156 research outputs found
The Observational Appearance of Slim Accretion Disks
We reexamine the hypothesis that the optical/UV/soft X-ray continuum of
Active Galactic Nuclei is thermal emission from an accretion disk. Previous
studies have shown that fitting the spectra with the standard, optically thick
and geometrically thin accretion disk models often led to luminosities which
contradict the basic assumptions adopted in the standard model. There is no
known reason why the accretion rates in AGN should not be larger than the thin
disk limit. In fact, more general, slim accretion disk models are
self-consistent even for moderately super-Eddington luminosities. We calculate
here spectra from a set of thin and slim, optically thick accretion disks. We
discuss the differences between the thin and slim disk models, stressing the
implications of these differences for the interpretation of the observed
properties of AGN. We found that the spectra can be fitted not only by models
with a high mass and a low accretion rate (as in the case of thin disk fitting)
but also by models with a low mass and a high accretion rate. In the first case
fitting the observed spectra in various redshift categories gives black hole
masses around 10^9 solar masses for a wide range of redshifts, and for
accretion rates ranging from 0.4 to 8 solar masses/year. In the second case the
accretion rate is around 10^2 solar masses/year for all AGN and the mass ranges
from 3*10^6 to 10^8 solar masses. Unlike the disks with a low accretion rate,
the spectra of the high-accretion-rate disks extend into the soft X-rays. A
comparison with observations shows that such disks could produce the soft X-ray
excesses claimed in some AGNs. We show also that the sequence of our models
with fixed mass and different accretion rates can explain the time evolution of
the observed spectra in Fairall 9.Comment: LaTeX file, 21 pages, 21 figures (on request from [email protected])
accepted by Astrophysical Journa
Studies of Thermally Unstable Accretion Disks around Black Holes with Adaptive Pseudospectral Domain Decomposition Method. II. Limit-Cycle Behavior in accretion disks around Kerr black holes
For the first time ever, we derive equations governing the time-evolution of
fully relativistic slim accretion disks in the Kerr metric, and numerically
construct their detailed non-stationary models. We discuss applications of
these general results to a possible limit-cycle behavior of thermally unstable
disks. Our equations and numerical method are applicable in a wide class of
possible viscosity prescriptions, but in this paper we use a diffusive form of
the "standard alpha prescription" that assumes the viscous torque is
proportional to the total pressure. In this particular case, we find that the
parameters which dominate the limit-cycle properties are the mass-supply rate
and the value of the alpha-viscosity parameter. Although the duration of the
cycle (or the outburst) does not exhibit any clear dependence on the black hole
spin, the maximal outburst luminosity (in the Eddington units) is positively
correlated with the spin value. We suggest a simple method for a rough estimate
of the black hole spin based on the maximal luminosity and the ratio of
outburst to cycle durations. We also discuss a temperature-luminosity relation
for the Kerr black hole accretion discs limit-cycle. Based on these results we
discuss the limit-cycle behavior observed in microquasar GRS 1915+105. We also
extend this study to several non-standard viscosity prescriptions, including a
"delayed heating" prescription recently stimulated by the recent MHD
simulations of accretion disks.Comment: 36 pages, 6 figures, 1 table. Accepted by ApJ
Why Low-Mass Black-Hole Binaries Are Transient
We consider transient behavior in low-mass X-ray binaries. In short-period
neutron-star systems (orbital period less than ~ 1d) irradiation of the
accretion disk by the central source suppresses this except at very low mass
transfer rates. Formation constraints however imply that a significant fraction
of these neutron star systems have nuclear-evolved main-sequence secondaries
and thus mass transfer rates low enough to be transient. But most short-period
low-mass black-hole systems will form with unevolved main-sequence companions
and have much higher mass transfer rates. The fact that essentially all of them
are nevertheless transient shows that irradiation is weaker, as a direct
consequence of the fundamental black-hole property - the lack of a hard stellar
surface.Comment: 13 pages (including 3 figures); accepted for publication in Ap
Interlayer Exchange Coupling in (Ga,Mn)As-based Superlattices
The interlayer coupling between (Ga,Mn)As ferromagnetic layers in
all-semiconductor superlattices is studied theoretically within a tight-binding
model, which takes into account the crystal, band and magnetic structure of the
constituent superlattice components. It is shown that the mechanism originally
introduced to describe the spin correlations in antiferromagnetic EuTe/PbTe
superlattices, explains the experimental results observed in ferromagnetic
semiconductor structures, i.e., both the antiferromagnetic coupling between
ferromagnetic layers in IV-VI (EuS/PbS and EuS/YbSe) superlattices as well as
the ferromagnetic interlayer coupling in III-V ((Ga,Mn)As/GaAs) multilayer
structures. The model allows also to predict (Ga,Mn)As-based structures, in
which an antiferromagnetic interlayer coupling could be expected.Comment: 4 pages, 3 figure
Studies of Thermally Unstable Accretion Disks around Black Holes with Adaptive Pseudo-Spectral Domain Decomposition Method I. Limit-Cycle Behavior in the Case of Moderate Viscosity
We present a numerical method for spatially 1.5-dimensional and
time-dependent studies of accretion disks around black holes, that is
originated from a combination of the standard pseudo-spectral method and the
adaptive domain decomposition method existing in the literature, but with a
number of improvements in both the numerical and physical senses. In
particular, we introduce a new treatment for the connection at the interfaces
of decomposed subdomains, construct an adaptive function for the mapping
between the Chebyshev-Gauss-Lobatto collocation points and the physical
collocation points in each subdomain, and modify the over-simplified
1-dimensional basic equations of accretion flows to account for the effects of
viscous stresses in both the azimuthal and radial directions. Our method is
verified by reproducing the best results obtained previously by Szuszkiewicz &
Miller on the limit-cycle behavior of thermally unstable accretion disks with
moderate viscosity. A new finding is that, according to our computations, the
Bernoulli function of the matter in such disks is always and everywhere
negative, so that outflows are unlikely to originate from these disks. We are
encouraged to study the more difficult case of thermally unstable accretion
disks with strong viscosity, and wish to report our results in a subsequent
paper.Comment: 29 pages, 8 figures, accepted by Ap
Dissipation Instability in the Accretion Disk
The model of a geometrically thin gaseous disk in the external gravitational
potential is considered. The dinamics of small nonaxisymmetric perturbations in
the plane of the accretion disk with dissipative effects is investigated. It is
showed, that conditions of development and parameters of unstable oscillation
modes in the opticaly thick accretion disk are strongly depended on the models
of viscosity and opacity.Comment: Plain TeX, 6 pages, 2 figures (GIF), Submitted to Astron. Astrophys.
Transaction
A Note on Bimodal Accretion Disks
The existence of bimodal disks is investigated. Following a simple argument
based on energetic considerations we show that stationary, bimodal accretion
disk models in which a Shakura--Sunyaev disk (SSD) at large radii matches an
advection dominated accretion flow (ADAF) at smaller radii are never possible
using the standard slim disk approach, unless some extra energy flux is
present. The same argument, however, predicts the possibility of a transition
from an outer Shapiro--Lightman--Eardley (SLE) disk to an ADAF, and from a SLE
disk to a SSD. Both types of solutions have been found.Comment: 9 pages including 9 figures, accepted for publication in The
Astrophysical Journa
Universal spectral shape of high accretion rate AGN
The spectra of quasars and NLS1 galaxies show surprising similarity in their
spectral shape. They seem to scale only with the accretion rate. This is in
contradiction with the simple expectations from the standard disk model which
predicts lower disk temperature for higher black hole mass. Here we consider
two mechanisms modifying the disk spectrum: the irradiation of the outer disk
due to the scattering of the flux by the extended ionized medium (warm absorber
and the development of the warm Comptonizing disk skin under the effect of the
radiation pressure instability. Those two mechanisms seem to lead to a spectrum
which indeed roughly scales, as observed, only with the accretion rate. The
scenario applies only to objects with relatively high luminosity to the
Eddington luminosity ratio for which disk evaporation is inefficient.Comment: 14 pages, 14 figures, 1 table, accepted for publication in A&
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