9,484 research outputs found
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
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
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
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
Evolution of the quasi-periodic oscillation frequency in GRO J1655-40 -- Implications for accretion disk dynamics
Low and intermediate frequency quasi-periodic oscillations (QPOs) are thought
to be due to oscillations of Comptonizing regions or hot blobs embedded in
Keplerian disks. Any movement of these perturbations is expected systematically
to change the QPO frequency. Our goal is to find systems where such a
systematic drifts have been observed. We also try to find the real cause of
such drifts and whether they shed some light on the accretion disk dynamics.
Using archival data of the recent outburst of GRO J1655-40, we report the
presence of such systematic drifts not only during the rising phase from the
25th of February 2005 to the 12th March 2005, when the QPO frequency
monotonically increased from 82mHz to 17.78Hz but also in the decline phase
from the 15th September 2005 to the 5th of October 2005, when the QPO frequency
decreased from 13.14Hz to 34mHz. We fitted the frequency drifts with the
propagatory oscillating shock solution. In the shock-oscillation solution, the
frequency is inversely proportional to the infall time scale from the shock
location. We obtained the shock location and strength through such a fit. The
astonishing smoothness of the variation of the QPO frequency over a period of
weeks directly supports the view that it may due to the drift of an oscillating
shock rather than the movements of a blob inside a differentially rotating
disk.Comment: 8 pages, 3 figures, Astronomy & Astrophysics (accepted
Length-scales of Dynamic Heterogeneity in a Driven Binary Colloid
Here we study characteristic length scales in an aqueous suspension of
symmetric oppositely charged colloid subject to a uniform electric field by
Brownian Dynamics simulations. We consider a sufficiently strong electric field
where the like charges in the system form macroscopic lanes. We construct
spatial correlation functions characterizing structural order and that of
particles of different mobilities in-plane transverse to the electric field at
a given time. We call these functions as equal time density correlation
function (ETDCF). The ETDCF between particles of different charges,
irrespective of mobilities, are called structural ETDCFs, while those between
particles of different mobilities are called the dynamic ETDCF. We extract the
characteristic length of correlation by fitting the envelopes of the ETDCFs to
exponential dependence. We find that structural ETDCF and the dynamical-ETDCFs
of the slow particles increase with time. This suggests that the slow particles
undergo microphase separation in the background of the fast particles which
drive the structural pattern in the plane transverse to the lanes. The ETDCFs
can be measured for colloidal systems directly following particle motion by
video-microscopy and may be useful to understand patterns out of equilibrium
A study of the atmosphere and ionosphere using satellite observations of 300-1400 Ang airglow
A study of the O(+) ion distribution reveals that the OII 834 A emission can be used to infer the O(+) density as a function of altitude. The ion temperature was obtained from these measurements. Variations of the ion density distributions were obtained as a function of latitude. Daytime observations show that the OII 834 A emissions contain the signature of the Appleton anomary. Analysis of the 300 to 900 A auroral spectra reveals a large number of OII features. Several pairs of OII features with a common upper state were used to obtain their branching ratios and compared with laboratory observations and theoretical calculations. Evidence for OIII emissions were also found in an aurora
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