26,905 research outputs found
Studies of dissipative standing shock waves around black holes
We investigate the dynamical structure of advective accretion flow around
stationary as well as rotating black holes. For a suitable choice of input
parameters, such as, accretion rate () and angular momentum
(), global accretion solution may include a shock wave. The post shock
flow is located at few tens of Schwarzchild radius and it is generally very hot
and dense. This successfully mimics the so called Compton cloud which is
believed to be responsible for emitting hard radiations. Due to the radiative
loss, a significant energy from the accreting matter is removed and the shock
moves forward towards the black hole in order to maintain the pressure balance
across it. We identify the effective area of the parameter space () which allows accretion flows to have some energy dissipation at
the shock . As the dissipation is increased, the parameter
space is reduced and finally disappears when the dissipation is reached its
critical value. The dissipation has a profound effect on the dynamics of
post-shock flow. By moving forward, an unstable shock whose oscillation causes
Quasi-Periodic Oscillations (QPOs) in the emitted radiation, will produce
oscillations of high frequency. Such an evolution of QPOs has been observed in
several black hole candidates during their outbursts.Comment: 13 pages, 5 figures, accepted by MNRA
Massloss from viscous advective disc
Rotating transonic flows are long known to admit standing or oscillating
shocks and that the excess thermal energy in the post shock flow drives a part
of the infalling matter as bipolar outflows. We compute massloss from a viscous
advective disc. We show that the mass outflow rate decreases with increasing
viscosity of the accretion disc, since viscosity weakens the centrifugal
barrier that generates the shock. We also show that the optical depth of the
post-shock matter decreases due to massloss which may soften the spectrum from
such a mass losing disc.Comment: 14 pages, 7 figures, accepted in New Astronom
Nonlinear transforms of momenta and Planck scale limit
Starting with the generators of the Poincar\'e group for arbitrary mass (m)
and spin (s) a nonunitary transformation is implemented to obtain momenta with
an absolute Planck scale limit. In the rest frame (for ) the transformed
energy coincides with the standard one, both being . As the latter tends to
infinity under Lorentz transformations the former tends to a finite upper limit
where is the Planck length and the
mass-dependent nonleading terms vanish exactly for zero rest mass.The invariant
is conserved for the transformed momenta. The speed of light continues
to be the absolute scale for velocities. We study various aspects of the
kinematics in which two absolute scales have been introduced in this specific
fashion. Precession of polarization and transformed position operators are
among them. A deformation of the Poincar\'e algebra to the SO(4,1) deSitter one
permits the implementation of our transformation in the latter case. A
supersymmetric extension of the Poincar\'e algebra is also studied in this
context.Comment: 10 pages, no figures, corrected some typo
Hydrodynamic Simulations of Oscillating Shock Waves in a Sub-Keplerian Accretion Flow Around Black Holes
We study the accretion processes on a black hole by 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.Comment: 19 pages, 13 figures, 1 Table MNRAS (In press
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
Identification of Shocks in the Spectra from Black Holes
We study the spectral properties of a low angular momentum flow as a function
of the shock strength, compression ratio, accretion rate and flow geometry. In
the absence of a satisfactory description of magnetic fields inside the
advective disk, we consider the presence of only stochastic fields and use the
ratio of the field energy to the gravitational energy density as a parameter.
We not only include `conventional' synchrotron emission and Comptonization by
Maxwell-Bolzmann electrons in the gas, but we also compute these effects due to
power-law electrons. For strong shocks, a bump is produced due to the
post-shock flow. A power-law spectral components due to the thermal and
non-thermal electrons appear after this bump.Comment: 8 pages, 5 figures, Astronomy and Space Science (in press),
Proceedings of the Hong Kong Conference (2004) Edited by Cheng and Romer
Adenine Abundance in a Collapsing Molecular Cloud
A vital ingredient of DNA molecule named adenine may be produced by
successive addition of HCN during molecular cloud collapse and star formation.
We compute its abundance in a collapsing cloud as a function of the reaction
rate and show that in much of the circumstances the resulting amount may be
sufficient to contaminate planets, comets and meteorites. We introduce a
-parameter which may be used to study the abundance where radiative
association takes place.Comment: Six pages and one figure. Accepted for Publication in Indian Journal
of Physics (April 1, 2000 issue
Dissipative accretion flows around a rotating black hole
We study the dynamical structure of a cooling dominated rotating accretion
flow around a spinning black hole. We show that non-linear phenomena such as
shock waves can be studied in terms of only three flow parameters, namely, the
specific energy (), the specific angular momentum () and the
accretion rate () of the flow. We present all possible accretion
solutions. We find that a significant region of the parameter space in the
plane allows global accretion shock solutions. The effective
area of the parameter space for which the Rankine-Hugoniot shocks are possible
is maximum when the flow is dissipation free. It decreases with the increase of
cooling effects and finally disappears when the cooling is high enough. We show
that shock forms further away when the black hole is rotating compared to the
solution around a Schwarzschild black hole with identical flow parameters at a
large distance. However, in a normalized sense, the flow parameters for which
the shocks form around the rotating black holes are produced shocks closer to
the hole. The location of the shock is also dictated by the cooling efficiency
in that higher the accretion rate (), the closer is the shock
location. We believe that some of the high frequency quasi-periodic
oscillations may be due to the flows with higher accretion rate around the
rotating black holes.Comment: 9 pages, 7 figures. To appear in MNRA
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