Power-law tails from dynamical comptonization in converging flows

The effects of bulk motion Comptonization on the spectral formation in a converging flow onto a black hole are investigated. The problem is tackled by means of both a fully relativistic, angle-dependent transfer code and a semianalytical, diffusion approximation method. We find that a power-law high-energy tail is a ubiquitous feature in converging flows and that the two approaches produce consistent results at large enough accretion rates when photon diffusion holds. Our semianalytical approach is based on an expansion in eigenfunctions of the diffusion equation. Contrary to previous investigations based on the same method, we find that although the power-law tail at extremely large energies is always dominated by the flatter spectral mode, the slope of the hard X-ray portion of the spectrum is dictated by the second mode and it approaches Gamma = 3 at large accretion rates, irrespective of the model parameters. The photon index in the tail is found to be largely independent on the spatial distribution of soft seed photons when the accretion rate is either quite low (less than or similar to5 in Eddington units) or sufficiently high (greater than or similar to10). On the other hand, the spatial distribution of source photons controls the photon index at intermediate accretion rates, when Gamma switches from the first to the second mode. Our analysis confirms that a hard tail with photon index Gamma < 3 is produced by the upscattering of primary photons onto infalling electrons if the central object is a black hole.