X-ray and Gamma-ray Emission from a High-Temperature Plasma and the Spectra of Accreting Black Holes

Abstract

We present our numerical computations of broadband radiation spectra forming in a layer of high-temperature (kTe ~ 50 keV) semitransparent (with a Thomson optical depth ~1-3) plasma with an electron density Ne ~ 10^{17}-10^{19} cm^{-3}, typical for the accretion disk regions surrounding a black hole in X-ray binaries. The computations take into account the bremsstrahlung processes of photon production and absorption and their Comptonization. We show that the intrinsic radiation of such a plasma is sufficient to explain the X-ray spectra observed in the low (hard) state of Galactic black holes. No commonly assumed additional soft (with E < 1 keV) photons to maintain Comptonization are required; moreover, their presence would lead to severe distortions of the spectrum compared to the observed one. In the hard X-ray range the forming power-law spectrum with a photon index alpha ~ 1.4-1.7 and an exponential cutoff at E > 50 keV, exceeds considerably the bremsstrahlung flux that might be expected from such a plasma layer in the limit of its small depth. This is a result of the multiple Compton scattering of bremsstrahlung photons. It is important that the power-law spectrum of such a plasma should extend in an invariable form downward along the energy axis to the UV and OIR ranges (1-3 eV). At energies E < 1 eV the depth for bremsstrahlung absorption grows and the spectrum becomes the Rayleigh-Jeans one. To explain the steeper alpha ~ 2.1-2.5 X-ray spectra observed from black holes in their high (soft) state, it is indeed necessary that a large number of external photons enter a hot layer. Such photons could be emitted by the surface of an outer cold disk whose inner edge during these states approaches the black hole. The OIR emission from systems in these states is associated with that of the outer disk, whereas during their low state it can be produced directly in its central region.

37 pages, 22 figures, 3 tables

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