X-rays and gamma-rays from accretion flows onto black holes in Seyferts and x-ray binaries

We review observations and theoretical models of X-ray/γ-ray spectra of radio-quiet Seyfert galaxies and of Galactic black-hole candidates (in the hard spectral state). The spectra observed from the two classes of objects share all their basic characteristics: an underlying power law with a high-energy cutoff above ∼ 200 keV, a Compton-reflection component with an Fe Kα line, and low-energy absorption by intervening cold matter. The X-ray energy spectral index, α, is typically in the range ∼ 0.8-1 in Seyfert spectra from Ginga, EXOSAT and OSSE. The hard-state spectra of black-hole candidates Cyg X-1 and GX 339-4 from simultaneous Ginga/OSSE observations have α ≃ 0.6-0.8. The Compton-reflection component corresponds to cold matter (e.g., inner or outer parts of an accretion disk) covering a solid angle of ∼ (0.4-1) × 2π as seen from the X-ray source. The broad-band spectra of both Seyferts and black-hole sources are well fitted by Compton upscattering of soft photons in thermal plasmas. Our fits yield the thermal plasma temperature of ∼ 100 keV and the Thomson optical depth of τ ∼ 1. A fraction of the luminosity emitted nonthermally appears to be small and it can be constrained to ≲ 15% in the Seyfert galaxy NGC 4151. All of the spectra are cut off before 511 keV, which is most likely due to a thermostatic role of e^± pair production in constraining the temperature and optical depth of the sources. The source geometry is compatible with a patchy corona above a cold disk in most Seyferts, but not in Cyg X-1. In the latter, the relative weakness of reflection is compatible with reflection of emission of a hot inner disk from outer disk regions.