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SPECTRAL PROPERTIES OF GALACTIC AND EXTRAGALACTIC BLACK HOLE CANDIDATES
CHAKRABARTI SANDIP K. The Korean Astronomical Society 1996 Journal of The Korean Astronomical Society Vol.29 No.suppl1
We review current theoretical understanding of the spectral properties (low and high states, transition of states, quasi-periodic oscillations etc.) of the low mass as well as supermassive black hole candidates.
Giri, Kinsuk,Chakrabarti, Sandip K.,Samanta, Madan M.,Ryu, D. Blackwell Publishing Ltd 2010 MONTHLY NOTICES- ROYAL ASTRONOMICAL SOCIETY Vol.403 No.1
<P>ABSTRACT</P><P>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.</P>
Studies of dissipative standing shock waves around black holes
Das, Santabrata,Chakrabarti, Sandip K.,Mondal, Soumen Blackwell Publishing Ltd 2010 MONTHLY NOTICES- ROYAL ASTRONOMICAL SOCIETY Vol.401 No.3
<P>ABSTRACT</P><P>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 <IMG SRC='http://onlinelibrary.wiley.com/store/10.1111/j.1365-2966.2009.15793.x/asset/equation/MNR_15793_mu1.gif?v=1&s=1f863aabc4afe4e815441d3668ba8bef331dbb46'/> and angular momentum (λ), a global accretion solution may include a shock wave. The post-shock flow is located at a few tens times the Schwarzchild radius and is generally very hot and dense. This successfully mimics the so-called Compton cloud, which is believed to be responsible for emitting hard radiation. Owing to the radiative loss, significant energy is removed from the accreting matter and the shock moves forward towards the black hole in order to maintain the pressure balance across it. We identify the effective area of parameter space <IMG SRC='http://onlinelibrary.wiley.com/store/10.1111/j.1365-2966.2009.15793.x/asset/equation/MNR_15793_mu2.gif?v=1&s=390e74728c22c2f001e31788bfde14ab9deeb019'/> that allows accretion flows to have some energy dissipation at the shock (Δε). As the dissipation is increased, the parameter space is reduced and finally disappears when the dissipation reaches a critical value. The dissipation has a profound effect on the dynamics of post-shock flow. By moving forward, an unstable shock, the oscillation of which 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.</P>