We study the spectral properties of a very general class of accretion disks which can be decomposed into three distinct components apart from a shock at r = rs: (1) An optically thick Keplerian disk on the equatorial plane (r > rs); (2) a sub-Keplerian optically thin halo above and below this disk r > rs and (3) a hot, optically slim τ ∼ 1 postshock region r < rs ∼ 5-10rg where rg is the Schwarzschild radius. The postshock halo intercepts soft photons from the Keplerian component and reradiates them as hard X-rays and γ rays after Comptonization. We solve two-temperature equations in the postshock region with Coulomb energy exchange between protons and electrons, and incorporating radiative processes such as bremsstrahlung and Comptonization. We also present the exact prescription to compute the reflection of the hard X-rays from the cool disk. We produce radiated spectra from both the disk components as functions of the accretion rates and compare them with the spectra of Galactic and extragalactic black hole candidates. We find that the transition from hard state to soft state is smoothly initiated by a single parameter, namely the mass accretion rate of the disk. In the soft state, when the postshock region is very optically thick and cooled down, bulk motion of the converging flow determines the spectral index to be about 1.5 in agreement with observations

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We extend our previous numerical simulation of accretion disks with shock waves when cooling e ects are also included. We consider bremsstrahlung and other power law processes: / T 2 to mimic cooling in our simulation. We employ Smoothed Particle Hydrodynamics technique as in the past. We observe that for a given angular momentum of the ow, the shock wave undergoes a steady,

We provide the complete set of global solutions of viscous transonic flows (VTFs) around black holes and neutron stars. These solutions describe the optically thick and optically thin flows from the horizon of the black hole or from the neutron star surface to the location where the flow joins with a Keplerian disk. We study the nature of the multiple sonic points as functions of advection, rotation, viscosity, heating and cooling. Stable shock waves, which join two transonic solutions, are found to be present in a large region of the parameter space. We classify the solutions in terms of whether or not the flow can have a standing shock wave. We find no new topology of solutions other than what are observed in our previous studies of isothermal VTFs. We particularly stress the importance of the boundary conditions and argue that we have the most complete solution of accretion and winds around black holes and neutron stars.

Context. The Galactic transient black hole candidate (BHC) GX 339-4 exhibited several outbursts at regular intervals of about two to three years in the Rossi X-ray Timing Explorer (RXTE) era. After remaining in an almost quiescent state for three long years, it again became X-ray active in January, 2010, continuing to be so over the next ∼14 months. Aims. We study the timing and spectral properties of the BHC during its recent outburst and understand the behavioral change in the accretion flow dynamics associated with the evolution of the various X-ray features. Methods. The detailed analysis of the temporal and spectral properties of the source during this outburst are carried out using archival data of the RXTE PCA instrument. We analyze a total of 236 observational intervals consisting of 419 days of data observed by RXTE, from 2010 January 12 to 2011 March 6. Results. Our study provides a comprehensive understanding of the mass accretion processes and properties of the accretion disk of the BHC. The PCA spectra of 2.5-25 keV are mainly fitted with a combination of two components, namely, a disk black body and a power-law. The entire outburst as observed by RXTE, is divided into four spectral states, namely, hard, hard-intermediate, softintermediate, and soft. Quasi-periodic oscillations (QPOs) were found in three out of the four states, namely hard, hard-intermediate, and soft-intermediate states. The QPO frequencies increase monotonically from 0.102 Hz to 5.692 Hz in the rising phase of the outburst, while during the declining phase QPO frequencies decrease monotonically from 6.420 to 1.149 Hz. The evolution pattern, i.e. the hardness-intensity diagram, of the present outburst can be reproduced by two different components of the flow of accreting material. Conclusions. The recent outburst of GX 339-4 gives us an opportunity to understand the evolution of the two-component accretion rates starting from the onset to the end of the outburst phase. We found that the QPO frequency variation could be explained by the propagating oscillatory shock model (POS) and the hardness versus intensity variation can be reproduced if we assume that higher viscosity causes the conversion of a low angular momentum disk component into a Keplerian component during the outburst phase. The decline phase starts because of the reduction in the viscosity.

We characterize the nature of thin, axisymmetric, inviscid, accretion flows of cold adiabatic gas with zero specific energy in the vicinity of a black hole by the specific angular momentum. Using two-dimensional hydrodynamic simulations in cylindrical geometry, we present various regimes in which the accretion flows behave distinctly differently. When the flow has a small angular momentum (λ < ∼ λ b ), most of the material is accreted into the black hole forming a quasi-spherical flow or a simple disk-like structure around it. When the flow has a large angular momentum (typically, larger than the marginally bound value, λ > ∼ λ mb ), almost no accretion into the black hole occurs. Instead, the flow produces a stable standing shock with one or more vortices behind it and is deflected away at the shock as a conical outgoing wind of higher entropy. If the flow has an angular momentum somewhat smaller than λ mb

We present results of numerical simulation of inviscid thick accretion disks and wind flows around black holes. We use Smoothed Particle Hydrodynamics (SPH) technique for this purpose. Formation of thick disks are found to be preceded by shock waves travelling away from the centrifugal barrier. For a large range of the parameter space, the travelling shock settles at a distance close to the location obtained by a one-and-a-half dimensional model of inviscid accretion disks. Occasionally, it is observed that accretion processes are aided by the formation of oblique shock waves, particularly in the initial transient phase. The post-shock region (where infall velocity suddenly becomes very small) resembles that of the usual model of thick accretion disk discussed in the literature, though they have considerable turbulence. The flow subsequently becomes supersonic before falling into the black hole. In a large number of cases which we simulate, we find the formation of strong winds which are hot and subsonic when originated from the disk surface very close to the black hole but become supersonic within a few tens of the Schwarzschild radius of the blackhole. In the case of accretion of high angular momentum flow, very little amount of matter is accreted directly onto the black hole. Most of the matter is, however, first squeezed to a small volume close to the black hole, and subsequently expands and is expelled as a strong wind. It is quite possible that this expulsion of matter and the formation of cosmic radio jets is aided by the shock heating in the inner parts of the accretion disks.