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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.
2017TNSCR1536....1G    Altcode:
  No abstract at ADS

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Title: ePESSTO Transient Classification Report for 2017-09-11
Authors: Gromadzki; Rybicki; Fraser; Callis; Yaron, O.; Knezevic, N.
2017TNSCR.994....1G    Altcode:
  No abstract at ADS

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Title: ePESSTO Transient Classification Report for 2017-09-12
Authors: Rybicki; Gromadzki; Fraser; Yaron, O.; Knezevic, N.
2017TNSCR.997....1R    Altcode:
  No abstract at ADS

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Title: Modeling Molecular Hyperfine Line Emission
Authors: Keto, Eric; Rybicki, George
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, NH<SUB>3</SUB>,
  N<SUB>2</SUB>H<SUP>+</SUP>, and C<SUP>17</SUP>O. 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 N<SUB>2</SUB>H<SUP>+</SUP>
  hyperfine lines made with approximate and more exact rates and find
  that satisfactory results are obtained.

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Title: Thermal X-Rays from Millisecond Pulsars: Constraining the
    Fundamental Properties of Neutron Stars
Authors: Bogdanov, Slavko; Grindlay, Jonathan E.; Rybicki, George B.
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<SUB>⊙</SUB>, the stellar radius is constrained to be R &gt; 9.4 km
  and R &gt; 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.

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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.
2008xru..confE..85H    Altcode:
  No abstract at ADS

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Title: Constraints on Neutron Star Properties from X-Ray Observations
    of Millisecond Pulsars
Authors: Bogdanov, Slavko; Rybicki, George B.; Grindlay, Jonathan E.
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 M<SUB>solar</SUB>,
  the model restricts the allowed stellar radii to R=6.8-13.8 km (90%
  confidence) and R&gt;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.

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Title: Comptonization in the X-ray Spectra of Radio Millisecond
    Pulsars
Authors: Bogdanov, Slavko; Grindlay, Jonathan E.; Rybicki, George B.
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 &gt;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.

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Title: X-Rays from Radio Millisecond Pulsars: Comptonized Thermal
    Radiation
Authors: Bogdanov, Slavko; Grindlay, Jonathan E.; Rybicki, George B.
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.

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Title: Improved Fokker-Planck Equation for Resonance-Line Scattering
Authors: Rybicki, George B.
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
  &amp; 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 &amp; Miralda-Escudé.

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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.
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
  M<SUB>solar</SUB> is assumed, our spectral fits require that its radius
  be in the range R<SUB>ns</SUB>=14.5<SUP>+1.8</SUP><SUB>-1.6</SUB> 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 M<SUB>ns</SUB>=2.20<SUP>+0.03</SUP><SUB>-0.16</SUB>
  M<SUB>solar</SUB> 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.

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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.
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 M<SUB>1</SUB>~8 M<SUB>solar</SUB>. 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 L<SUB>X</SUB>~3.5×10<SUP>30</SUP>
  ergs s<SUP>-1</SUP> (0.3-7 keV). In our search for a thermal
  component, we fitted our Chandra data to a power-law model (with
  slope and N<SUB>H</SUB> 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, N<SUB>H</SUB>~1.2×10<SUP>20</SUP>
  cm<SUP>-2</SUP>, we obtained very strong limits on a
  hypothetical thermal source: kT<SUB>∞</SUB>&lt;0.011 keV and
  L<SUB>∞,th</SUB>&lt;9.4×10<SUP>30</SUP> ergs s<SUP>-1</SUP> (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
  &gt;~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 M<SUB>solar</SUB> compact object that might
  be masked by the interstellar medium. However, our upper limit on the
  total luminosity of J1118 of 1.3×10<SUP>31</SUP> ergs s<SUP>-1</SUP>
  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.

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Title: Radiative Transfer and Starless Cores
Authors: Keto, Eric; Rybicki, George B.; Bergin, Edwin A.; Plume, René
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 N<SUB>2</SUB>H<SUP>+</SUP>
  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 κ<SUB>1.3mm</SUB>=0.04 cm<SUP>2</SUP> g<SUP>-1</SUP>,
  consistent with models of dust grains that have opacities enhanced by
  ice mantles and fluffy aggregrates.

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Title: Bondi Accretion and the Problem of the Missing Isolated
    Neutron Stars
Authors: Perna, Rosalba; Narayan, Ramesh; Rybicki, George; Stella,
   Luigi; Treves, Aldo
2003ApJ...594..936P    Altcode: 2003astro.ph..5421P
  A large number of neutron stars (NSs), ~10<SUP>9</SUP>, 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.

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Title: A New Kinetic Equation for Compton Scattering
Authors: Rybicki, George B.
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.

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Title: The Structure of Self-gravitating Polytropic Systems with n
    around 5
Authors: Medvedev, Mikhail V.; Rybicki, George
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, ψ<SUB>n+ɛ</SUB>(x) of the nonlinear
  Poisson equation with the polytropic index n+ɛ, given the solution
  ψ<SUB>n</SUB>(x) with the polytropic index n, for any positive or
  negative ɛ, such that |ɛ|&lt;&lt;1. A similar technique has been
  developed independently by Seidov &amp; Kuzakhmedov. Application of
  this method to the spherically symmetric stellar polytropes with n~=5
  yields the solutions that describe spatially bound systems, if n&lt;5,
  and the formation of a second core, if n&gt;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.

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Title: Scattered Lyα Radiation around Sources before Cosmological
    Reionization
Authors: Loeb, Abraham; Rybicki, George B.
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 z<SUB>s</SUB>~10 scatter over a characteristic angular
  radius of ~15" around the source and compose a line that is broadened
  and redshifted by ~10<SUP>3</SUP> km s<SUP>-1</SUP> 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.

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Title: Polarization of the Lyα Halos around Sources before
    Cosmological Reionization
Authors: Rybicki, George B.; Loeb, Abraham
1999ApJ...520L..79R    Altcode: 1999astro.ph..3291R
  In Loeb &amp; 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.

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Title: Radiative Transfer
Authors: Rybicki, George B.
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.

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Title: Magnification Ratio of the Fluctuating Light in Gravitational
    Lens 0957+561
Authors: Press, William H.; Rybicki, George B.
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.

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Title: Spectral line signatures of relativistic disks
Authors: Rybicki, George B.; Bromley, Benjamin C.
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.

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Title: Emission Line Formation in a Relativistic Accretion Disk
Authors: Rybicki, George B.; Bromley, Benjamin C.
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.

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Title: Properties of Statistical Equilibrium Equations: Positivity
    and Uniqueness
Authors: Rybicki, George B.
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.

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Title: Desperately Seeking Non-Gaussianity: The Light Curve of
    0957+561
Authors: Press, W. H.; Rybicki, G. B.
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.

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Title: Radiative Transfer
Authors: Rybicki, George B.
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.

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Title: Deprojection of axially symmetric objects
Authors: Kochanek, Christopher S.; Rybicki, George B.
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.
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.
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\(n<SUP>3</SUP>\) 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.
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&lt;&lt;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.
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.
1994NASCP3278..149W    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Study of Reverberation Lags and Spreads in NGC 5548
Authors: Rybicki, George B.; Kleyna, Jan T.
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.
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<SUP>-1</SUP>. 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.
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 &lt;z &lt;4.3 without
  actually resolving any single cloud. We find that the mean absorption
  increases with z approximately as a power law (1 + Z)<SUP>gamma</SUP>
  + 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.
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.
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.
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.
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.
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 R<SUB>II</SUB> 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.
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 τ<SUB>AB</SUB> =
  548<SUP>+19</SUP><SUB>-16</SUB> days (95% C.L.). Combined analysis of
  the optical and radio data yield τ<SUB>AB</SUB> = 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.
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.
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.
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 CO<SUB>2</SUB> 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
1987BAAS...19..702O    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Radiation driven instabilities
Authors: Rybicki, George B.
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.
1987sowi.conf..177O    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Deprojection of Galaxies - how much can BE Learned
Authors: Rybicki, G. B.
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.
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.
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.
1986BAAS...18..953O    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Radiative Processes in Astrophysics
Authors: Rybicki, George B.; Lightman, Alan P.
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.
1986AN....307..170R    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: A Gridless Fourier Method
Authors: Rybicki, G. B.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
1984BAAS...16Q.725R    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Escape probability methods.
Authors: Rybicki, G. B.
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.
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.
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.
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.
1983ApJ...271..888H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Radiative Processes in Astrophysics
Authors: Rybicki, G. B.; Lightman, A. P.; Porter, A.
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.
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.
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.
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.
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.
1982ApL....22...73R    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Comptonization by cold electrons
Authors: Lightman, A. P.; Lamb, D. Q.; Rybicki, G. B.
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.
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.
1981BAAS...13..852R    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Line Formation in Accretion Disks
Authors: Carroll, D.; Hummer, D. G.; Rybicki, G. B.
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.
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.
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.
1980BAAS...12..798H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Inverse Compton reflection - The steady-state theory
Authors: Lightman, A. P.; Rybicki, G. B.
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.
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.
1980S&T....59..509R    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Radiative Processes in Astrophysics
Authors: Rybicki, G. B.; Lightman, A. P.
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.
1979BAAS...11..788V    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Inverse Compton Reflection.
Authors: Lightman, A. P.; Rybicki, G. B.
1979BAAS...11Q.793L    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Inverse Compton reflection: time-dependent theory.
Authors: Lightman, A. P.; Rybicki, G. B.
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.
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<SUP>+</SUP> fine-structure levels.
Authors: Flannery, B. P.; Rybicki, G. B.; Sarazin, C. L.
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.
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.
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.
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.
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.
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.
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.
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.
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.
1976BAAS....8R.546H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Galactic Mass Determinations from Incomplete Rotation Curves
Authors: Schaefer, M. M.; Rybicki, G.; Lecar, M.
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 <SUP>2</SUP>, (B) the mass density is
  zero, and (C) the mass density falls off as 1/r <SUP>3</SUP>. 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.
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.
1975MNRAS.170..423R    Altcode:
  This note presents evidence that the claims advanced by Wilson, Tung
  &amp; Sen regarding the adequacy of Wilson &amp; 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 &lt;
  &lt;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.
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: R<SUB>o</SUB> From Galactic Disk Models.
Authors: Rybicki, G.; Lecar, M.; Schaefer, M.
1974BAAS....6..453R    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Wiener Filtering of Sampled Astronomical Spectra.
Authors: Rybicki, G.; Harrison, P.
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
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.
1973BAAS....5..413N    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Relaxation Times in Strictly Disk Systems
Authors: Rybicki, G. B.
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.
1972lfpm.conf..145R    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Exact Statistical Mechanics of a One-Dimensional
    Self-Gravitating System
Authors: Rybicki, G. B.
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.
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.
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
  <SUP>2</SUP> 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
    R<SUB>II-A</SUB>(x, xÿ) and of the associated scattering integral.
Authors: Adams, T. F.; Hummer, D. G.; Rybicki, G. B.
1971JQSRT..11.1365A    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: A modified Feautrier method.
Authors: Rybicki, G. B.
1971JQSRT..11..589R    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The searchlight problem with isotropic scattering.
Authors: Rybicki, G. B.
1971JQSRT..11..827R    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Formation of Spectral Lines
Authors: Hummer, D. G.; Rybicki, G.
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.
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&lt; , 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.
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.
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.
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.
1968rla..conf..213H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Redshifted Line Profiles from Differentially Expanding
    Atmospheres
Authors: Hummer, D. G.; Rybicki, G. B.
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.
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.
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.
1967IAUS...28..471R    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Computational Methods for Non-LTE Line_transfer Problems
Authors: Hummer, D. G.; Rybicki, G.
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.
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.
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.
1965SAOSR.174..149R    Altcode:
  No abstract at ADS