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 10 12K. 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 ~10 9K. For large accretion rates (tau Tap1) the protons deviate significantly from adiabaticity, because of the energy transfer to the electrons. The temperature at the Schwarzschild radius decreases by a factor ~2. The X-ray spectra of Seyfert galaxies and quasar and of Cyg X-1 can be ascribed to Comptonization of synchrotron photons by the thermal electrons in the vicinity of the black hole.