Spherical accretion onto black holes - A complete analysis of stationary solutions

Abstract

The problem of stationary, spherical accretion onto a Schwarzschild hole is here reinvestigated by the construction of a self-consistent model which incorporates all relevant physical processes taking place in an astro-physical plasma, apart from the presence of magnetic fields and dissipative processes. In particular, transfer of radiation through the accreting gas is treated in full generality using a completely relativistic formalism. A careful analysis of critical points and boundary conditions for radiation hydrodynamics equations is performed. The complete topology of solutions in the accretion rate-luminosity plane is obtained, showing the existence of two distinct branches of models with very different emission properties: stationary accretion reveals, therefore, a bimodal behavior. By means of a self-consistent study of the effects of Compton heating, both the upper and the lower bounds for the existence of high-luminosity solutions were derived. The stability of the two possible accretion regimes is also briefly discussed.