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.