Andrzej A. Zdziarski1, W. Neil Johnson2, Juri Poutanen3, Pawel Magdziarz4, and Marek Gierlinski4
1N. Copernicus Astronomical Center, Bartycka 18,
00-716 Warsaw, Poland
2E. O. Hulburt Center for Space Research, Naval
Research Lab, Washington, DC 20375, USA
3Uppsala Observatory, Box 515, S-75120 Uppsala, Sweden
4Astronomical Observatory, Jagiellonian University,
Orla 171, 30-244 Cracow, Poland
We review observations and theoretical models of X-ray/gamma-ray spectra of radio-quiet Seyfert galaxies and of Galactic black-hole candidates (in the hard spectral state). The observed spectra share all their basic components: an underlying power law, a Compton-reflection component with an Fe K line, low-energy absorption by intervening cold matter, and a high-energy cutoff above 200 keV. The X-ray energy spectral index, 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 spectral index 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) * 2 pi as seen from the X-ray source. The spectra are cut off in soft gamma-rays above 200 keV. 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 1. A fraction of the luminosity emitted nonthermally appears to be small and it can be constrained to about 15% in the Seyfert galaxy NGC 4151. The spectra are cut off before 511 keV, which is strongly suggestive of a thermostatic role of electron-positon 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 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.