  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-alpha line,
low-energy absorption by intervening cold matter, and a high-energy cutoff
above ~ 200 keV. The X-ray energy spectral index, alpha, 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 alpha ~ 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) times 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 tau 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. The spectra are cut off
before 511 keV, which is strongly suggestive of a thermostatic role of e+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 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.
