6,002 research outputs found
Satellite observations of thought experiments close to a black hole
Since black holes are `black', methods of their identification must
necessarily be indirect. Due to very special boundary condition on the horizon,
the advective flow behaves in a particular way, which includes formation of
centrifugal pressure dominated boundary layer or CENBOL where much of the
infall energy is released and outflows are generated. The observational aspects
of black holes must depend on the steady and time-dependent properties of this
boundary layer. Several observational results are written down in this review
which seem to support the predictions of thought experiments based on this
advective accretion/outflow model. In future, when gravitational waves are
detected, some other predictions of this model could be tested as well.Comment: Published in Classical and Quantum Gravity, v. 17, No. 12, p. 2427,
200
Black Hole Accretion: From Quasars to Nano-Quasars
In this review we shall comment on a few recent findings which strengthen the
view that the black hole accretion has substantial amount of sub-Keplerian
component. The manifestation of this component is many fold. We discuss some of
them. A general outline of the complex structure that emerges from the
multitude of observations is presented. A detailed outline of what might be
going on in outburst sources is also discussed. The relationship amount the
spectral and timing properties can be best understood by this picture. We claim
that the sub-Keplerian advective disk paradigm is a complete package. Since
signatures of sub-Keplerian motion is already increasing in the literature, the
whole package must be correct.Comment: 7 Pages, 2 Figures, Proceeding of the 2nd Kolkata Conference on
"Observational Evidence for the Black Holes in the Universe", Published in
AIP, 200
Spectral Properties of Accretion Disks Around Black Holes II -- Sub-Keplerian Flows With and Without Shocks
Close to a black hole, the density of the sub-Keplerian accreting matter
becomes higher compared to a spherical flow due to the presence of a
centrifugal barrier independent of whether or not a standing shock actually
forms. This hot dense flow intercepts soft photons from a cold Keplerian disk
and reprocesses them to form high energy X-rays and gamma rays. We study the
spectral properties of various models of accretion disks where a Keplerian disk
on the equatorial plane may or may not be flanked by a sub-Keplerian disk and
the sub-Keplerian flow may or may not possess standing shocks. From comparison
with the spectra, we believe that the observed properties could be explained
better when both the components (Keplerian and sub-Keplerian) are
simultaneously present close to a black hole, even though the sub-Keplerian
halo component may have been produced out of the Keplerian disk itself at
larger radii. We are able to understand soft and hard states of black hole
candidates, properties of X-ray novae outbursts, and quasi-periodic
oscillations of black hole candidates using these two component models. We fit
spectra of X-ray novae GS1124-68 and GS2000+25 and satisfactorily reproduce the
light curves of these objects.Comment: 15 Latex pages plus 12 figures. Macros included. Astrophysical
Journal (In press
Solutions of Two Dimensional Viscous Accretion and Winds In Kerr Black Hole Geometry
We extend our previous studies of shock waves and shock-free solutions in
thin accretion and winds in pseudo-Newtonian geometry to the case when the flow
is ``two-dimensional'' and around a ``Kerr black hole''. We present equations
for fully general relativistic viscous transonic flows and classify the
parameter space according to whether or not shocks form in an inviscid flow. We
discuss the behaviors of shear, angular momentum distribution, heating and
cooling in viscous flows. We obtain a very significant result: we find that in
weak viscosity limit the presence of the standing shock waves is more generic
in the sense that flows away from the equatorial plane can produce shock waves
in a wider range of parameter space. Similar conclusion also holds when the
angular momentum of the black hole is increased. Generally, our conclusions
regarding the shape of the shock waves are found to agree with results of the
existing numerical simulations of the two dimensional accretion in
Schwarzschild geometry. In a strong viscosity limit, the shocks may be located
farther out or disappear completely as in the pseudo-Newtonian geometry.Comment: 22 page Latex files and 7 figures. Special ApJ macro aasp4.sty
included. Appearing in ApJ on Nov. 1st 199
Computation of outflow rates from accretion disks around black holes
We self-consistently estimate the outflow rate from the accretion rates of an
accretion disk around a black hole in which both the Keplerian and the
sub-Keplerian matter flows simultaneously. While Keplerian matter supplies
soft-photons, hot sub-Keplerian matter supplies thermal electrons. The
temperature of the hot electrons is decided by the degree of inverse
Comptonization of the soft photons. If we consider only thermally-driven flows
from the centrifugal pressure-supported boundary layer around a black hole, we
find that when the thermal electrons are cooled down, either because of the
absence of the boundary layer (low compression ratio), or when the surface of
the boundary layer is formed very far away, the outflow rate is negligible. For
an intermediate size of this boundary layer the outflow rate is maximal. Since
the temperature of the thermal electrons also decides the spectral state of a
black hole, we predict that the outflow rate should be directly related to the
spectral state.Comment: 9 pages, 5 figure
Mass Outflow Rate From Accretion Discs around Compact Objects
We compute mass outflow rates from accretion disks around compact objects,
such as neutron stars and black holes. These computations are done using
combinations of exact transonic inflow and outflow solutions which may or may
not form standing shock waves. Assuming that the bulk of the outflow is from
the effective boundary layers of these objects, we find that the ratio of the
outflow rate and inflow rate varies anywhere from a few percent to even close
to a hundred percent (i.e., close to disk evacuation case) depending on the
initial parameters of the disk, the degree of compression of matter near the
centrifugal barrier, and the polytropic index of the flow. Our result, in
general, matches with the outflow rates obtained through a fully time-dependent
numerical simulation. In some region of the parameter space when the standing
shock does not form, our results indicate that the disk may be evacuated and
may produce quiescence states.Comment: 30 Latex pages and 13 figures. crckapb.sty; Published in Class.
Quantum Grav. Vol. 16. No. 12. Pg. 387
Correlation among QPO frequencies and Quiescence-state Duration in Black Hole Candidate GRS 1915+105
We discover a definite correlation between the frequency of the
quasi-periodic oscillations (QPO) in quiescence states and the duration of the
quiescence state of the transient X-ray source GRS 1915+105. We find that while
the QPO frequency can be explained with the oscillation of shocks in accretion
flows, the switching of burst to quiescence states (and vice versa) and their
duration can be explained by assuming an outflow from the post-shock region.
The duration of the quiescence state is inversely related to the QPO-frequency.
We derive this relation. We also find the correlation between the observed low
(Hz) and the intermediate (1-10Hz) QPO frequencies. Our
analytical solutions are verified by analyzing several days of public-domain
data from RXTE.Comment: Latex, 13 pages with 3 figures; Accepted for Publication in
Astrophysical Journal Letter
QPO Evolution in 2005 Outburst of the Galactic Nano Quasar GRO J1655-40
GRO J1655-40 showed significant X-ray activity in the last week of February,
2005 and remained active for the next 260 days. The rising and the decline
phases of this particular outburst show evidence for systematic movements of
the Comptonizing region, assumed to be a CENBOL, which causes the
Quasi-periodic Oscillations or QPOs. We present both the spectral and the
timing results of the RXTE/PCA data taken from these two hard spectral states.
Assuming that the QPOs originate from an oscillating shock CENBOL, we show how
the shock slowly moves in through the accretion flow during the rising phase at
a constant velocity and accelerate away outward during the later part of the
decline phase. By fitting the observed frequencies with our solution, we
extract time variation of various disk parameters such as the shock locations,
velocity etc.Comment: 5 Pages, 2 Figures, Proceeding of the 2nd Kolkata Conference on
"Observational Evidence for the Black Holes in the Universe", Published in
AIP, 200
- …