Time--dependent analysis of spherical accretion onto black holes

Results are presented from a time--dependent, numerical investigation of spherical accretion onto black holes, within the framework of relativistic radiation hydrodynamics. We have studied the stability of self--consistent, stationary solutions of black hole accretion with respect to thermal and radiative perturbations and also the non--linear evolution of unstable, high temperature models, heated by the hard radiation produced by the accretion flow itself in the inner region near to the horizon. In some cases, a hydrodynamic shock forms at around $10^3$--$10^4$ Schwarzschild radii, where Compton heating exceeds radiative cooling. The calculations were made using a suitably designed radiation hydrodynamics code, in which radiative transfer is handled by means of the PSTF moment formalism and which contains an original treatment of the radiation temperature equation.