October, 2013: MNARS
Quasi-periodic oscillations in a radiative transonic flow: results of a coupled Monte Carlo–TVD simulation
Author: Sudip K. Garain, Himadri Ghosh and Sandip K. Chakrabarti
Extract: Quasi-Periodic Oscillation frequency is analysed from the simulated light curves for different flow parameters. A time-dependent coupled hydrodynamic-radiative transfer simulation code is used for these simulations.Click here to get the paper
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2013, MNRAS, 430, 2836
Hydrodynamic simulation of two-component advective flows around black holes
Author: Giri, Kinsuk; Chakrabarti, Sandip K.
Extract:
Click here to get the paper
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2012, ApJ, 758, 114G
Effects of Compton Cooling on Outflow in a Two-component Accretion Flow around a Black Hole: Results of a Coupled Monte Carlo Total Variation Diminishing Simulation
Author: Garain, Sudip K.; Ghosh, Himadri; Chakrabarti, Sandip K.
Extract:Click here to get the paper
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October, 2013, 2013MNRAS.tmp.2643G
Title: Quasi-periodic oscillations in a
radiative transonic flow: results of a coupled Monte Carlo-TVD
simulation
Author: Sudip K. Garain, Himadri Ghosh
& Sandip K. Chakrabarti
Key Findings:
Low-
and intermediate-frequency quasi-periodic oscillations (QPOs) in
black hole candidates are believed to be due to oscillations of the
Comptonizing regions in an accretion flow.
Assuming
that the general structure of an accretion disc is a two-component
advective flow
(TCAF),
we numerically simulate the light curves emitted from an accretion
disc for different
accretion
rates and find how the QPO frequencies vary. We use a standard
Keplerian disc
residing
at the equatorial plane as a source of soft photons. These soft
photons, after suffering
multiple
scattering with the hot electrons of the low angular momentum,
sub-Keplerian, flow
emerge
out as hard radiation. The hydrodynamic and thermal properties of the
electron cloud
is
simulated using a total variation diminishing (TVD ) code. The TVD
code is then coupled with a
radiative
transfer code which simulates the energy exchange between the
electron and radiation
using
Monte Carlo technique. The resulting localized heating and cooling
are included also.
We
find that the QPO frequency increases and the spectrum becomes softer
as we increase the
Keplerian
disc rate. However, the spectrum becomes harder if we increase the
sub-Keplerian
accretion
rate. We find that an earlier prediction that QPOs occur when the
infall time-scale
roughly
matches with the cooling time-scale, originally obtained using a
power-law cooling,
remains
valid even for Compton cooling. Our findings agree with the general
observations of
low-frequency
QPOs in black hole candidates.
=========================================================
2013MNRAS.430.2836G
(January, 2013)
Title:
Hydrodynamic simulation of two-component advective
flows around black holes
Author:
Kinsuk Giri & Sandip K. Chakrabarti
Key
findings: We carry out a series
of numerical simulations of viscous accretion flows having a
reasonable spatial
distribution of the viscosity parameter. We add the power-law cooling
throughout the flow. We show that, in agreement with the theoretical
solutions of viscous transonic flows,
matter
having the viscosity parameter above a critical value becomes a
Keplerian disc while
matter
having lesser viscosity remains a low angular momentum, sub-Keplerian
flow. The
latter
component produces centrifugal pressure supported shock waves. Thus,
for instance, a
flow
having sufficiently high viscosity on the equatorial plane and low
viscosity above and
below
would produce a two-component advective flow where a Keplerian disc
is surrounded
by
a rapidly infalling sub-Keplerian halo. We find that the post-shock
region of the relatively
cooler
Keplerian disc is evaporated and the overall configuration is quite
stable. This agrees
with
the theoretical model with two components, which attempt to explain
the spectral and
timing
properties of black hole candidates.
=========================================================
2013ASInC...8...51G
(Recent Trends in the Study of Compact Objects: Theory and
Observation
ASI
Conference Series, 2013)
Title:
Compton cooling and its effects on spectral and hydrodynamic
properties of an accretion flow around a black hole: results of a
coupled monte carlo TVD simulation
Author:
H. Ghosh, S. K. Garain & S. K. Chakrabarti
Key
findings: We investigate the effects of cooling of the
Compton cloud on the outflow formation rate in an accretion disk
around a black hole using a time-dependent coupled hydrodynamics -
radiative transfer sumulation. We show that as a result of
inverse-Comptonization of the soft photons, originating from the
Keplerian disk immersed into an accreting sub-Keplerian flow (halo),
by the hot Compton cloud, the cloud becomes cooler with the increase
in the disk rate. As the resultant thermal pressure is reduced, the
post-shock region collapses and the outflow rate is also reduced. We
also find a direct correlation between the spectral states and the
outflow rates of an accreting black hole.
=========================================================
2013ASInC...8...15G
(Recent Trends in the Study of Compact Objects: Theory and
Observation
ASI
Conference Series, 2013)
(http://bulletin.astron-soc.in/asics_vol008/015_kinsuk.pdf)
Title:
Role of viscosity and cooling in hydrodynamic
simulation of Two Component Accretion Flow (TCAF) around black holes
Author:
Kinsuk Giri & S. K. Chakrabarti
Key
findings: We carry out numerical simulation of
accretion flows around a black hole in presence of both viscous and
cooling effects. Instead of using a constant α parameter throughout
the simulation grid, we assume that α is maximum on the equatorial
plane and gradually goes down in the perpendicular direction We show
that when the injected sub-Keplerian flow angular momentum is high
enough and/or the viscosity and also cooling is high enough, Two
Component Accretion Flow (TCAF) would be formed, otherwise the
sub-Keplerian flow would remain sub-Keplerian. We see that a
Keplerian disk is produced on the equatorial plane. Time variations
of the total, Keplerian and Sub-Keplerian matter are studied with
respect to various flow parameters.
=========================================================
2013ASInC...8...51G
(Recent Trends in the Study of Compact Objects: Theory and
Observation
ASI
Conference Series, 2013)
Title:
Compton cooling and its effects on spectral and hydrodynamic
properties of an accretion flow around a black hole: results of a
coupled monte carlo TVD simulation
Author:
H. Ghosh, S. K. Garain & S. K. Chakrabarti
Key findings:
=========================================================
2013ASInC...8...15G
(Recent Trends in
the Study of Compact Objects: Theory and Observation
ASI
Conference Series, 2013)
Title:
Role of viscosity and cooling in hydrodynamic simulation of Two
Component Accretion Flow (TCAF) around black holes
Author:
Kinsuk Giri & S. K. Chakrabarti
=========================================================
(http://iopscience.iop.org/0004-637X/758/2/114/pdf/apj_758_2_114.pdf)
Title:
Effects of Compton Cooling on Outflow in a
Two-component Accretion Flow around a Black Hole: Results of a
Coupled Monte Carlo Total Variation Diminishing Simulation
Author:
Sudip K. Garain, Himadri Ghosh & Sandip K. Chakrabarti
=========================================================
(http://mnras.oxfordjournals.org/content/421/1/666.full.pdf+html)
Title:
Hydrodynamic simulations of viscous accretion flows
around black holes
Author:
Giri, Kinsuk & Chakrabarti Sandip K.
Key findings: We study the
time evolution of a rotating, axisymmetric, viscous accretion flow
around black holes using a grid-based finite difference method. We
use the Shakura-Sunyaev viscosity prescription. However, we compare
with the results obtained when all the three independent components
of the viscous stress are kept. We show that the centrifugal pressure
supported shocks became weaker with the inclusion of viscosity. The
shock is formed farther out when the viscosity is increased. When the
viscosity is above a critical value, the shock disappears altogether
and the flow becomes subsonic and Keplerian everywhere except in a
region close to the horizon, where it remains supersonic. We also
find that as the viscosity is increased, the amount of outflowing
matter in the wind is decreased to less than a percentage of the
inflow matter. Since the post-shock region could act as a reservoir
of hot electrons or the so-called 'Compton cloud', the size of which
changes with viscosity, the spectral properties are expected to
depend on viscosity strongly: the harder states are dominated by low
angular momentum and the low-viscosity flow with significant outflows
while the softer states are dominated by the high-viscosity Keplerian
flow having very few outflows.
=========================================================
(http://mnras.oxfordjournals.org/content/416/2/959.full.pdf+html)
Title:
Effects of Compton cooling on the hydrodynamic and the
spectral properties of a two-component accretion flow around a black
hole
Author:
Ghosh, Himadri; Garain, Sudip K.; Giri, Kinsuk &
Chakrabarti, Sandip K.
Key
findings: We carry out a time-dependent numerical
simulation where both the hydrodynamics and the radiative transfer
are coupled together. We consider a two-component accretion flow in
which the Keplerian disc is immersed inside an accreting low angular
momentum flow (halo) around a black hole. The injected soft photons
from the Keplerian disc are reprocessed by the electrons in the halo.
We show that in presence of an axisymmetric soft-photon source the
spherically symmetric Bondi flow loses its symmetry and becomes
axisymmetric. The low angular momentum flow was observed to slow down
close to the axis and formed a centrifugal barrier which added new
features into the spectrum. Using the Monte Carlo method, we
generated the radiated spectra as functions of the accretion rates.
We find that the transitions from a hard state to a soft state is
determined by the mass accretion rates of the disc and the halo. We
separate out the signature of the bulk motion Comptonization and
discuss its significance. We study how the net spectrum is
contributed by photons suffering different number of scatterings and
spending different amounts of time inside the Compton cloud. We study
the directional dependence of the emitted spectrum as well.
=========================================================
(http://mnras.oxfordjournals.org/content/403/1/516.full.pdf+html)
Title:
Hydrodynamic simulations of oscillating shock waves in
a sub-Keplerian accretion flow around black holes
Author:
Giri, Kinsuk; Chakrabarti, Sandip K.; Samanta, Madan M.
& Ryu, D.
Key
findings: We study the accretion processes on a black
hole by a numerical simulation. We use a grid-based finite difference
code for this purpose. We scan the parameter space spanned by the
specific energy and the angular momentum and compare the
time-dependent solutions with those obtained from theoretical
considerations. We found several important results. (a) The
time-dependent flow behaves close to a constant height model flow in
the pre-shock region and a flow with vertical equilibrium in the
post-shock region. (c) The infall time-scale in the post-shock region
is several times higher than the free-fall time-scale. (b) There are
two discontinuities in the flow, one being just outside of the inner
sonic point. Turbulence plays a major role in determining the
locations of these discontinuities. (d) The two discontinuities
oscillate with two different frequencies and behave as a coupled
harmonic oscillator. A Fourier analysis of the variation of the outer
shock location indicates higher power at the lower frequency and
lower power at the higher frequency. The opposite is true when the
analysis of the inner shock is made. These behaviours will have
implications in the spectral and timing properties of black hole
candidates.
=========================================================
(http://www.worldscientific.com/doi/abs/10.1142/S0218271810016555)
Title:
Monte Carlo Simulations of the Thermal Comptonization
Process in a Two-Component Accretion Flow around a Black Hole in the
Presence of AN Outflow
Author:
Ghosh, Himadri; Garain, Sudip K.; Chakrabarti, Sandip
K. & Laurent, Philippe
Key
findings: A black hole accretion may have both the
Keplerian and the sub-Keplerian component. In the so-called
Chakrabarti–Titarchuk scenario, the Keplerian component supplies
low-energy (soft) photons while the sub-Keplerian component supplies
hot electrons which exchange their energy with the soft photons
through Comptonization or inverse Comptonization processes. In the
sub-Keplerian component, a shock is generally produced due to the
centrifugal force. The postshock region is known as the CENtrifugal
pressure–supported BOundary Layer (CENBOL). In this paper, we
compute the effects of the thermal and the bulk motion Comptonization
on the soft photons emitted from a Keplerian disk by the CENBOL, the
preshock sub-Keplerian disk and the outflowing jet. We study the
emerging spectrum when the converging inflow and the diverging
outflow (generated from the CENBOL) are simultaneously present. From
the strength of the shock, we calculate the percentage of matter
being carried away by the outflow and determine how the emerging
spectrum depends on the outflow rate. The preshock sub-Keplerian flow
is also found to Comptonize the soft photons significantly. The
interplay between the up-scattering and down-scattering effects
determines the effective shape of the emerging spectrum. By
simulating several cases with various inflow parameters, we conclude
that whether the preshock flow, or the postshock CENBOL or the
emerging jet is dominant in shaping the emerging spectrum depends
strongly on the geometry of the flow and the strength of the shock in
the sub-Keplerian flow.
=========================================================
(http://www.worldscientific.com/doi/abs/10.1142/S0218271809015242)
Title:
Monte Carlo Simulations of the Thermal Comptonization
Process in a Two-Component Accretion Flow around a Black Hole
Author:
Ghosh, Himadri; Chakrabarti, Sandip K.;
Laurent, Philippe
Key
findings: We compute the effects of thermal
Comptonization of soft photons emitted from a Keplerian disk around a
black hole by the postshock region of a sub-Keplerian flow, known as
the CENtrifugal-pressure-dominated BOundary Layer (CENBOL). We show
that the spectral state transitions of black hole candidates could be
explained either by varying the outer boundary of the CENBOL, which
also happens to be the inner edge of the Keplerian disk, or by
changing the central density of the CENBOL, which is governed by the
rate of the sub-Keplerian flow. We confirm the conclusions of the
previous theoretical studies that the interplay between the intensity
of the soft photons emitted by the Keplerian flow, the optical depth
and electron temperature of the Comptonizing cloud is responsible for
the state transitions in a black hole.
=========================================================
(http://www.worldscientific.com/doi/abs/10.1142/9789812704030_0112)
Title : QPOs from Radial and
Vertical Oscillation of Shocks in Advective Accretion Flows
Author :
Chakrabarti, Sandip K.; Acharyya, Kinsuk;
Molteni, Diego
Key findings : We present
results of several numerical simulations of two dimensional advective
flows which include cooling processes. We show that the computed
light curve is similar to the χ state in GRS 1915+105. The power
density spectrum (PDS) also shows presence of QPOs near the break
frequency.
=========================================================
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