Astronomy and Astrophysics – Astrophysics
Scientific paper
2005-07-25
Mon.Not.Roy.Astron.Soc.365:606-614,2006
Astronomy and Astrophysics
Astrophysics
10 pages, 4 figures, revised version, accepted for publication in MNRAS
Scientific paper
10.1111/j.1365-2966.2005.09752.x
We study spectra generated by Comptonization of soft photons by cold electrons radially free-falling onto 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 Gamma >~ 3. In contrast to some previous studies, we find that these power-law spectra extend only to energies << 511 keV. We indicate several effects resulting in generic cutoffs 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 cutoff) 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, Gamma >~ 4.
Niedzwiecki Andrzej
Zdziarski Andrzej A.
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