Speaker
Description
We present our numerical computations of broadband radiation spectra formed in a layer of high-temperature ($kT_{e}\sim 50$ keV) semitransparent plasma (with a Thomson optical depth of $\tau_{T}\sim 1-3$) with an electron number density of $N_{e}\sim 10^{17}-10^{19}\ \mbox{cm}^{-3}$, characteristic of 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 subsequent Comptonization. We show that the intrinsic radiation of such a high-temperature plasma is sufficient to explain the X-ray spectra observed in the low (hard) state of galactic black hole candidates and X-ray novae. None of the generally assumed additional soft photons (with energies of $h\nu\leq 1$ keV) are required to maintain Comptonization; moreover, their presence would lead to strong distortions of the spectrum compared to the observed one or would require very fine-tuning of the plasma parameters. In the hard X-ray range, the resulting power-law radiation spectrum with a photon index of $\alpha\sim 1.4-1.7$, exponentially decaying at $\geq 50$ keV, significantly exceeds the bremsstrahlung flux that would be expected from a similar plasma layer in the limit of its very small depth. This is the result of multiple inverse Compton scattering of bremsstrahlung photons. Importantly, the power-law spectrum of the radiation of such a high-temperature plasma should, according to computations, extend unchanged down the energy axis all the way to the infrared, optical, and ultraviolet bands ($h\nu\sim 1-3$ eV). At energies of $h\nu\leq 1$ eV, the depth for bremsstrahlung absorption rapidly increases, and the radiation spectrum becomes Rayleigh-Jeans. To explain the steeper ($\alpha\sim 2.1-2.5$) X-ray spectra observed in accreting black holes in the high (soft or two-component) state, it is indeed necessary for a large number of soft external photons to enter the hot cloud, in addition to the photons of the plasma's own bremsstrahlung. Such photons could be emitted by the surface of the outer, dense, cold accretion disk, the inner edge of which, during these states, characterized by a strong soft component in the X-ray spectrum, approaches the black hole as closely as possible. The optical and infrared radiation of systems in these states is associated specifically with the emission from the outer disk, whereas during their low states, it can be generated directly in the hot, inflated central region of the disk. Under favorable circumstances (disk size and inclination), the low-frequency radiation from this region can significantly exceed in flux and luminosity the emission from the outer, cool regions of the accretion disk.