25 research outputs found
Where is a Marginally Stable Last Circular Orbit in Super-Critical Accretion Flow?
Impressed by the widespread misunderstanding of the issue, we return to the
old question of the location of the inner edge of accretion disk around black
hole. We recall the fundamental results obtained in the 1970's and 1980's by
Warsaw and Kyoto research groups that proved, in particular, that the inner
edge does not coincide with the location of the innermost stable Keplerian
circular orbit. We give some novel illustrations of this particular point and
of some other fundamental results obtained by Warsaw and Kyoto groups. To
investigate the flow dynamics of the inner edge of accretion disk, we carefully
solve the structure of the transonic flow and plot the effective potential
profile based on the angular-momentum distribution calculated numerically. We
show that the flow does not have a potential minimum for accretion rates, {\dot
M} > 10 L_E/c^2 (with L_E being the Eddington luminosity and
being the speed of light). This property is realized even in relatively
small viscosity parameters
(i.e., \alpha ~ 0.01), because of the effect of pressure gradient. In
conclusion, the argument based on the last circular orbit of a test particle
cannot give a correct inner boundary of the super-critical flow and the inner
edge should be determined in connection with radiation efficiency. The same
argument can apply to optically thin ADAF. The interpretation of the observed
QPO frequencies should be re-considered, since the assumption of Kepler
rotation velocity can grossly over- or underestimate the disk rotation
velocity, depending on the magnitude of viscosity.Comment: 7 pages, 3 figures, accepted for PAS
Eclipsing light curves for accretion flows around a rotating black hole and atmospheric effects of the companion star
We calculate eclipsing light curves for accretion flows around a rotating
black hole taking into account the atmospheric effects of the companion star.
In the cases of no atmospheric effects, the light curves contain the
information of the black hole spin because most of the X-ray photons around 1
keV usually come from the blueshifted part of the accretion flow near the black
hole shadow, and the size and the position of the black hole shadow depend on
the spin. In these cases, when most of the emission comes from the vicinity of
the event horizon, the light curves become asymmetric at ingress and egress. We
next investigate the atmospheric absorption and scattering effects of the
companion stars. By using the solar-type atmospheric model, we have taken into
account the atmospheric effects of the companion star, such as the
photoionization by HI and HeI. We found that the eclipsing light curves
observed at 1 keV possibly contain the information of the black hole spin.
However, in our atmospheric model, the effects of the atmosphere are much
larger than the effects of the black hole spin. Therefore, even in the case
that the light curves contain the information of the black hole spin, it may be
difficult to extract the information of the black hole spin if we do not have
the realistic atmospheric profiles, such as the temperature, and the number
densities for several elements. Even in such cases, the light-curve asymmetries
due to the rotation of the accretion disc exist. Only when we have the reliable
atmospheric model, in principle, the information of the strong-gravity regions,
such as the black hole spin, can be obtained from the eclipsing light curves.Comment: Takahashi R., Watarai K., 2007, MNRAS, 374, 151
Eclipsing Light-Curve Asymmetry for Black-Hole Accretion Flows
We propose an eclipsing light-curve diagnosis for black-hole accretion flows.
When emission from an inner accretion disk around a black hole is occulted by a
companion star, the observed light curve becomes asymmetric at ingress and
egress on a time scale of 0.1-1 seconds. This light-curve analysis provides a
means of verifying the relativistic properties of the accretion flow, based on
the special/general relativistic effects of black holes. The ``skewness'' for
the eclipsing light curve of a thin disk is , whereas that of a slim
disk is , since the innermost part is self-occulted by the disk's outer
rim.Comment: 7 pages, 4 figures, PASJ accepte
Slim Disk: Viscosity Prescriptions and Observational Implications
We examine the effects of the different viscosity prescriptions and the
magnitude of the viscosity parameter, , on the structure of the slim
disk, and discuss the observational implications on accretion-flow into a
stellar-mass black hole. For the range of we
calculate the disk spectra and from spectral fitting we derive \Tin, maximum
temperature of the disk, \Rin, the size of the region emitting blackbody
radiation with \Tin, and , the
slope of the effective temperature distribution. It was founded that the
estimated \Tin slightly increases as increases. This is because the
larger the magnitude of viscosity is, the larger becomes the accretion velocity
and, hence, the more enhanced becomes advective energy transport, which means
less efficient radiative cooling and thus higher temperatures. Furthermore we
check different viscosity prescriptions with the form of the viscous stress
tensor of , where is
the ratio of gas pressure to total pressure, and is a parameter (). For we have previously found that as luminosity approaches
the Eddington, L_{\rm E, \Rin decreases below 3\rg (with
being Schwarzschild radius) and the effective temperature becomes flatter,
. Such a slim-disk nature does not appear when
is large, , even at . Hence, the temperature of
the innermost region of the disk sensitively depends on the value. We can
rule out the case with large , since it will not be able to
produce a drop in \Rin with an increase in luminosity as was observed in an
ultraluminous X-ray source, IC~342, source 1.Comment: 8 pages, 7 figures, accepted for PAS
Accretion Disk Spectra of the Ultra-luminous X-ray Sources in Nearby Spiral Galaxies and Galactic Superluminal Jet Sources
Ultra-luminous Compact X-ray Sources (ULXs) in nearby spiral galaxies and
Galactic superluminal jet sources share the common spectral characteristic that
they have unusually high disk temperatures which cannot be explained in the
framework of the standard optically thick accretion disk in the Schwarzschild
metric. On the other hand, the standard accretion disk around the Kerr black
hole might explain the observed high disk temperature, as the inner radius of
the Kerr disk gets smaller and the disk temperature can be consequently higher.
However, we point out that the observable Kerr disk spectra becomes
significantly harder than Schwarzschild disk spectra only when the disk is
highly inclined. This is because the emission from the innermost part of the
accretion disk is Doppler-boosted for an edge-on Kerr disk, while hardly seen
for a face-on disk. The Galactic superluminal jet sources are known to be
highly inclined systems, thus their energy spectra may be explained with the
standard Kerr disk with known black hole masses. For ULXs, on the other hand,
the standard Kerr disk model seems implausible, since it is highly unlikely
that their accretion disks are preferentially inclined, and, if edge-on Kerr
disk model is applied, the black hole mass becomes unreasonably large (> 300
M_solar). Instead, the slim disk (advection dominated optically thick disk)
model is likely to explain the observed super-Eddington luminosities, hard
energy spectra, and spectral variations of ULXs. We suggest that ULXs are
accreting black holes with a few tens of solar mass, which is not unexpected
from the standard stellar evolution scenario, and that their X-ray emission is
from the slim disk shining at super-Eddington luminosities.Comment: ApJ, accepte
New Analytical Formula for Supercritical Accretion Flows
We examine a new family of global analytic solutions for optically thick
accretion disks, which includes the supercritical accretion regime. We found
that the ratio of the advection cooling rate, , to the viscous
heating rate, , i.e., , can be
represented by an analytical form dependent on the radius and the mass
accretion rate. The new analytic solutions can be characterized by the
photon-trapping radius, \rtrap, inside which the accretion time is less than
the photon diffusion time in the vertical direction; the nature of the
solutions changes significantly as this radius is crossed. Inside the trapping
radius,
approaches , which corresponds to the advection-dominated
limit (), whereas outside the trapping radius, the radial dependence
of changes to , which corresponds to the
radiative-cooling-dominated limit. The analytical formula for derived here
smoothly connects these two regimes. The set of new analytic solutions
reproduces well the global disk structure obtained by numerical integration
over a wide range of mass accretion rates, including the supercritical
accretion regime. In particular, the effective temperature profiles for our new
solutions are in good agreement with those obtained from numerical solutions.
Therefore, the new solutions will provide a useful tool not only for evaluating
the observational properties of accretion flows, but also for investigating the
mass evolution of black holes in the presence of supercritical accretion flows.Comment: 14 pages, 7 figures, accepted for publication in the Astrophysical
Journa
Model for Relaxation Oscillations of Luminous Accretion Disk in GRS1915+105: Variable Inner Edge
To understand the bursting behavior of the microquasar GRS 1915+105, we
calculate time evolution of a luminous, optically thick accretion disk around a
stellar mass black hole undergoing limit-cycle oscillations between the high-
and low- luminosity states. We, especially, carefully solve the behavior of the
innermost part of the disk, since it produces significant number of photons
during the burst, and fit the theoretical spectra with the multi-color disk
model. The fitting parameters are \Tin (the maximum disk temperature) and
\Rin (the innermost radius of the disk). We find an abrupt, transient
increase in \Tin and a temporary decrease in \Rin during a burst, which are
actually observed in GRS 1915+105. The precise behavior is subject to the
viscosity prescription. We prescribe the radial-azimuthal component of
viscosity stress tensor to be ,
with being the height integrated pressure, and being the
parameter, and and being the total pressure and gas pressure
on the equatorial plane, respectively. Model with can produce the
overall time changes of \Tin and \Rin, but cannot give an excellent fit to
the observed amplitudes. Model with , on the other hand, gives the
right amplitudes, but the changes of \Tin and \Rin are smaller. Although
precise matching is left as future work, we may conclude that the basic
properties of the bursts of GRS 1915+105 can be explained by our ``limit-cycle
oscillation'' model. It is then required that the spectral hardening factor at
high luminosities should be about 3 at around the Eddington luminosity instead
of less than 2 as is usually assumed.Comment: 11 pages, 5 figures, accepted for publication in Ap
Hot Disk Corona and Magnetic Turbulence in Radio-Quiet Active Galactic Nuclei: Observational Constraints
We compile a sample consisting of 56 radio-quiet active galactic nuclei so as
to investigate statistical properties of hot corona of accretion disks from
{\em ASCA} observations. The black-hole masses in the sample are estimated via
several popular methods and the bolometric luminosities from the
multi-wavelength continuum. This allows us to estimate the Eddington ratio
() so that the undergoing physical
processes can be tested via hard X-ray data. We find a strong correlation
between and as
with a multivariate regression. This
indicates that the release of gravitational energy in the hot corona is
controlled by the Eddington ratio. On the other hand, the correlation between
the hard X-ray spectral index () and depends critically on
the types of objects: is nearly constant ()
in broad-line Seyfert 1's (BLS1s), whereas in narrow-line Seyfert 1's (NLS1s), although not very significant.
We can set constraints on the forms of magnetic stress tensor on the condition
that is proportional to the fraction of gravitational
energy dissipated in the hot corona and that is proportional to magnetic
energy density in the disk. We find that the shear stress tensor
is favored by the correlation in the present
sample, where is the gas pressure