Two-temperature model of spherical accretion onto a black hole

Spherical accretion onto a black hole is studied, considering a two-temperature plasma model. The ion and electron temperature profiles are described by two energy balance equations coupled by an energy exchange term including only Coulomb interactions between the two populations. For accretion rates smaller than the Eddington one, the proton temperature profile is closely adiabAtic. at the Schwarzschild radius the temperature approaches 1012 K. The electron temperature at large radii is close to that of protons. Approaching the black hole, the cooling mechanisms, namely opaque synchrotron radiation and multiple Compton scattering, limit the electron temperature to approx.109 K. For large accretion rates (tau/sub T/roughly-equal1) the protons deviate significantly from adiabatiity, because of the energy transfer to the electrons. The temperature at the Schwarzschild radius decreases by a factor approx.2. The threshold temperature for pion production which was just reached in the very thin cases is not met for tau/sub T/roughly-equal1. The maximum achievable γ-ray flux from π0-decay is approx.2 x 1033 ergs s-1 for a 10 M/sub sun/ hole. The electron component always contributes most (by many orders of magnitude) to the overall luminosity with an efficiency epsilon = L/Mc2roughly-equal5 x 10-3. The spectrum is basically a power law extending from the synchrotron transparency frequency at the inner radius (1012--1013 Hz) to 3kT (approx.1 MeV) with spectral index varying from 1 to 0.5 for tau/sub T/ = 10-2 and tau/sub T/ = 1, respectively. All results are essentially independent of the black hole mass