Transonic flow solutions with explicit cooling and viscosity

In this chapter, we discuss about the transonic flows around black hole binaries in presence of explicit cooling and viscosity and their observational aspects. Accreting compact objects are powerful X-ray emitters. The X-ray flux arises from two key accretion flow components: the geometrically thin standard accretion disk and an inner X-ray corona. The flow that feeds these objects is transonic in nature and the structure of the flow appears to depend on the heating and cooling mechanisms, where the whole system can flatten to a disk or puff up into a sphere, radiate efficiently or advect all of its energy. Sometimes the inner region varies quasi-periodically without changing much its total accretion rates. Thus the phenomenology of X-ray observations of accreting compact object is very rich and intensely studied. To go beyond the standard model of a stable accretion flow, with no corona and only a parametric viscosity of unknown origin, it is necessary to rely on a promising physical solution. In a Two Component Advective Flow solution, a high-viscosity Keplerian disk is flanked by a low angular momentum and low-viscosity flow that forms a centrifugal pressure-supported shock. The post-shock region which behaves as a Compton cloud, upscatters soft photons from the Keplerian disk. The shock wave forms under the satisfaction of Rankine-Hugoniot conditions. This shock may be steady or oscillating depending on whether flow has cooling or not. To get the full transonic solution we coupled hydrodynamics and radiative transfer in presence of cooling, heating and viscosity mechanisms.