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NASA NTRS1983

Compton heated winds and coronae above accretion disks. I Dynamics

X rays emitted in the inner part of an accretion disk system can heat the surface of the disk farther out, producing a corona and possibly driving off a strong wind. The dynamics of Compton-heated coronae and winds are analyzed using an approximate two-dimensional technique to estimate the mass loss rate as a function of distance from the source of X rays. The findings have important dynamical implications for accretion disks in quasars, active galactic nuclei, X ray binaries, and cataclysmic variables. These include: mass loss from the disk possibly comparable with or exceeding the net accretion rate onto the central compact object, which may lead to unstable accretion; sufficient angular momentum loss in some cases to truncate the disk in a semidetached binary at a smaller radius than that predicted by tidal truncation theories; and combined static plus ram pressure in the wind adequate to confine line-emitting clouds in quasars and Seyfert galaxies.

Begelman, M. C.↗

NASA NTRS1992

Accretion disks in active galactic nuclei - Vertically averaged models

Simple models of accretion disks surrounding supermassive black holes in AGN are constructed. By considering several different opacity laws, it is shown that some conclusions arrived at by previous investigators are model-dependent. A simple technique for solving the full vertically averaged disk equations, taking into account both self-gravity and central object gravity and gas pressure and radiation pressure, is presented. The self-consistency of the standard non-self-gravitating, geometrically thin accretion disk models is discussed, and the findings of Lin and Shields (1986) that such models are valid only in a restricted parameter space are confirmed.

Cannizzo, John K.↗

NASA NTRS1987

Constraints on quasar accretion disks from the optical/ultraviolet/soft X-ray big bump

The simplest accretion-disk models have difficulties explaining the optical/UV/soft-X-ray 'big bump' in quasars. Here more realistic models are investigated, incorporating opacity and inclination effects while retaining an analytic form for ease of computation. It is found that opacity effects can explain the uniform 20,000-30,000 K 'maximum disk temperature' derived by previous workers. The observed spectral turndown would be the result of the onset of electron scattering and should not be identified with the hottest part of the disk. The frequency at which this spectral turndown occurs is only weakly dependent on the accretion-disk parameters. These opacity effects also allow high-frequency EUV and soft-X-ray extensions of the big bump without exceeding the Eddington limit strongly. For a wide range of disk parameters, the EUV spectra of quasar disks should have spectral slopes near -1.

Czerny, Bozena↗

NASA NTRS1993

A model of an X-ray-illuminated accretion disk and corona

The X-ray-illuminated surface of the accretion disk in a low-mass X-ray Binary (LMXRB) and the X-ray-heated corona above the disk produce optical, UV, and soft X-ray emission lines. This paper presents 1D models of the emission line spectra and the vertical temperature and density structures at different radii. The models include a detailed treatment of the important atomic processes and an escape probability treatment of radiative transfer. Soker and Raymond (1993) use the density structure predicted by these models for a 2D Monte Carlo simulation of the photon scattering in the accretion disk corona (ADC) to examine the effects of the ADC on the angular distribution of X-rays and the flux of X-rays incident on the outer disk. This paper concentrates on the emission line fluxes for various elemental abundances and disk parameters. The UV lines of the classic LMXRBs are consistent with the model predictions. Some CNO processing is necessary to account for the nitrogen and helium abundances in Sco X-1 and other LMXRBs. Comparison of the models with observed spectra also points to a soft X-ray component with luminosity comparable to the hard X-rays. The models predict a substantial luminosity in the group of highly ionized iron lines near 100 A.

Raymond, John C.↗

NASA NTRS2009

X-ray Reflected Spectra from Accretion Disk Models. I. Constant Density Atmospheres

We present new models for illuminated accretion disks, their structure and reprocessed emission. We consider the effects of incident X-rays on the surface of an accretion disk by solving simultaneously the equations of radiative transfer, energy balance and ionization equilibrium over a large range of column densities. We assume plane-parallel geometry and azimuthal symmetry, such that each calculation corresponds to a ring at a given distance from the central object. Our models include recent and complete atomic data for K-shell of the iron and oxygen isonuclear sequences. We examine the effect on the spectrum of fluorescent Ka line emission and absorption in the emitted spectrum. We also explore the dependence of the spectrum on the strength of the incident X-rays and other input parameters, and discuss the importance of Comptonization on the emitted spectrum.

Garcia, Javier↗

NASA NTRS1984

The evolution of the inner regions of viscous accretion disks surrounding neutron stars

The structure and evolution of the inner regions of an accretion disk around a neutron star have been investigated with respect to variations in the mass flow rate. In order to examine the sensitivity of the general response of the disk to the form of viscous dissipation, several theoretical constraints on disk evolution are proposed on the basis of current accretion disk theory. It is shown through a series of calculations that under certain circumstances nonlocal radial energy transfer can stabilize regions of the disk which appear to be unstable by local analysis. When viscous stress was scaled with the total pressure of the disk, global analysis was found to be consistent with local analysis, and instability was manifested in luminosity fluctuations and in bursts of less than 10 s. Analysis of the bursts in their limit cycles showed that the disk remained optically thick and geometrically thin throughout its evolution. Substantial agreement was found between the theoretical results and the available observational data.

Taam, R. E.↗

NASA NTRS1999

The Radio Jets and Accretion Disk in NGC 4261

The structure of AGN accretion disks on sub-parsec scales can be probed through free-free absorption of synchrotron emission from the base of symmetric radio jets.

radio jets accretion disk NGC AGN↗

NASA NTRS1977

On the accretion disk model of QSOs

The accretion-disk model for the optical continuum of QSOs has been investigated using the standard viscous-stress ('alpha-P') formalism. It is found that with a maximum allowable mass of 10 billion solar masses, the standard model with sub-Eddington flow falls a factor of 10 short of the luminosity of the brightest QSOs. Models with the higher mass-flow rates required to reach the maximum luminosities produce most of their energy in an optically thin region which has not yet been self-consistently treated. The disk model with sub-Eddington flow has difficulty in producing a power law of the correct slope over the optical band. It is suggested that nonthermal processes may be needed to explain the optical properties of QSOs.

Callahan, P. S.↗

NASA NTRS1989

Particle dynamics and gamma-ray emission in the magnetospheres of neutron stars with accretion disks

Whether the existence of cold accretion disks around isolated, magnetized neutron stars might result in a distinct observational signature due to the Compton drag interaction between the electrostatically accelerated changes and the thermal radiation emitted by these disks. The particle dynamics resulting from the combined effects of electrostatic acceleration and the Compton drag interaction are determined. In the course of their deceleration, the charged particles boost the thermal disk radiation to gamma-ray energies, which may result in a significant spectral component. Cold disks are too underluminous to significantly retard the flow, and if such disks are present in pulsars, only a small fraction of the particle energy is transferred to the radiation field. The results also indicate that steady, unidirectional particle acceleration is unlikely to be the mechanism operating in gamma-ray burst sources.

Melia, Fulvio↗

NASA NTRS1990

On neglect of nonlinear momentum terms in solar nebula accretion disk models

The standard model for accretion disks (e.g., Pringle, 1981; Lin and Papaloizou, 1985) parameterizes the radial eddy angular momentum flux which is quadratically nonlinear in velocity, solely by a viscous diffusion process which is linear in velocity. This paper shows that the eddy angular momentum flux, in addition to a gradient dissipative (viscous) flux, involves a countergradient accelerating flux. Therefore, the momentum parameterization in the standard model is in general unjustified. The formalism of the parameterization can be justified only by recognizing that the eddy viscosity used in the standard model is in fact a pseudoviscosity which includes the effects of countergradient fluxes, and which therefore may have a magnitude less than the eddy diffusivity K for mixing trace constituents in the disk.

Prinn, Ronald G.↗

NASA NTRS2013

Can the 62 Day X-ray Period of ULX M82 X-1 Be Due to a Precessing Accretion Disk?

We have analyzed all the archival RXTE/PCA monitoring observations of the ultraluminous X-ray source (ULX) M82 X-1 in order to study the properties of its previously discovered 62 day X-ray period (Kaaret & Feng 2007). Based on the high coherence of the modulation it has been argued that the observed period is the orbital period of the binary. Utilizing a much longer data set than in previous studies we find: (1) The phase-resolved X-ray (3-15 keV) energy spectra - modeled with a thermal accretion disk and a power-law corona - suggest that the accretion disk's contribution to the total flux is responsible for the overall periodic modulation while the power-law flux remains approximately constant with phase. (2) Suggestive evidence for a sudden phase shift-of approximately 0.3 in phase (20 days)-between the first and the second halves of the light curve separated by roughly 1000 days. If confirmed, the implied timescale to change the period is approx. = 10 yrs, which is exceptionally fast for an orbital phenomenon. These independent pieces of evidence are consistent with the 62 day period being due to a precessing accretion disk, similar to the so-called super-orbital periods observed in systems like Her X-1, LMC X-4, and SS433. However, the timing evidence for a change in the period needs to be confirmed with additional observations. This should be possible with further monitoring of M82 with instruments such as the X-ray telescope (XRT) on board Swift.

ULX M82↗

NASA NTRS2012

Effects of Accretion Disks on Spins and Eccentricities of Binaries, and Implications for Gravitational Waves

Effects of accretion disks on spins and eccentricities of binaries, and implications for gravitational waves. John Baker Space-based gravitational wave observations will allow exquisitely precise measurements of massive black hole binary properties. Through several recently suggested processes, these properties may depend on interactions with accretion disks through the merger process. I will discuss ways that accretion may influence those binary properties which may be probed by gravitational-wave observations.

Baker, John↗

NASA NTRS1992

Herbig Ae/Be stars - Intermediate-mass stars surrounded by massive circumstellar accretion disks

The proposition that Herbig Ae/Be stars are young intermediate mass stars surrounded by optically thick accretion disks is explored. From a study of 47 such objects, a subset of 30 stars is identified whose spectral energy distributions can be interpreted convincingly in terms of pre-main sequence stars surrounded by massive optically thick circumstellar accretion disks. Constraints on the physical properties of the disks, such as size, mass, accretion rate, lifetime, and radial structure are derived from the photometric data.

Hillenbrand, Lynne A.↗

NASA NTRS1992

The formation of coronal regions in accretion disks

A mechanism for heating the hot low-density gas or winds above accretion disks, which is similar to that used to model chromospheric heating in the sun and other stars, is proposed. Sound waves propagating through an accretion disk are refracted away from the central plane by the strong density gradient. As they move into regions of lower density, the sound waves accelerate to form shocks, which heat the gas, leading to the formation of a hot low-density region. The steeper density gradients present in disks make the process more efficient than in stellar atmospheres. Results of hydrodynamical simulations show that waves with frequencies similar to the local Keplerian frequency lead to the most efficient heating. For the optically thin region modeled in the present study, the result is the formation of coronal regions with densities less than about 10 exp 10/cu cm, and the temperatures ranging from 10 exp 4 to 6 K over a few scale heights. It is argued that the hot low-density gas which results from the shock heating is responsible for the observed UV lines from cataclysmic variables, as well as the spatially coincidental lines of H and He I.

Murray, Stephen D.↗

NASA NTRS1987

The wind-disk interaction in X-ray burst sources. II - Collimation by thick accretion disks

The wind-disk interaction model in type I cosmic X-ray bursts is here examined in the case where the accretion disk surrounding the neutron star is geometrically thick. The funnel structure along the rotation axis of such a system may collimate the radiatively driven wind following a thermonuclear flash in the star's surface layers, and could thereby enhance the radiative flux (diffusive plus advective) along the beam direction. It is shown that for plausible conditions, this mechanism can account for the super-Eddington fluxes observed in some bursts. The overall time scale for variability, the color temperature, and the spectral shape in this picture are consistent with the observed properties of type I X-ray bursts.

Melia, Fulvio↗

NASA NTRS1994

An upper limit to the electrodynamic power output from a thin accretion disk around a black hole

We examine the electrodynamic power output from a thin accretion disk surrounding a Kerr black hole. We derive an upper limit for the total power. We find that this limit is only moderately sensitive to most of the relativistic correction factors. In fact, this limit is less sensitive to the change in the black hole spin than one would expect from the change in the inner radius of the accretion disk. The disk around a rapidly rotating black hole will produce more nonthermal radiation than a similar disk around a slowly rotating black hole of identical mass, but the difference is only a factor of approximately 1.7.

Park, Seok Jae↗

NASA NTRS1997

Advection in Accretion Disk Boundary Layers

Recent numerical simulations of accretion disk boundary layers have shown qualitatively the importance tance of advected energy in the inner region of the disk. In this short paper we present quantitative results of advective boundary layers in the optically thick regime. Numerical results are obtained for various systems, by means of a one-dimensional time-dependent numerical code. At high accretion mass rates, dot-M approx. = 10(exp -4) solar mass/yr, or low values of the viscosity parameter, alpha approx. = 0.001-0.01 (characteristic of FU Orionis systems and some symbiotic stars), the optical thickness in the inner part of the disk becomes very large (tau much greater than 1). The disk, unable to cool efficiently, becomes geometrically thick (H/r approx. = 0.5). The energy dissipated in the dynamical boundary layer is radiated outward to larger radii and advected into the star. The boundary-layer luminosity is only a fraction of its expected value; the rest of the energy is advected into the star. The fraction of the advected energy is zeta = L(sub adv)/L(sub acc) approx. = 0.1 in symbiotic stars (accretion onto a low-mass main-sequence star) and zeta approx. = 0.2 in FU Ori systems (accretion onto a pre-main sequence star).

Godon, Patrick↗

NASA NTRS1988

Gamma-ray burst reprocessing in an accretion disk

The observational consequences of the reprocessing of gamma-ray burst (GRB) photons by an accretion disk surrounding a neutron star are explored. Reprocessing of gamma rays by cold disks driven by dynamical viscosity of nonrelativistic, degenerate electrons results in an optical/UV fluence relative to the gamma-ray fluence at earth of greater than 0.001, for GRB energies of (1-10) x 10 to the 37th ergs, and distances of 15-250 pc. The spectrum peaks in the optical/UV with a soft X-ray cutoff due to the inner edge of the disk, which lies very near the surface of the neutron star for magnetic fields less than 1 TG. The optical/UV peak is partially due to reprocessing in the outer part of the cold disk, which remains optically thick to gamma rays. At these distances, a positive identification of a quiescent counterpart is expected at an IR (K) sensitivity less than 2 microJy. On the other hand, compact binary systems with standard alpha disks and accretion rates resulting from Roche-lobe overflow must be located at least several hundred pc away in order to have escaped detection down to the current IR sensitivity. The optical/UV fluence resulting from reprocessing in these systems is deficient by at least an order of magnitude compared to the observed values.

Melia, Fulvio↗