7,160 research outputs found
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
Impact Parameter Dependent Parton Distributions for a Relativistic Composite System
We investigate the impact parameter dependent parton distributions for a
relativistic composite system in light-front framework. We express them in
terms of overlaps of light-cone wave functions for a self consistent two-body
spin-1/2 state, namely an electron dressed with a photon in QED. The pdfs are
distorted in the transverse space for transverse polarization of the state at
one loop level.Comment: 4 pages, presented at Lightcone 2004, August 16-20, Amsterdam,
Netherland
Nucleosynthesis in Advective Accretion Disks Around Galactic and Extra-Galactic Black Holes
We compute the nucleosynthesis of materials inside advective disks around
black holes. We show that composition of incoming matter can change
significantly depending on the accretion rate and accretion disks. These works
are improvements on the earlier works in thick accretion disks of Chakrabarti,
Jin & Arnett (1987) in presence of advection in the flow.Comment: Latex pages including figures. Kluwer Style files included. Appearing
in `Observational Evidence for Black Holes in the Universe', ed. Sandip K.
Chakrabarti, Kluwer Academic Publishers (DORDRECHT: Holland
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
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