The effects of bulk motion comptonization on the spectral formation in a converging flow onto a black hole are discussed and new results are presented. The problem has been tackled by means of both a fully relativistic, angle-dependent trasfer code and a semi-analytical, 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. 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 (< 5 in Eddington units) or sufficiently high (> 10). Our analysis confirms that a hard tail with photon index Γ < 3 is produced by the up-scattering of primary photons onto infalling electrons if the central object is a black hole.
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