Compton scattering in a converging fluid flow - III Spherical supercritical accretion

Radiative transfer in spherical, supercritical accretion on to a massive black hole is considered. Particular emphasis is placed on the case of non-adiabatic flow in which electron scattering in the converging flow is the dominant source of opacity and photon heating. In escaping diffusively, the photons, which are produced mainly at the trapping radius, undergo ∼(c/u_(tr))^2 scatterings (where utr is the velocity of the flow at the trapping radius), each one giving a secular fractional energy increase ∼(u_(tr)/c)^2 and a total average increase of order unity. The emitted radiation spectrum will be a superposition of the locally produced spectra at low frequencies and a power-law at high frequencies. For gas accreting radially with the free-fall speed, the spectral index observed in the outflowing radiation is α~2. Both thermal and non-thermal emission processes are discussed. The conditions under which bulk heating of the photons is important are specified