2006MNRAS.365..606N

We study spectra generated by Comptonization of soft photons by cold electrons radially free falling on to a black hole. We use a Monte Carlo method involving a fully relativistic description of Comptonization in the Kerr space-time. In agreement with previous studies, we find that Comptonization on the bulk motion of free fall gives rise to power-law spectra with the photon index of Γ≳ 3. In contrast to some previous studies, we find that these power-law spectra extend only to energies ≪m_ec². We indicate several effects resulting in generic cut-offs of such spectra at several tens of keV, regardless of any specific values of physical parameters in the model. This inefficiency of producing photons with energies ≳100 keV rules out bulk motion Comptonization as a main radiative process in soft spectral states of black hole binaries. The normalization of the power law (below the cut-off) with respect to the peak of the blackbody emission of the surrounding disc is typically very low, except for models with an overlap between the disc and the plasma, in which case the spectra are very soft, Γ≳ 4.