Abstract
We present the results of our numerical computations of the broadband radiation spectra forming in a layer of high-temperature ( \(kT_{\textrm{e}}\sim 50\) keV) semitransparent (with a Thomson optical depth \(\tau_{\textrm{T}}\sim 1{-}3\) ) plasma with an electron density \(N_{\textrm{e}}\sim 10^{17}{-}10^{19}\textrm{ cm}^{-3}\) typical for the accretion disk regions surrounding a black hole in X-ray binaries. In our computations we took into account the bremsstrahlung processes of the production (and absorption) of photons and their subsequent Comptonization. The intrinsic radiation from such a high-temperature plasma is shown to be enough to explain the X-ray spectra observed in the low (hard) state of Galactic black hole candidates and X-ray novae. No commonly assumed additional soft (with energies \(h\nu\lesssim 1\) keV) photons to maintain Comptonization are required; moreover, their presence would lead to severe distortions of the spectrum compared to the observed one or would require a very fine tuning of plasma parameters. In the hard X-ray range the forming power-law radiation spectrum with a photon index \(\alpha\sim 1.4{-}1.7\) , which has an exponential cutoff at energies \(h\nu\gtrsim 100\) keV, exceeds considerably the bremsstrahlung flux that might be expected from such a plasma layer in the limit of an infinitely small optical depth. This is a result of the multiple inverse Compton scattering of bremsstrahlung photons. It is important that, according to our computations, the power-law radiation spectrum for such a high-temperature plasma should extend in an invariable form downward along the energy axis to the ultraviolet, optical, and infrared ranges ( \(h\nu\sim 1{-}3\) eV). At energies \(h\nu\lesssim 1\) eV the optical depth for bremsstrahlung absorption grows rapidly and the radiation spectrum becomes the Rayleigh–Jeans one. To explain the steeper ( \(\alpha\sim 2.1{-}2.5\) ) X-ray spectra observed from accreting black holes in their high (soft or two-component) state, it is indeed necessary that a large number of soft photons additional to the intrinsic plasma bremsstrahlung photons enter a hot cloud. Such photons could be emitted by the surface of an outer dense and cold accretion disk whose inner edge during these states characterized by a powerful soft component in the X-ray spectrum approaches the black hole maximally closely. The optical and infrared emission from systems in these states is associated precisely with the emission from the outer disk, whereas during their low state it can be produced directly in the hot central disk region bloated by instabilities. Under favorable conditions (disk size and inclination) the low-frequency emission from this region can noticeably exceed in flux and luminosity the emission from the outer cold accretion disk regions.
