constrain parameters such as the disk geometry and the rotation
of the central black hole. The formation of the spectral line in a
disk generally has been modeled with simple assumptions such as local
isotropy of emission. Here we consider line formation in the presence
of velocity gradients induced by the differential flow in the disk. In
this case the emission can have anisotropy in the form of an azimuthal
dependence relative to the local principle axes of shear. Since the
physical conditions in a disk are uncertain in detail, we investigate
this effect with simple parameterized models based on Sobolev theory to
highlight the overall character of the changes in the line profile. We
find that velocity gradients generally cause a relative increase of
flux in the red wing, hence the inner radius of the disk would be
underestimated if the effect were not taken into consideration. If the
inner radius is used as a signature of black hole rotation, as when
the disk is not emissive within the marginally stable circular orbit,
then the inferred rotation would be overestimated in cases where the
emissivity of the disk has fairly shallow fall-off with radius. If the
disk were emissive even within the marginally stable orbit, then the
local azimuthal anisotropy of emission produces features in the line
profile which distinguish rotating from nonrotating black holes.
Title: Properties of Statistical Equilibrium Equations: Positivity
and Uniqueness
Authors: Rybicki, George B.
Bibcode: 1997ApJ...479..357R
Altcode:
While linear statistical equilibrium equations play an important role
in the description of nonequilibrium processes in astrophysics, some of
their basic mathematical properties, such as uniqueness and positivity,
have not been fully explored. In this paper these properties are related
to concepts of connectivity from the theory of continuous-time Markov
chains. For the irreducible case (in which every state is connected to
every other state, either directly or through intermediate states),
the solution is shown to be positive and unique when one positive
normalization condition is provided. It is then shown how a general
linear statistical equilibrium problem can be reduced by dividing the
system into inessential and essential states and then partitioning
the latter into separate irreducible subproblems. It is shown that:
(1) The inessential states all have zero populations. If a positive
normalization condition is imposed separately on each irreducible
subproblem, then (2) the essential states all have positive populations
and (3) the overall solution is unique.
Title: Desperately Seeking Non-Gaussianity: The Light Curve of
0957+561
Authors: Press, W. H.; Rybicki, G. B.
Bibcode: 1997ASSL..218...61P
Altcode: 1997ats..proc...61P
Non-Gaussianity, or its close relative non-stationarity, is frequently
invoked as the explanation for a wide variety of anomalies in time
series astronomical data. For the case of the gravitational lens
0957+561, recent high quality data by Kundic et al. invite us to ask,
in a quantitative fashion, whether or not any signs of non-Gaussianity
are in evidence. Applying theoretical analysis, we first show that,
for a wide range of plausible models, non-Gaussianity may be exceedingly
hard to detect even if it is grossly present in the underlying physics
of the quasar accretion disk. We show, in particular, that for any
chance of success, the chosen detection statistic must be finely tuned
to both the statistical properties of the quasar, and to the noise
model of the data. We give examples of such statistics. We then give
results from analyzing the available data.
Title: Radiative Transfer
Authors: Rybicki, George B.
Bibcode: 1996JApA...17...95R
Altcode:
Chandrasekhar's work in radiative transfer theory began in 1944 and
culminated with the publication of his influential treatise Radiative
Transfer in 1950. In this review his major contributions to radiative
transfer will be recounted and evaluated. These include his development
of the discrete ordinates method, the invariance principles, and his
formulation and solution of the transfer equation for polarized light.
Title: Deprojection of axially symmetric objects
Authors: Kochanek, Christopher S.; Rybicki, George B.
Bibcode: 1996MNRAS.280.1257K
Altcode: 1995astro.ph.10076K
The deprojection of axisymmetric density distributions is generally
indeterminate to within the addition of certain axisymmetric
distributions (konus densities) that are invisible in projection. The
known class of konus densities is expanded considerably here through
the introduction of semikonus functions. These functions are closed
with respect to multiplication in ordinary space, and the real part of
an arbitrary polynomial of semikonus functions is a konus function. This
property facilitates the construction of semikonus (and konus) functions
with tailored properties, such as asymptotic forms. We also develop a
simple technique for constructing several classes of konus distributions
with arbitrary density profiles in the equatorial plane.
Title: A Distant Photosphere: CBR Spectral Distortions from the
Recombination Epoch
Authors: Rybicki, G. B.; dell'Antonio, I. P.
Bibcode: 1996AAS...188.3202R
Altcode: 1996BAAS...28..866R
The spectral distortions introduced during the hydrogen recombination
epoch in the early universe are features of the CBR that could,
in principle, determine a host of cosmological parameters, such as
Omega , Omega_B , and H_0. Previous calculations have indicated
that such distortions will extremely small due the small ratio
of baryons to photons in the universe. However, because of their
potential importance, it seems worthwhile to do a more complete
calculation of the distortions, taking special care to include any
physical effects that might affect their magnitude. Such a calculation
provides an instructive example of how classical techniques of stellar
atmosphere theory can be applied to significantly different regimes of
physical parameters and boundary conditions. For example, here atomic
transition rates are almost completely dominated by thermal radiative
processes induced by the CBR itself, and the ``photosphere'' exists
more in time than in space. Several improvements in the calculation
will be described. An new formalism is developed for determining the
spectral distortions due to the hydrogen lines, which is based on
a perturbation expansion of the excited level populations away from
their thermodynamic equilibrium values. The temperature difference
between the radiation and electrons is taken into account. A new type
of visibility function is introduced that describes where photons were
originally generated by true absorption processes, rather than where
they were last Thomson scattered. Improved values for the relic ionized
fraction, matter temperature, and the visibility function are found
for a range of cosmological parameters. Despite these improvements, the
spectral deviations found were not significantly different from previous
estimates, and are still several orders of magnitude below the expected
backgrounds, well below detectability for the forseeable future.
Title: Class of Fast Methods for Processing Irregularly Sampled or
Otherwise InhomogeneousOne-Dimensional Data
Authors: Rybicki, George B.; Press, William H.
Bibcode: 1995PhRvL..74.1060R
Altcode: 1994comp.gas..5004R
With the ansatz that a data set's correlation matrix has a certain
parametrized form (one general enough, however, to allow the arbitrary
specification of a slowly varying decorrelation distance and population
variance), the general machinery of Wiener or optimal filtering can
be reduced from O\(n3\) to O\(n\) operations, where n is
the size of the data set. The implied vast increase in computational
speed can allow many common suboptimal or heuristic data analysis
methods to be replaced by fast, relatively sophisticated, statistical
algorithms. Three examples are given: data rectification, high- or
low-pass filtering, and linear least-squares fitting to a model with
unaligned data points.
Title: An accelerated lambda iteration method for multilevel radiative
transfer. III. Noncoherent electron scattering
Authors: Rybicki, G. B.; Hummer, D. G.
Bibcode: 1994A&A...290..553R
Altcode: 1994astro.ph..4019R
Since the mass of the electron is very small relative to atomic masses,
Thomson scattering of low-energy photons (hν<<m_e_c^2^) produces
thermal Doppler frequency shifts that are much larger than atomic
Doppler widths. A method is developed here to evaluate the electron
scattering emissivity from a given radiation field which is considerably
faster than previous methods based on straightforward evaluation of the
scattering integral. This procedure is implemented in our multilevel
radiative code (MALI), which now takes full account of the effects of
noncoherent electron scattering on level populations, as well as on
the emergent spectrum. Calculations using model atmospheres of hot,
low-gravity stars display not only the expected broad wings of strong
emission lines but also effects arising from the scattering of photons
across continuum edges. In extreme cases this leads to significant
shifts of the ionization equilibrium of helium.
Title: The Time Development of a Resonance Line in the Expanding
Universe
Authors: Rybicki, George B.; dell'Antonio, Ian P.
Bibcode: 1994ApJ...427..603R
Altcode: 1993astro.ph.12006R
The time-dependent spectral profile of a resonance line in a
homogeneous expanding medium is studied by numerically solving an
improved Fokker-Planck diffusion equation. The solutions are used to
determine the time required to reach a quasi-static solution near the
line center. A simple scaling law for this relaxation time is derived
and is fitted to the numerical results. The results are applied to
the case of Lyman alpha scattering during primordial recombination of
hydrogen. For a wide range of cosmological models it is found that
the relaxation times are smaller than the recombination timescale,
although not by a very large factor. Thus the standard assumption of
a quasi-static solution in cosmological recombination calculations
is reasonably valid, and should not cause substantial errors in the
solutions.
Title: Performance, Defect Behavior and Carrier Enhancement in Low
Energy Proton Irradiated p+nn+ InP Solar Cells
Authors: Weinberg, I.; Rybicki, G. C.; Vargas-Aburto, C.; Jain, R. K.;
Scheiman, D.
Bibcode: 1994NASCP3278..149W
Altcode:
No abstract at ADS
Title: Study of Reverberation Lags and Spreads in NGC 5548
Authors: Rybicki, George B.; Kleyna, Jan T.
Bibcode: 1994ASPC...69...85R
Altcode: 1994rmbl.conf...85R
No abstract at ADS
Title: Properties of High-Redshift Lyman-Alpha Clouds. II. Statistical
Properties of the Clouds
Authors: Press, William H.; Rybicki, George B.
Bibcode: 1993ApJ...418..585P
Altcode: 1993astro.ph..3017P
Curve-of-growth analysis, applied to the Lyman series absorption ratios
deduced in our previous paper, yields a measurement of the logarithmic
slope of distribution of Lyman-α clouds in column density N. The
observed exponential distribution of the clouds' equivalent widths W
is then shown to require a broad distribution of velocity parameters b,
extending up to 80 km s-1. We show how the exponential itself
emerges in a natural way. An absolute normalization for the differential
distribution of cloud numbers in Z, N, and b is obtained. By detailed
analysis of absorption fluctuations along the line of sight (including
correlations among neighboring spectral frequency bins) we are able
to put upper limits on the cloud-cloud correlation function ζ on
several megaparsec length scales. We show that observed b values,
if thermal, are incompatible, in several different ways, with the
hypothesis of equilibrium heating and ionization by a background UV
flux. Either a significant component of b is due to bulk motion (which
we argue against on several grounds), or else the clouds are out of
equilibrium, and hotter than is implied by their ionization state,
a situation which could be indicative of recent adiabatic collapse.
Title: Properties of High-Redshift Lyman-Alpha Clouds. I. Statistical
Analysis of the Schneider-Schmidt-Gunn Quasars
Authors: Press, William H.; Rybicki, George B.; Schneider, Donald P.
Bibcode: 1993ApJ...414...64P
Altcode: 1993astro.ph..3016P
Techniques for statistical analysis of the Lyman-α forest in
high-redshift quasars are developed, and applied to the low-resolution
(25 A) spectra of 29 of the 33 quasars in the Schneider-Schmidt-Gunn
(SSG) sample. We extrapolate each quasar's continuum shortward of
Lyman-α emission, then consider each spectral bin of each quasar
to be an (approximately) independent measurement of the absorption
due to the Lyman-α clouds. With several thousand such measurements
thus available, we can obtain good determinations of some interesting
properties of clouds in the redshift range 2.5 <z <4.3 without
actually resolving any single cloud. We find that the mean absorption
increases with z approximately as a power law (1 + Z)gamma
+ 1^ with γ = 2.46 +/- 0.37. The mean ratio of Lyman-α to Lyman-β
absorption in the clouds is 0.476 +/- 0.054. We also detect, and
obtain ratios, for Lyman-γ, δ, and possibly ɛ. We are also able to
quantify the fluctuations of the absorption around its mean, and find
that these are comparable to, or perhaps slightly larger than, that
expected from an uncorrelated distribution of clouds. The techniques
in this paper, which include the use of bootstrap resampling of the
quasar sample to obtain estimated errors and error covariances, and
a mathematical treatment of absorption from a (possibly nonuniform)
stochastic distribution of lines, should be applicable to future,
more extensive, data sets.
Title: A perturbation particle method for stability studies of
stellar systems
Authors: Wachlin, Felipe C.; Rybicki, George B.; Muzzio, Juan C.
Bibcode: 1993MNRAS.262.1007W
Altcode:
We discuss a numerical method for investigating the stability of stellar
systems that uses an analytic distribution function to describe a
stellar system in equilibrium and 'perturbation particles' to represent
departures from that equilibrium state. The particles are used only
to represent the perturbation, and statistical fluctuations due to
the finite number of particles are therefore much less severe than
in full N-body codes. We provide a general description of the method,
recipes for particular aspects of its implementation, and an example
of its application to a simple model with known analytical solution.
Title: Spectral Distortions in the CMB from Recombination.
Authors: Rybicki, G. B.; dell'Antonio, I. P.
Bibcode: 1993ASPC...51..548R
Altcode: 1993obco.symp..548R
No abstract at ADS
Title: Interpolation, Realization, and Reconstruction of Noisy,
Irregularly Sampled Data
Authors: Rybicki, George B.; Press, William H.
Bibcode: 1992ApJ...398..169R
Altcode:
Various statistical procedures related to linear prediction and
optimal filtering are developed for general, irregularly sampled, data
sets. The data set may be a function of time, a spatial sample, or an
unordered set. In the case of time series, the underlying process may
be low-frequency divergent (weakly nonstationary). Explicit formulas
are given for (i) maximum likelihood reconstruction (interpolation)
with estimation of uncertainties, (ii) reconstruction by unbiased
estimators (Gauss-Markov), (iii) unconstrained Monte Carlo realization
of the underlying process, (iv) Monte Carlo realizations constrained
by measured data, and (v) simultaneous reconstruction and determination
of unknown linear parameters.
Title: An accelerated lambda iteration method for multilevel radiative
transfer. II. Overlapping transitions with full continuum.
Authors: Rybicki, G. B.; Hummer, D. G.
Bibcode: 1992A&A...262..209R
Altcode:
The ALI method has been generalized to include treatment of overlapping,
active continuum transfer and overlapping lines. The linearity of
the iterative equations is maintained in this method by consistent
use of the psi operator rather than the lambda operator. The method
has been applied to a sample problem of pure helium, which includes 23
levels, 31 lines, 22 continua, three stages of ionization, and electron
scattering. Velocity fields of order of a few Doppler widths were also
included by means of an observer's frame formulation. The convergence
of the solutions was found to be comparable to that achieved in the
previous pure line method.
Title: The Sobolev Approximation for Line Formation with Partial
Frequency Redistribution
Authors: Hummer, D. G.; Rybicki, G. B.
Bibcode: 1992ApJ...387..248H
Altcode:
Attention is given to the formation of a spectral line in a uniformly
expanding infinite medium in the Sobolev approximation, with emphasis
on the various mechanisms for frequency redistribution. Numerical and
analytic solutions of the transfer equation are presented of a number
of redistribution functions and their approximations, including type I
and type II partial redistribution, coherent scattering and complete
redistribution, and the Fokker-Planck and uncorrelated approximation
to the RII function. The solutions for the mean intensity
are shown to depend very much on the type of redistribution mechanism,
while for the frequency-weighted mean intensity, which enters the
rate equations, this dependence is weak. It is inferred that use of
Sobolev escape probabilities based on complete redistribution can be
an adequate approximation for many calculations for which only the
radiative excitation rates are needed.
Title: The Time Delay of Gravitational Lens 0957+561. II. Analysis
of Radio Data and Combined Optical-Radio Analysis
Authors: Press, William H.; Rybicki, George B.; Hewitt, Jacqueline N.
Bibcode: 1992ApJ...385..416P
Altcode:
The method for determining the time delay of gravitational lens
0957+561, which we previously applied to published optical data, is here
extended and applied to the radio data of Lehar et al. The radio data
are relatively free of the windowing effects of seasonal observing. For
the radio data alone, our analysis gives the result τAB =
548+19-16 days (95% C.L.). Combined analysis of
the optical and radio data yield τAB = 540±12 days (95%
C.L.). We have also fitted for a linear drift between the two components
(possibly an indication of microlensing). The radio data are consistent
with zero drift (as expected theoretically), while the optical data
show a small but marginally significant drift at the 2.4 σ level.
Title: The Time Delay of Gravitational Lens 0957+561. I. Methodology
and Analysis of Optical Photometric Data
Authors: Press, William H.; Rybicki, George B.; Hewitt, Jacqueline N.
Bibcode: 1992ApJ...385..404P
Altcode:
When a data set's underlying covariance function (autocorrelation)
can be estimated, one can construct from the data set a global X^2^
statistic that takes into account the expected statistical relationship
between all pairs of points. Incorporating a projection operator,
the statistic is independent of the underlying process mean and
total variance, which are poorly determined (or ill-defined) for
low-frequency divergent signals. Minimizing this X^2^ gives (1) an
optimal reconstruction of the underlying signal, (2) standard errors
on that reconstruction, and (3) an optimal determination of additional
model parameters, such as the time delay and magnitude differences of
two different sets of observations. Applying this methodology to the
gravitational lens 0957+561, and using previously published optical
data, we obtain the value 536^+14^_-12_ days (95% confidence interval)
for the delay, consistent with the less precise radio value of Lehar et
al., but inconsistent with previous optical determinations, including
those using the same optical data. We find that the existence of a
time delay is highly significant (1% level). Monte Carlo simulations
demonstrate the effect of seasonal and monthly windowing on the optical
data, and show why previous analyses are likely to have given erroneous
results.
Title: A perturbation particle method for stability studies of
stellar systems
Authors: Wachlin, F. C.; Rybicki, G. B.; Muzzio, J. C.
Bibcode: 1992BAAA...37...67W
Altcode:
In order to investigate the stability of stellar systems, we discuss
a numerical method that uses an analytic distribution function to
describe a stellar system in equilibrium and "perturbation particles"
to represent departures from that equilibrium state. Thus, all the
particles are used only to represent the perturbation, and statistical
fluctuations due to the finite number of particles are much less
severe than in full N-body codes. We provide a general description of
the method, recipes for particular aspects of its implementation, and
an example of its application to a simple model with known analytical
solution.
Title: The solution of radiative transfer problems in molecular bands
without the LTE assumption by accelerated lambda iteration methods.
Authors: Kutepov, A. A.; Kunze, D.; Hummer, D. G.; Rybicki, G. B.
Bibcode: 1991JQSRT..46..347K
Altcode:
An iterative method based on the use of approximate transfer (or Λ)
operators, which was designed initially to solve multilevel NLTE line
formation problems in stellar atmospheres, is adapted and applied to
the solution of the NLTE molecular band radiative transfer in planetary
atmospheres. The matrices to be constructed and inverted are much
smaller than those used in the traditional Curtis matrix technique,
which makes possible the treatment of more realistic problems (including
rotational NLTE, overlapping of lines in the bands and overlapping of
bands with continuua) using relatively small computers. This technique
converges much more rapidly than straightforward iteration between
the transfer equation and the equations of statistical equilibrium
(Λ-iteration). A test application of this new technique to the solution
of NLTE radiative transfer problems for optically-thick and thin bands
(the 4.3 μm CO2 band in the Venusian atmosphere and the
4.7 and 2.3 μm CO bands in the Earth's atmosphere) is described.
Title: An accelerated lambda iteration method for multilevel
radiative transfer.
Authors: Rybicki, G. B.; Hummer, D. G.
Bibcode: 1991A&A...245..171R
Altcode:
A method is presented for solving multilevel transfer problems when
nonoverlapping lines and background continuum are present and active
continuum transfer is absent. An approximate lambda operator is employed
to derive linear, 'preconditioned', statistical-equilibrium equations. A
method is described for finding the diagonal elements of the 'true'
numerical lambda operator, and therefore for obtaining the coefficients
of the equations. Iterations of the preconditioned equations, in
conjunction with the transfer equation's formal solution, are used
to solve linear equations. Some multilevel problems are considered,
including an eleven-level neutral helium atom. Diagonal and tridiagonal
approximate lambda operators are utilized in the problems to examine the
convergence properties of the method, and it is found to be effective
for the line transfer problems.
Title: Instabilities in Line-driven Stellar Winds. V. Effect of an
Optically Thick Continuum
Authors: Owocki, Stanley P.; Rybicki, George B.
Bibcode: 1991ApJ...368..261O
Altcode:
Earlier analyses of the linear instability of line-driven stellar winds
are extended to the case, relevant to Wolf-Rayet stars, in which the
continuum remains optically thick well above the sonic point. It
is found that an optically thick flow driven by pure scattering
lines is stabilized by the drag effect of the diffuse, scattered
radiation. However, even a relatively small photon destruction
probability can cause a flow with continuum optical thickness much
greater than 1 to remain unstable, with a given growth rate. The
implications of these results for the variability characteristics of
winds from Wolf-Rayet stars are briefly discussed.
Title: Recent Advances in Computational Methods
Authors: Rybicki, G. B.
Bibcode: 1991ASIC..341....1R
Altcode: 1991sabc.conf....1R
No abstract at ADS
Title: Lyman-Alpha Trapping in Hydrogen During Recombination
Authors: Rybicki, G. B.; dell'Antonio, I. P.
Bibcode: 1990BAAS...22.1214R
Altcode:
No abstract at ADS
Title: Instabilities in Line-driven Stellar Winds. IV. Linear
Perturbations in Three Dimensions
Authors: Rybicki, G. B.; Owocki, S. P.; Castor, J. I.
Bibcode: 1990ApJ...349..274R
Altcode:
Nonradial wave propagation in line-driven stellar winds is analyzed
including both finite disk effects and the line-drag effect of scattered
line radiation. Within the local (WKB) analysis the results apply to
wavelengths both longer and shorter than the Sobolev length. The finite
disk causes short-wavelength waves with lateral velocity polarization
to be unstable in the idealized case of pure absorption; however, the
growth rates are smaller than for radially polarized waves, and the
instability is damped by a relatively small amount of scattering. Hence,
in realistic stellar winds, where the driving is primarily by scattering
lines, perturbations with an arbitrary mixture of lateral and radial
polarizations at the wind base should quickly become nearly radially
polarized farther out in the wind. The implications of these results
are discussed, both for interpretation of observational signatures of
wind structure and for theoretical calculations aimed at modeling the
nonlinear evolution of wind instabilities.
Title: Fast Algorithm for Spectral Analysis of Unevenly Sampled Data
Authors: Press, William H.; Rybicki, George B.
Bibcode: 1989ApJ...338..277P
Altcode:
The Lomb-Scargle method performs spectral analysis on unevenly
sampled data and is known to be a powerful way to find, and test the
significance of, weak periodic signals. The method has previously
been thought to be 'slow', requiring of order 10(2)N(2) operations
to analyze N data points. We show that Fast Fourier Transforms (FFTs)
can be used in a novel way to make the computation of order 10(2)N log
N. Despite its use of the FFT, the algorithm is in no way equivalent
to conventional FFT periodogram analysis.
Title: Physics of Instabilities in Radiatively Driven Stellar Winds
Authors: Owocki, S. P.; Castor, J. I.; Rybicki, G. B.
Bibcode: 1989ASSL..157..291O
Altcode: 1989IAUCo.113..291O; 1989plbv.coll..291O
No abstract at ADS
Title: Time-dependent Models of Radiatively Driven Stellar
Winds. I. Nonlinear Evolution of Instabilities for a Pure Absorption
Model
Authors: Owocki, Stanley P.; Castor, John I.; Rybicki, George B.
Bibcode: 1988ApJ...335..914O
Altcode:
The authors describe results of numerical radiation-hydrodynamics
simulations of the nonlinear evolution of instabilities in radiatively
driven stellar winds. The wind is idealized as a spherically
symmetric, isothermal flow driven by pure absorption of stellar
radiation in a fixed ensemble of spectral lines. The simulations
indicate that there is a strong tendency for the unstable flow to
form rather sharp rarefactions in which the highest speed material
has very low density. The growth of wave perturbations thus remains
nearly exponential well beyond the linear regime, until the waves
are kinematically steepened into strong shocks. The strongest shocks
here are reverse shocks that arise to decelerate high-speed, rarefied
flow as it impacts slower material that has been compressed into
dense shells. The subsequent wind evolution shows a slow decay of the
shocks and the gradual thermal decompression and interaction of the
dense shells.
Title: A Study of the Intrinsic Shapes of Elliptical Galaxies
Authors: Rybicki, G. B.; Liu, C. T.
Bibcode: 1988BAAS...20.1038R
Altcode:
No abstract at ADS
Title: Time-Dependent Mass Loss from Hot Stars With and Without
Radiative Driving
Authors: Castor, John I.; Owocki, Stanley P.; Rybicki, George B.
Bibcode: 1988ASSL..148..229C
Altcode: 1988pmls.conf..229C
A numerical hydrodynamics code is used to investigate two aspects of
the winds of hot stars. The first is the question of the instability
of the massive radiatively-driven wind of an O star that is caused
by the line shape mechanism: modulation of the radiation force by
velocity fluctuations. The evolution of this instability is studied in
a model O star wind, and is found to lead to wave structures that are
compatible with observations of wind instabilities. The other area of
investigation is of main-sequence B star winds.
Title: Shock Formation from the Nonlinear Evolution of Instabilities
in Line-Driven Stellar Winds
Authors: Owocki, S. P.; Castor, J. I.; Rybicki, G. B.
Bibcode: 1987BAAS...19..702O
Altcode:
No abstract at ADS
Title: Radiation driven instabilities
Authors: Rybicki, George B.
Bibcode: 1987ASSL..136..175R
Altcode: 1987ilet.work..175R
Various radiation driven instabilities have been shown to operate in
the atmospheres and winds of early-type luminous stars. The strongest
of these occur in the supersonic parts of the winds, where as many
as one hundred e-folds of linear growth can occur during a typical
outflow time. The nonlinear growth of such instabilities can possibly
account for the observed superionization and X-ray emission in these
stars. Developments in the linear theory of these instabilities is
reviewed.
Title: Shock Formation from the Nonlinear Evolution of Instabilities
in Line-Driven Stellar Winds
Authors: Owocki, S.; Castor, J. I.; Rybicki, G. B.
Bibcode: 1987sowi.conf..177O
Altcode:
No abstract at ADS
Title: Deprojection of Galaxies - how much can BE Learned
Authors: Rybicki, G. B.
Bibcode: 1987IAUS..127..397R
Altcode:
A general discussion, based on the "Fourier Slice Theorem", is given
for the problem of deprojecting the observed light distribution of
galaxies to obtain their intrinsic three dimensional light distribution
or "shape". Several results are obtained : (1) A model-independent
deprojection of an axially symmetric galaxy is shown to be possible
only if the symmetry axis lies in the plane of the sky. (2) A simple
criterion is given to test whether two different galaxies can have the
same intrinsic shape, based solely on their observed projections. (3)
It is shown that a homogeneous class of galaxies can be deprojected
using a sufficiently large number of projections of random perspective.
Title: Nonlinear dynamics of instabilities in line-driven stellar
winds
Authors: Owocki, S. P.; Castor, J. I.; Rybicki, G. B.
Bibcode: 1987ASSL..136..269O
Altcode: 1987ilet.work..269O
The authors have been developing a numerical radiation-hydrodynamics
program in order to study the nonlinear evolution of instabilities in
line-driven winds from luminous, early-type stars. Initial tests of
the code indicate that the velocity structure of nonlinear pulses in
such a wind may be quite different than assumed in previous analyses.
Title: Instabilities in Line-driven Stellar Winds. III. Wave
Propagation in the Case of Pure Line Absorption
Authors: Owocki, S. P.; Rybicki, G. B.
Bibcode: 1986ApJ...309..127O
Altcode:
The spatial and temporal evolution of small-amplitude velocity
perturbations is examined in the idealized case of a stellar wind that
is driven by pure line absorption of the star's continuum radiation. It
is established that the instability in the supersonic region is of the
advective type relative to the star, but of the absolute type relative
to the wind itself. It is also shown that the inward propagation of
information in such a wind is limited to the sound speed, in contrast
to the theory of Abbott, which predicts inward propagation faster than
sound. This apparent contradiction is resolved through an extensive
discussion of the analytically soluble case of zero sound speed.
Title: A Time-Dependent Line-Driven Wind Model Not Based on the
Sobolev Approximation
Authors: Owocki, S. P.; Castor, J. I.; Rybicki, G. B.
Bibcode: 1986BAAS...18..953O
Altcode:
No abstract at ADS
Title: Radiative Processes in Astrophysics
Authors: Rybicki, George B.; Lightman, Alan P.
Bibcode: 1986rpa..book.....R
Altcode:
Radiative Processes in Astrophysics: This clear, straightforward, and
fundamental introduction is designed to present-from a physicist's
point of view-radiation processes and their applications to
astrophysical phenomena and space science. It covers such topics as
radiative transfer theory, relativistic covariance and kinematics,
bremsstrahlung radiation, synchrotron radiation, Compton scattering,
some plasma effects, and radiative transitions in atoms. Discussion
begins with first principles, physically motivating and deriving all
results rather than merely presenting finished formulae. However, a
reasonably good physics background (introductory quantum mechanics,
intermediate electromagnetic theory, special relativity, and some
statistical mechanics) is required. Much of this prerequisite material
is provided by brief reviews, making the book a self-contained
reference for workers in the field as well as the ideal text for
senior or first-year graduate students of astronomy, astrophysics,
and related physics courses. Radiative Processes in Astrophysics also
contains about 75 problems, with solutions, illustrating applications
of the material and methods for calculating results. This important and
integral section emphasizes physical intuition by presenting important
results that are used throughout the main text; it is here that most
of the practical astrophysical applications become apparent.
Title: Book-Review - Radiative Processes in Astrophysics
Authors: Rybicki, G. B.; Lightman, A. P.; Paul, H. G.
Bibcode: 1986AN....307..170R
Altcode:
No abstract at ADS
Title: A Gridless Fourier Method
Authors: Rybicki, G. B.
Bibcode: 1986LNP...267..181R
Altcode: 1986ussd.conf..181R
No abstract at ADS
Title: Instabilities in line-driven stellar winds. II - Effect
of scattering.
Authors: Owocki, S. P.; Rybicki, G. B.
Bibcode: 1985ApJ...299..265O
Altcode:
An earlier analysis (Owocki and Rybicki) of the linear instability
of line-driven stellar winds is extended to take proper account of
the dynamical effect of scattered radiation. The principal findings
are as follows: (1) the drag effect of the mean scattered radiation
does indeed greatly reduce the contribution of scattering lines to the
instability at the very base of the wind, but the instability growth
rate associated with such lines rapidly increases as the flow moves
outward from the base, reaching more than 50 percent of the growth
rate for pure absorption lines within a stellar radius of the surface,
and eventually reaching 80 percent of that rate at large radii; (2)
perturbations in the scattered radiation field may be important for the
propagation of wind disturbances, but they have little effect on the
wind instability; (3) the contribution of a strongly shadowed line to
the wind instability is often reduced compared to that of an unshadowed
line, but its effect is not one of damping in the outer parts of the
wind. The primary conclusion derived from these results is thus that,
even when all scattering effects are taken into account, the bulk of
the flow in a line-driven stellar wind is still highly unstable.
Title: Book-Review - Radiative Processes in Astrophysics
Authors: Rybicki, G. B.; Lightman, A. P.
Bibcode: 1985S&T....70R.451R
Altcode:
No abstract at ADS
Title: The HEAO-2 Guest Investigator Program: Non-linear growth of
instabilities in line-driven stellar winds
Authors: Rybicki, G. B.
Bibcode: 1985STIN...8535840R
Altcode:
The linear instability of line-driven stellar winds to take proper
account of the dynamical effect of scattered radiation were analyzed. It
is found that: (1) the drag effect of the mean scattered radiation
does greatly reduce the contribution of scattering lines to the
instability at the very base of the wind, but the instability growth
rate associated with such lines rapidly increases as the flow moves
outward from the base, reaching more than 50% of the growth rate for
pure absorption lines within a stellar radius of the surface, and
eventually reaching 80% of that rate at large radii; (2) perturbations
in the scattered radiation field may be important for the propagation of
wind disturbances, but they have little effect on the wind instability;
and (3) the contribution of strongly shadowed lines to the wind
instability is often reduced compared to that of unshadowed lines,
but their overall effect is not one of damping in the outer parts of
the wind. It is concluded that, even when all scattering effects are
taken into account, the bulk of the flow in a line-driven stellar wind
is still highly unstable.
Title: The Sobolev approximation for line formation with continuous
opacity
Authors: Hummer, D. G.; Rybicki, G. B.
Bibcode: 1985ApJ...293..258H
Altcode:
The Sobolev approximation for line-formation problems in atmospheres
with high-speed flows is generalized to include the effects of
continuum absorption and emission in the region of the line. The
result is very simple, being expressed entirely in terms of known
functions with the exception of one quantity of order unity, which
is tabulated. Comparison with accurate numerical solutions for simple
problems in plane-parallel geometry shows the approximation to be quite
accurate in those regions of the atmosphere where the conditions for
the validity of the approximation are satisfied. A three-dimensional
version of the theory is given that applies to general geometries.
Title: Escape probability methods.
Authors: Rybicki, G. B.
Bibcode: 1985ASIC..152..199R
Altcode: 1985pssl.proc..199R
The physical foundations of escape probability methods, and methods
derived from them, are briefly reviewed. First-order escape probability
methods, the core saturation method, second-order escape probability
methods, and Scharmer's method are discussed.
Title: Effect of scattering on instabilities in line-driven stellar
winds.
Authors: Owocki, S. P.; Rybicki, G. B.
Bibcode: 1985NASCP2358..221O
Altcode: 1985onhm.rept..221O
Line driven O-B stellar winds are unstable to perturbations of short
spatial wavelength and the growth rates for such instabilities are very
rapid. The nature of the nonlinear development of this instability
is unknown, but might possibly be one of blobs of gas driven through
ambient gas or a quasiregular train of outward moving shocks. In either
case the resulting dissipation of mechanical energy might explain the
observed anomalous heating n O-B stars as evidenced by their X-ray
emission and high ionization state. It also might explain the observed
fine structure of the absorption lines and their time variability. The
driving due to the absorption of the stellar continuum flux was
considered and the effects of the diffuse, scattered radiation field
were neglected. It was shown that under certain special conditions
the effect of scattering could reduce the instability growth rate to
zero. A stability analysis that includes scattering, but that uses
the more physically realistic assumption of complete redistribution
instead of coherent scattering, and that includes the effects of
transverse velocity gradients, which become important as the flow
moves away from the stellar surface is presented. It is found that
the instability is eliminated right at the base of the wind, but
that as the flow moves outward the instability rate rapidly becomes
equal to a substantial fraction of the calculated value, the fraction
asymptotically reaching 80% at large radii. Since this still implies
many e folds in a characteristic outflow time, the primary conclusion
that these winds are highly unstable is unchanged.
Title: X-ray measurements of the aspherical mass distribution in
the clusterof galaxies A 2256.
Authors: Fabricant, D.; Rybicki, G.; Gorenstein, P.
Bibcode: 1985xra..conf..381F
Altcode: 1984xra..conf..381F
The authors describe a technique for deriving the underlying mass
distribution from X-ray surface brightness maps of rich clusters of
galaxies with a smooth X-ray morphology and approximate bilateral
symmetry. They apply this technique to A 2256 and find significant
departures from spherical symmetry.
Title: Radiative processes in astrophysics.
Authors: Rybicki, G. B.; Lightman, A. P.
Bibcode: 1985rpa..book.....R
Altcode:
No abstract at ADS
Title: Pannel discussion on radiative transfer methods.
Authors: Kalkofen, W.; Linsky, J.; Rybicki, G.; Scharmer, G.;
Weherse, R.
Bibcode: 1985ASIC..152..233K
Altcode: 1985pssl.proc..233K
No abstract at ADS
Title: X-ray measurements of the nonspherical mass distribution in
the cluster of galaxies A 2256.
Authors: Fabricant, D.; Rybicki, G.; Gorenstein, P.
Bibcode: 1984ApJ...286..186F
Altcode:
The authors describe a technique for the analysis of X-ray surface
brightness maps of rich clusters of galaxies with a smooth X-ray
morphology and appropriate bilateral symmetry to yield an accurate
estimate of the underlying cluster mass distribution. This method
assumes that the X-ray emitting gas is in hydrostatic equilibrium
in the gravitational potential of the cluster, and that the cluster
possesses an axis of symmetry that lies in the plane of the sky. The
errors associated with the latter assumption are discussed. The authors
apply this technique to the cluster of galaxies A2256 and find that
the mass distribution departs significantly from spherical symmetry,
with axial ratios of approximately 1.6. This is to be compared with
the axial ratio of 2 - 3.3 found for the galaxy distribution. They
find that the surfaces of constant total density are more complex than
similar oblate or prolate spheroids with a fixed center. The radially
integrated mass derived from the X-ray data is consistent with that
estimated from the galaxy surface density and velocity dispersion.
Title: Effect of Scattering on the Instability of Radiation-Driven
Stellar Winds
Authors: Rybicki, G. B.; Owocki, S. P.
Bibcode: 1984BAAS...16R.993R
Altcode:
No abstract at ADS
Title: Instabilities in line-driven stellar winds. I. Dependence on
perturbation wavelength.
Authors: Owocki, S. P.; Rybicki, G. B.
Bibcode: 1984ApJ...284..337O
Altcode:
An analytical study is presented of the stability of absorption
line-driven flows, such as found in stellar winds, in the presence
of small-amplitude disturbances. A generalized calculation of the
perturbed direct extinction force is performed and the evolution of
the perturbation into the nonlinear regime and the dynamical results
of perturbation in the scattered radiation field are examined. An
expression is derived for the wavenumber variation of the perturbed line
force from the milieu of nonoverlapping lines which have a power-law
distribution in opacity. A linear dispersion analysis is carried out
to model the growth and propagation of radiative-acoustic waves in
absorption line-driven flows, which are found unstable to perturbations
that may be smaller than the Sobolev length. No damping mechanism was
found that would eliminate the absorption line-driven flows.
Title: Half-range moment methods for radiative transfer in spherical
geometry. II - Implementation of the method
Authors: Martin, P. G.; Rogers, C.; Rybicki, G. B.
Bibcode: 1984ApJ...284..317M
Altcode:
The accuracy of a new numerical method for problems of radiative
transfer in spherical geometry, based on half-range power moments
of the specific intensity, is implemented and demonstrated. Use of
a Ricatti transformation allows an accurate, though computationally
inefficient, radial integration of the moment equations, so that the
convergence of the method as the number of moments is increased can
be isolated. The method was checked against both analytic (exact)
solutions and results from several other numerical methods, for a
variety of problems including those with central sources or with
an isotropic external radiation field and including anisotropic and
nonconservative scattering. Because this method is well adapted to
the boundary conditions, takes account of possible outward peaking in
the intensity, and has a consistent treatment of the source function,
good solutions can be obtained using only a few moments. Thus, when the
radial integration is treated with a fast finite-difference scheme,
this half-range moment method offers an efficient solution to a wide
range of problems in radiative transfer.
Title: Effect of Scattered Radiation on the Instability of Hot
Star Winds
Authors: Rybicki, G. B.; Owocki, S. P.
Bibcode: 1984BAAS...16Q.725R
Altcode:
No abstract at ADS
Title: Escape probability methods.
Authors: Rybicki, G. B.
Bibcode: 1984mrt..book...21R
Altcode: 1984mrt..conf...21R
Escape probability methods for the approximate treatment of non-LTE
line and continuum transfer have recently undergone considerable
development. The author reviews these methods, starting from a detailed
description of the escape probabilities in static and moving media,
and proceeding through the original (first-order) escape probability
methods, the core saturation method, the newer (second-order) escape
probability methods, and Scharmer's method.
Title: The specific luminosity of a three-dimensional medium in
terms of the escape probability
Authors: Rybicki, G. B.; Hummer, D. G.
Bibcode: 1983ApJ...274..380R
Altcode:
Radiation emitted by a three-dimensional medium and received by a
distant observer is expressed in terms of the specific luminosity,
which can be evaluated in terms of escape probability functions for
the medium. This approach is simpler in many cases than an integration
over the plane of the sky, because it allows any symmetries of the
problem to be more fully exploited. Integral theorems are derived
involving averaged escape probabilities that generalize a theorem
of Irons. Analogous results are presented for a three-dimensional
line-emitting region in the Sobolev (or large velocity gradient)
limit. To illustrate the theory, results for expanding spherical media
and differently rotating thin disks are discussed. Computationally
useful results for several related escape probability functions
are given.
Title: Instabilities in Line-Driven Stellar Winds
Authors: Rybicki, G. B.; Owocki, S. P.
Bibcode: 1983BAAS...15..968R
Altcode:
No abstract at ADS
Title: Measurements of the Nonspherical Mass Distribution in Galaxy
Clusters from X-ray Maps
Authors: Fabricant, D.; Gorenstein, P.; Rybicki, G.
Bibcode: 1983BAAS...15..974F
Altcode:
No abstract at ADS
Title: Erratum - Second-Order Escape Probability Approximations in
Radiative Transfer
Authors: Hummer, D. G.; Rybicki, G. B.
Bibcode: 1983ApJ...271..888H
Altcode:
No abstract at ADS
Title: Book-Review - Radiative Processes in Astrophysics
Authors: Rybicki, G. B.; Lightman, A. P.; Porter, A.
Bibcode: 1983S&T....65..336R
Altcode:
No abstract at ADS
Title: Second-order escape probability approximations in radiative
transfer
Authors: Hummer, D. G.; Rybicki, G. B.
Bibcode: 1982ApJ...263..925H
Altcode:
Second-order escape probability approximations make some allowance
for the transfer of radiation between the point where a photon is
created and that where it escapes or is absorbed. An approximation of
this kind has recently been formulated by Puetter et al. (1982) for
planar atmospheres of finite thickness, in the form of a first-order
differential equation relating the integrated mean intensity to the
source function. Two alternative normalizations to the one proposed by
these authors are given, the first of these enforces global conservation
of photons in each transition, and the second gives reasonably accurate
results with much less computational effort than the first. These
results, along with the first-order approximation and a second-order
result of Ivanov (1972), are compared with accurate numerical results
for static isothermal planar atmospheres. The maximum error for the
photon-conserving approximation is less than 25 percent for Doppler
and 7 percent for Lorentz broadening.
Title: The structure and evolution of galacto-detonation waves -
Some analytic results in sequential star formation models of spiral
galaxies
Authors: Cowie, L. L.; Rybicki, G. B.
Bibcode: 1982ApJ...260..504C
Altcode:
Waves of star formation in a uniform, differentially rotating disk
galaxy are treated analytically as a propagating detonation wave
front. It is shown, that if single solitary waves could be excited,
they would evolve asymptotically to one of two stable spiral forms,
each of which rotates with a fixed pattern speed. Simple numerical
solutions confirm these results. However, the pattern of waves that
develop naturally from an initially localized disturbance is more
complex and dies out within a few rotation periods. These results
suggest a conclusive observational test for deciding whether sequential
star formation is an important determinant of spiral structure in some
class of galaxies.
Title: The Stability of Line-Radiation-Driven Stellar Winds
Authors: Owocki, S. P.; Rybicki, G.
Bibcode: 1982BAAS...14..920O
Altcode:
No abstract at ADS
Title: A unified treatment of escape probabilities in static and
moving media. I - Plane geometry.
Authors: Hummer, D. G.; Rybicki, G. B.
Bibcode: 1982ApJ...254..767H
Altcode:
An expression giving the escape probability for photons in a spectral
line formed in a planar atmosphere with an arbitrary monotonic velocity
law is derived and evaluated. For a small velocity gradient, the usual
static result is recovered; for large velocity gradients the Sobolev
result is obtained, but only at optical depths sufficiently large that
the static part of the escape probability is negligible. Extensive
numerical results for the escape-probability function for a constant
velocity gradient are given for Doppler, Voigt (a = 0.001, 0.01) and
Lorentz profiles. The use of these results for flows with nonconstant
gradients is discussed.
Title: Book-Review - Radiative Processes in Astrophysics
Authors: Rybicki, G. B.; Lightman, A. P.; Begelman, M. C.
Bibcode: 1982ApL....22...73R
Altcode:
No abstract at ADS
Title: Comptonization by cold electrons
Authors: Lightman, A. P.; Lamb, D. Q.; Rybicki, G. B.
Bibcode: 1981ApJ...248..738L
Altcode:
Analytic approximations to the Green's functions for scattering
of photons by cold electrons in finite media of various geometries
are derived. The probabilities of a photon to escape spherical and
plane-parallel, semi-infinite media after n scatterings are computed,
and the results are compared to those previously obtained by the
'scattering time' method. Green's functions are then obtained by
combining these probabilities with the wavelength distribution after n
scatterings. To illustrate the results, the emergent spectra from the
input blackbody and bremsstrahlung spectra in the geometries mentioned
above are computed.
Title: Enhancement of Passive Diffusion and Suppression of Heat Flux
in a Fluid with Time Varying Shear
Authors: Press, W. H.; Rybicki, G. B.
Bibcode: 1981ApJ...248..751P
Altcode:
Diffusion and heat conductivity within stars are shown to be
substantially affected by fluid shear motions, even when these motions
do not mix fluid elements (i.e., even when no fluid element wanders far
from its equilibrium position). Internal gravity waves (g-modes) are
one example of such a nonmixing flow. A formalism, based on Lagrangian
flow coordinates, is developed to describe the effect in Boussinesq
approximation. The diffusion of passive contaminants and of specific
entropy is found to be generally enhanced, while the conduction of
heat in stably stratified layers is suppressed (the fluid motions
power a thermodynamic refrigerator). Explicit formulae are derived for
flows which can be approximated as having spatially constant velocity
gradients, and also for interval gravity waves. In the latter case the
mechanical dissipation and thermodynamic efficiencies are calculated.
Title: Waves of Sequential Star Formation
Authors: Rybicki, G. B.; Cowie, L. L.
Bibcode: 1981BAAS...13..852R
Altcode:
No abstract at ADS
Title: Line Formation in Accretion Disks
Authors: Carroll, D.; Hummer, D. G.; Rybicki, G. B.
Bibcode: 1981BAAS...13Q.818C
Altcode:
No abstract at ADS
Title: Book-Review - Radiative Processes in Astrophysics
Authors: Rybicki, G. B.; Lightman, A. P.; Tayler, R. J.
Bibcode: 1981Natur.289..729R
Altcode:
No abstract at ADS
Title: Ultraviolet pumping of Si/+/ fine-structure levels
Authors: Flannery, B. P.; Rybicki, G. B.; Sarazin, C. L.
Bibcode: 1980ApJS...44..539F
Altcode:
The effects of ultraviolet pumping on the population of the doublet
fine-structure levels of Si(+) are investigated, and an efficient
method for the numerical solution of the multi-line, multi-level
radiative transfer problem is presented. Following a review of the
atomic properties of Si(+), analytic results are presented for
optically thin pumping through eight UV levels with 16 lines in
the wavelength range 990-1820 A. Consideration is then given to the
radiative transfer problem for pumping in an optically thick medium,
and the method employed for the numerical solution of the problem
is introduced which is based on a core saturation approximation for
the statistical equilibrium equations. Numerical results are then
presented for a grid of homogeneous plane parallel nebulae illuminated
by a point source, which are characterized by a wide range of electron
densities, UV source strengths and line optical depths. Applications
of the results to Stromgren spheres and an absorption system in the
quasar PKS 0237-23 are discussed.
Title: Escape-Probability Method for Flows of Arbitrary Velocity
Amplitude
Authors: Hummer, D. G.; Rybicki, G. B.
Bibcode: 1980BAAS...12..798H
Altcode:
No abstract at ADS
Title: Inverse Compton reflection - The steady-state theory
Authors: Lightman, A. P.; Rybicki, G. B.
Bibcode: 1980ApJ...236..928L
Altcode:
The steady-state theory of inverse Compton scattering in a finite
medium of thermal electrons is developed. It is shown that there is an
approximate separation of the spatial and energy transport problems. For
an initial soft photon input the energy transport portion is solved
using a combination of the central limit theorem and the saturated
Wien spectrum. The spatial transport portion is reduced to the study
of the distribution of n-times scattered intensities. For the case
of reflection of external photons, an n to the -3/2 law is shown
to represent asymptotically the probabilities of emergence after
n scatterings. The emergent spectrum is a quasi-power law, with a
slope that depends only weakly on plasma parameters. In particular,
the effects of an inhomogeneous temperature distribution on the
reflected spectrum are shown to be of minor importance. The flux due
to internally generated photons is also derived and is compared with
that due to reflection.
Title: The penetration of diffuse ultraviolet radiation into
interstellar clouds
Authors: Flannery, B. P.; Roberge, W.; Rybicki, G. B.
Bibcode: 1980ApJ...236..598F
Altcode:
It is shown that the solution of the transfer equation appropriate for
models of the penetration of diffuse UV radiation into interstellar
clouds, subject to attenuation by coherent, nonconservative, anisotropic
scattering from grains, can be expressed analytically, with arbitrary
accuracy, by means of the spherical harmonics method. Models
of plane-parallel and homogeneous spherical clouds are given as
functions of three parameters: the central optical depth, the single
scattering albedo, and the parameter in the Henyey-Greenstein phase
function. These models qualitatively confirm the results of earlier
Monte Carlo simulations of dust scattering, but reveal quantitative
discrepancies: the earlier results overestimated the actual mean
intensity, often by more than an order of magnitude.
Title: Book-Review - Radiative Processes in Astrophysics
Authors: Rybicki, G. B.; Lightman, A. P.
Bibcode: 1980S&T....59..509R
Altcode:
No abstract at ADS
Title: Book-Review - Radiative Processes in Astrophysics
Authors: Rybicki, G. B.; Lightman, A. P.
Bibcode: 1980Sci...210Q.568R
Altcode:
No abstract at ADS
Title: Interpretation of X-ray Spectra Observed in Tails of X-ray
Bursts.
Authors: van Paradijs, J.; Rybicki, G.; Lamb, D. Q.
Bibcode: 1979BAAS...11..788V
Altcode:
No abstract at ADS
Title: Inverse Compton Reflection.
Authors: Lightman, A. P.; Rybicki, G. B.
Bibcode: 1979BAAS...11Q.793L
Altcode:
No abstract at ADS
Title: Inverse Compton reflection: time-dependent theory.
Authors: Lightman, A. P.; Rybicki, G. B.
Bibcode: 1979ApJ...232..882L
Altcode:
Consideration is given to time-dependent behavior in inverse Compton
reflection in which an external flux of soft photons is incident upon
a hot, thermal plasma of large scattering depth and small absorption
depth. Fluctuations are assumed to arise from variability in the
input of soft photons. The treatment is largely analytic. Spectral
evolution during a fluctuation is calculated in detail and it is shown
how such observables as the frequency-dependent rise time and the
spectral hardness ratio may be used to derive the plasma parameters
and soft photon frequency, or perhaps rule out an inverse Compton
model altogether. Some of the results are qualitatively applicable
to inverse Compton processes in general. Such considerations may be
relevant to models for rapidly fluctuating X-ray sources.
Title: An asymptotic limit to optical pumping in an opaque region.
Authors: Sarazin, C. L.; Rybicki, G. B.; Flannery, B. P.
Bibcode: 1979ApJ...230..456S
Altcode:
An asymptotic limit to the optical pumping of low-lying levels of
an ion or molecule in an optically thick medium has been found. The
net rate of optical pumping approaches zero when optical depth is
sufficiently large, even though individual line intensities generally
increase as the optical depth increases. The optical pumping rate nears
zero because the lines approach a detailed balance resulting from
proton conservation; this detailed balance occurs for any geometry,
line profiles or redistribution processes. An intermediate domain
of large optical depths (the pumping-dominated limit) in which level
populations are identical to those in an optically thin region is also
found. Results of this optical pumping study may be applied to the
excitation of atomic fine-structure levels in H II regions and quasars.
Title: Ultraviolet pumping of N+ fine-structure levels.
Authors: Flannery, B. P.; Rybicki, G. B.; Sarazin, C. L.
Bibcode: 1979ApJ...229.1057F
Altcode:
The effects of radiative pumping, through the six-line UV 3D - 3P
multiplet at 1085 A, on the population of the fine-structure levels of
N(+) are investigated. Results are presented for both optically thin
and optically thick media. For the more important case of an optically
thick nebula, an efficient method is outlined for the numerical solution
of the multiline multilevel radiative-transfer problem in a model H II
region described by plane-parallel geometry, point source illumination,
and homogeneous physical conditions. The method is applied to construct
a grid of models for N(+) spanning a wide range of values for the
electron density, the strength of the UV source, and the line optical
depth of the nebula. In general it is found that radiative pumping
and transfer effects can significantly alter the level populations,
but the effect often mimics results obtained without radiative
pumping with a different electron density. For Stroemgren spheres
around early-type stars it is found as a result of transfer effects,
the role of pumping is never likely to be important. Infrared emission
from the fine-structure decays at 122 and 240 microns is also discussed.
Title: X-rays from active galactic nuclei: inverse Compton reflection.
Authors: Lightman, A. P.; Rybicki, G. B.
Bibcode: 1979ApJ...229L..15L
Altcode:
The radiative process of inverse Compton reflection, in which an
external flux of soft photons is incident upon a hot plasma of large
scattering depth and small absorption depth is investigated. Over a
large range of frequency, the emergent spectrum is a universal, quasi
power law, whose slope is insensitive to any parameters. Inverse Compton
reflection also produces strong energy amplification. This process may
have implications for models of X-ray sources in active galactic nuclei.
Title: On the distance from quasars to absorbing clouds.
Authors: Sarazin, C. L.; Flannery, B. P.; Rybicki, G. B.
Bibcode: 1979ApJ...227L.113S
Altcode:
A lower limit to the distance from a quasar to an absorbing cloud
is derived for the case where the quasar spectrum shows resonance
absorption lines but not lines from excited fine-structure levels. As
first discussed by Bahcall (1967), these limits arise from the
requirement that the rate of UV radiative excitation from the continuum
emission of the quasar be small. Previous calculations, which assumed
that the clouds were optically thin in the lines, found that the clouds
must be at least a kiloparsec away from the quasar. Distance limits
as derived for the case where the clouds are optically thick, and it
is found that the clouds may be within a few parsecs of the quasar.
Title: Radiative processes in astrophysics
Authors: Rybicki, George B.; Lightman, Alan P.
Bibcode: 1979rpa..book.....R
Altcode: 1979QB461.R88......
This clear, straightforward, fundamental introduction to radiative
processes in astrophysics is designed to present - from a physicist's
viewpoint - radiation processes and their applications to astrophysical
phenomena and space science. The book covers such topics as radiative
transfer theory, relativistic covariance and kinematics, bremsstrahlung
radiation, Compton scattering, some plasma effects, and radiative
transitions in atoms. The discussion begins with first principles,
physically motivating and deriving all results rather than merely
presenting finished formulas. Much of the prerequisite material is
provided by brief reviews, making the book a self-contained reference
tool. Also included are about 75 problems with solutions, illustrating
applications of the material and methods for calculating results
Title: Book-Review - Radiative Processes in Astrophysics
Authors: Rybicki, G. B.; Lightman, A. P.
Bibcode: 1979AstQ....3..199R
Altcode:
No abstract at ADS
Title: Optical Pumping and Fine Structure Absorption in Quasars.
Authors: Sarazin, C. L.; Flannery, B. P.; Rybicki, G. B.
Bibcode: 1978BAAS...10..449S
Altcode:
No abstract at ADS
Title: A generalization of the Sobolev method for flows with nonlocal
radiative coupling.
Authors: Rybicki, G. B.; Hummer, D. G.
Bibcode: 1978ApJ...219..654R
Altcode:
The escape-probability technique of Sobolev for solving radiative
transfer problems in moving atmospheres is extended to treat flows
in which the line-of-sight component of the flow velocity is not
monotonic. A completely general geometrical configuration and flow
velocity field are considered; an integral equation is derived for
configurations in which a surface is intersected an arbitrary number
of times. For the case of just two intersections, it is shown that an
iterative solution always converges rapidly. Numerical results for
inverse power-law velocity fields demonstrate the magnitude of the
radiative coupling between distant parts of the atmosphere.
Title: Integrals of the transfer equation. I. Quadratic integrals
for monochromatic, isotropic scattering.
Authors: Rybicki, G. B.
Bibcode: 1977ApJ...213..165R
Altcode:
Quadratic integrals of the transfer equation are introduced for
the case of monochromatic isotropic scattering in a plane-parallel
atmosphere. These integrals are described as natural generalizations to
all depths in the atmosphere of a certain class of results exemplified
by the Hopf-Bronstein relation and the square-root law of Frisch and
Frisch (1975). Two quadratic integrals (Q and R) are constructed on
the basis of a fundamental equation, the Q-integral is used to derive
and generalize the cited relation and law for the type of scattering
considered, and the mean intensity is determined at the boundary of two
half-spaces having different albedoes and source distributions. The
R-integral, regarded as a generalization of the flux integral to
nonconservative atmospheres, is applied to the case of an isotropic
point source of radiation situated between two slabs. Some special
inhomogeneous source distributions are also examined.
Title: The interpretation of Line Profiles
Authors: Rybicki, G. B.
Bibcode: 1977ebhs.coll..191R
Altcode: 1977IAUCo..36..191R
No abstract at ADS
Title: Escape Probability Method for Line Transfer in Accelerating
Inflows and Decelerating Outflows.
Authors: Hummer, D. G.; Rybicki, G. B.
Bibcode: 1976BAAS....8R.546H
Altcode:
No abstract at ADS
Title: Galactic Mass Determinations from Incomplete Rotation Curves
Authors: Schaefer, M. M.; Rybicki, G.; Lecar, M.
Bibcode: 1976Ap&SS..41....3S
Altcode:
Given an incomplete rotation curve of a spiral galaxy, various
assumptions about the Galaxy beyond the last observed point are made:
(A) the force falls off as 1/r 2, (B) the mass density is
zero, and (C) the mass density falls off as 1/r 3. The
msss distributions obtained from each of these assumptions are all
well behaved, and it is impossible to choose the correct curve from
considerations of the resulting mass distributions alone. The correct
mass distribution in the disk system of a galaxy cannot be deduced
from an incomplete rotation curve.
Title: Effect of weak turbulence in spectral line formation
Authors: Rybicki, G.
Bibcode: 1976pmas.conf..189R
Altcode:
No abstract at ADS
Title: A note on the ' peaking effect ' in spherical-geometry
transfer problems
Authors: Rybicki, G. B.; Hummer, D. G.
Bibcode: 1975MNRAS.170..423R
Altcode:
This note presents evidence that the claims advanced by Wilson, Tung
& Sen regarding the adequacy of Wilson & Sen's half-range
moment method for treating the outward peaking of the radiation field
in a spherical system are unjustified. In particular, the emergent
intensity obtained by Wilson et al. is shown to be negative for 0 <
<0.5 and greatly in error for larger values of . A discussion is
presented of the essential indeterminacy of the Wilson-Sen half-range
method. It is suggested that the good values obtained by Wilson et
al. for the mean intensity and the Eddington factor arise from their
choice of the arbitrary function A(r) to include the known asymptotic
forms of the source function.
Title: Transfer of line radiation in differentially expanding
atmospheres, IV: The two-level atom in plane parallel geometry solved
by the Feautrier method.
Authors: Noerdlinger, P. D.; Rybicki, G. B.
Bibcode: 1974ApJ...193..651N
Altcode:
We have solved the radiative transfer problem for a two-level atom in
plane-parallel geometry in an expanding atmosphere with Schuster-type
boundary conditions, by applying the Feautrier method in the fluid rest
frame. The line profiles are relatively smooth and very approximately
symmetric about their minima, except when the temperature in the
atmosphere exceeds that of the photosphere, in which case a redward
emission wing appears. Radiation pressure is evaluated and is found
to compress the inner part of the atmosphere and to expand the outer
part. This suggests that radiation pressure will disrupt QSO absorption
clouds, as found by Williams for nonexpanding clouds. The method is
stable, accurate, and fast for all optical depths and expansion rates
tested, which comprise a vast range.
Title: Ro From Galactic Disk Models.
Authors: Rybicki, G.; Lecar, M.; Schaefer, M.
Bibcode: 1974BAAS....6..453R
Altcode:
No abstract at ADS
Title: Wiener Filtering of Sampled Astronomical Spectra.
Authors: Rybicki, G.; Harrison, P.
Bibcode: 1974BAAS....6..306R
Altcode:
No abstract at ADS
Title: A Method of Computing the Gravitational Field of an Axially
Symmetric Flat Galaxy
Authors: Schaefer, Mary M.; Lecar, Myron; Rybicki, George
Bibcode: 1973Ap&SS..25..357S
Altcode:
A method has been developed for computing the gravitational force field
of an axially symmetric flat galaxy from its surface mass density. The
method is simple to program, fast, and accurate. An inversion formula is
derived that allows computation of surface density from rotation curves
by use of any method that converts density to force. The method is
compared with a method of Clutton-Brock that utilizes Hankel transforms
of Laguerre functions.
Title: Line Formation in Expanding Atmospheres.
Authors: Noerdlinger, P. D.; Rybicki, G. B.
Bibcode: 1973BAAS....5..413N
Altcode:
No abstract at ADS
Title: Relaxation Times in Strictly Disk Systems
Authors: Rybicki, G. B.
Bibcode: 1972ASSL...31...22R
Altcode: 1972gnbp.coll...22R; 1971IAUCo..10...22R
No abstract at ADS
Title: A Novel Approach to the Solution of Multilevel Transfer
Problems
Authors: Rybicki, G. B.
Bibcode: 1972lfpm.conf..145R
Altcode:
No abstract at ADS
Title: Exact Statistical Mechanics of a One-Dimensional
Self-Gravitating System
Authors: Rybicki, G. B.
Bibcode: 1972ASSL...31..194R
Altcode: 1972gnbp.coll..194R; 1971IAUCo..10..194R
No abstract at ADS
Title: Exact Statistical Mechanics of a One-Dimensional
Self-Gravitating System (Papers appear in the Proceedings of IAU
Colloquium No. 10 Gravitational N-Body Problem (ed. by Myron Lecar),
R. Reidel Publ. Co. , Dordrecht-Holland.)
Authors: Rybicki, George B.
Bibcode: 1971Ap&SS..14...56R
Altcode:
The statistical mechanics of an isolated self-gravitating system
consisting ofN uniform mass sheets is considered using both canonical
and microcanonical ensembles. The one-particle distribution function is
found in closed form. The limit for large numbers of sheets with fixed
total mass and energy is taken and is shown to yield the isothermal
solution of the Vlasov equation. The order of magnitude of the approach
to Vlasov theory is found to be 0(1/N). Numerical results for spatial
density and velocity distributions are given.
Title: Relaxation Times in Strictly Disk Systems (Papers appear in
the Proceedings of IAU Colloquium No. 10 Gravitational N-Body Problem
(ed. by Myron Lecar), R. Reidel Publ. Co. , Dordrecht-Holland.)
Authors: Rybicki, George B.
Bibcode: 1971Ap&SS..14...15R
Altcode:
It is shown that the time of relaxation by particle encounters
of self-gravitating systems in the plane interacting by 1/r
2 forces is of the same order of magnitude as the mean orbit
time. There-fore such a system does not have a Vlasov limit for large
numbers of particles, unless appeal is made to some non-zero thickness
of the disk. The relevance of this results to numerical experiments
on galactic structure is discussed.
Title: Numerical evaluation of the redistribution function
RII-A(x, xÿ) and of the associated scattering integral.
Authors: Adams, T. F.; Hummer, D. G.; Rybicki, G. B.
Bibcode: 1971JQSRT..11.1365A
Altcode:
No abstract at ADS
Title: A modified Feautrier method.
Authors: Rybicki, G. B.
Bibcode: 1971JQSRT..11..589R
Altcode:
No abstract at ADS
Title: The searchlight problem with isotropic scattering.
Authors: Rybicki, G. B.
Bibcode: 1971JQSRT..11..827R
Altcode:
No abstract at ADS
Title: The Formation of Spectral Lines
Authors: Hummer, D. G.; Rybicki, G.
Bibcode: 1971ARA&A...9..237H
Altcode:
No abstract at ADS
Title: Radiative transfer in spherically symmetric systems. The
conservative grey case
Authors: Hummer, D. G.; Rybicki, G. B.
Bibcode: 1971MNRAS.152....1H
Altcode:
A practical computational method is presented for the solution of
radiative transfer problems in spherically symmetric systems. This
procedure involves iteration on the `Eddington factor ` f = K/J and
is designed to handle the outward peaking of the radiation field in
extended spherical systems. Extensive numerical results are obtained
and discussed for systems in which `cp = o< , for n = 3/2, 2 and 3.
Title: Non-coherent scattering - VII. Frequencydependent
thermalization lengths and scattering with continuous absorption
Authors: Hummer, D. G.; Rybicki, G. B.
Bibcode: 1970MNRAS.150..419H
Altcode:
Our previous definition of the thermalization length is generalized to
cases in which the initial photon frequency is prescribed. This permits
treatment of cases in which the emissivity does not have a unique
frequency dependence. This definition is then applied to the case of
a line formed by scattering in the presence of continuous opacity. In
this case the emissivity in the line has a unique frequency dependence
but the total emissivity does not. Equations for the distribution of
thermalization distances are derived both by a diagrammatic technique
and by use of resolvents. Median thermalization lengths are defined in
terms of these distributions. Extensive numerical results are reported
and the utility of this approach is discussed.
Title: Theoretical Methods of Treating Line Formation Problems in
Steady-State Extended Atmospheres (introductory Paper)
Authors: Rybicki, G.
Bibcode: 1970sfss.coll...87R
Altcode: 1970IAUCo...2...87R
No abstract at ADS
Title: Non-coherent scattering-V. Thermalization distances and their
distribution function
Authors: Rybicki, G. B.; Hummer, D. G.
Bibcode: 1969MNRAS.144..313R
Altcode:
The distribution function for thermalization distances is derived
for an infinite atmosphere with a plane source. Precise definitions
of the thermalization length are discussed from the point of view of
representing the distribution by a single characteristic length; of
these a definition in terms of the median of the distribution seems
to be most useful. The distribution of longest flights is derived
and shown to provide a good approximation to the distribution of
thermalization lengths at large distances from the source. Extensive
numerical illustrations are provided.
Title: Line Formation in Differentially Moving Media with Temperature
Gradients
Authors: Hummer, D. G.; Rybicki, G. B.
Bibcode: 1968rla..conf..213H
Altcode:
No abstract at ADS
Title: Redshifted Line Profiles from Differentially Expanding
Atmospheres
Authors: Hummer, D. G.; Rybicki, G. B.
Bibcode: 1968ApJ...153L.107H
Altcode:
Accurate numerical solutions of the non-LTE radiative-transfer problem
with non-coherent scatter- ing have been obtained using a generalization
of the Riccati method of Rybicki and Hummer (1967). Differential
expansion in the direction of the observer is found to produce
significantly redshifted line profiles
Title: Comment on "the Emergent Flux from a Fully Illuminated Slab"
Authors: Rybicki, G.
Bibcode: 1968ApJ...152..351R
Altcode:
No abstract at ADS
Title: Spectral Line Formation in Variable-Property Media: the
Riccati Method
Authors: Rybicki, G. B.; Hummer, D. G.
Bibcode: 1967ApJ...150..607R
Altcode:
A numerical method, based on a generalized Riccati transformation of
Rybicki and Usher, is presented for the integration of the radiative
transfer equation for spectral line formation by non-coherent scattering
in inbomogeneous plane-parallel media. A model atom with two discrete
levels is assumed. An asymptotic theory is developed which permits
the application of this method to problems involving semi-infinite
media. Numerical results obtained by the Riccati method are used to
discuss the effects of spatial variations in the Doppler width on
lines formed by pure Doppler broadening.
Title: Radiative Transfer in Turbulent Atmospheres: the Diagnostic
Problem
Authors: Rybicki, G. B.
Bibcode: 1967IAUS...28..471R
Altcode:
No abstract at ADS
Title: Computational Methods for Non-LTE Line_transfer Problems
Authors: Hummer, D. G.; Rybicki, G.
Bibcode: 1967MComP...7...53H
Altcode:
No abstract at ADS
Title: The Generalized Riccati Transformation as a Simple Alternative
to Invariant Imbedding
Authors: Rybicki, George B.; Usher, Peter D.
Bibcode: 1966ApJ...146..871R
Altcode:
A general linear two-point boundary-value problem of the type
encountered in transfer theory is reduced to a set of initial-value
problems by a transformation to uncoupled variables. The matrix of
this transformation is shown to be the usual reflection matrix of
invariant imbedding while the transformed variables describe the
effect of non-homogeneous terms in the equations This method yields
the interior solution as well as the solution at the boundary. It
is shown that the method provides a suitable numerical procedure for
subcritical problems. By a slight adaptation the method is shown to
work for the most general type of linear boundary conditions. The
relationship of this method to invariant imbedding and the relative
advantages of each are discussed.
Title: Transfer of Radiation in Stochastic Media
Authors: Rybicki, G. B.
Bibcode: 1965SAOSR.180.....R
Altcode:
Radiative-transfer equations are derived for a medium with small
stochastically defined opacity and energy fluctuations. These equations
provide relations between the correlation functions connecting these
fluctuations and the induced fluctuations in the radiation field. The
theory is shown to provide a description of the solar atmosphere,
which has statistically defined inhomogeneities due to an underlying
convection zone. A simple inhomogeneous solar model atmosphere is
defined by giving the statistical distribution of energy sources in the
atmosphere. The assumptions of local thermodynamic equilibrium and a
depth independent gray opacity are made. It is shown how the solutions
to the stochastic transfer equations in this case may be conveniently
organized through the use of certain Green's functions. Useful
analytic approximations for these Green's functions are obtained by
use of the invariance results of Sobolev, transform techniques, and
the kernel approximation. In particular, it is shown that the spatial
autocorrelation function of the emergent intensity may be related to
the autocorrelation function of the energy fluctuations.
Title: A New Differential Equation Approach to Transfer Problems
Authors: Rybicki, G. B.
Bibcode: 1965SAOSR.174..149R
Altcode:
No abstract at ADS