  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
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. Our semi-analytical 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 really 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 rate, 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 (< 5 in Eddington units) or sufficiently high (> 10).
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 up-scattering of primary photons onto
infalling electrons if the central object is a black hole.
