Shocks in spherically accreting black holes - A model for classical quasars
Spherically symmetric accretion onto black holes is thought to be very inefficient if the flow is nondissipative because of the inadequacy of compression heating and the short time available to radiate. It is shown that standing shock solutions exist where the downstream protons are heated to temperatures at which the fluid becomes almost collisionless. The material then moves inwards at the much slower diffusion rate given by the p-e energy exchange time scale. For a given accretion rate, a self-consistent shock radius is found by matching the flow equations above and below that value, showing that such shocks can be self-sustaining. Efficiencies reaching up to 0.1-0.3 can be achieved, with most of the luminosity in the hard X-ray and gamma-ray, as well as optical-IR ranges. This provides a new model for the classical (radio-quiet) quasars and active galactic nuclei and possibly for galactic sources such as Cyg X-1.