Quasi-Spherical Accretion onto the Black Hole: The Virial Regime

Extract

We are studying the slow (ṁ < 1) spherical accretion of a gas onto a supermassive (M ≈ 10⁸M _⊙) black hole in the presence of a strong tangled magnetic field. In the core with radius 2.5 r_g < r < 10 r_g protons are isotropized due to scattering in magnetic field, but are not thermalized since the characteristic time of pp — Coulomb scattering is less than the infall time. A proton moves in the electron gas with a friction due to pe — scattering, gradually transferring energy to electrons. The standard equations for the proton gas allow the virial regime of accretion when the kinetic energy of the proton is a function of a distance only E_k (r) = (2/5) m_pc ² (r_g /r). The model is relevant to the slow subsonic settling of matter onto the black hole, as, for example, in the upstream region after the shock standing at a distance r ∼ 20 r_g (Mészàros and Ostriker 1983). Electrons are thermalized and are cooling predominantly by bremsstrahlung radiation. For ṁ ≲ 0.1 the core is transparent for bremsstrahlung photons. In agreement with Park (1990) the e⁺e ⁻ - pair production is found to be insignificant. The equilibrium between the energy release in pe — scattering and the bremsstrahlung radiation results in the almost isothermal core with the temperature T _e ≈ 4 m_e , which slightly increases towards the inner edge of the core. The only role of magnetic field is the isotropization of the proton gas, as the synchrotron radiation is strongly self-absorbed. Therefore the model is insensitive to the precise value of H.