HIGH-ENERGY SPECTRAL COMPLEXITY FROM THERMAL-GRADIENTS IN BLACK-HOLE ATMOSPHERES

We show that Compton scattering of soft photons with energies similar to 100 eV in thermally stratified black hole accretion plasmas with te mperatures in the range similar to 100 keV-1 MeV can give rise to an X -ray spectral hardening above similar to 10 keV. This could produce th e hardening observed in the X-ray spectra of black holes, which is gen erally attributed to reflection or partial covering of the incident co ntinuum source by cold optically thick matter. in addition, we show th at the presence of very hot (kT similar or equal to 1 MeV) cores in pl asmas leads to spectra exhibiting high-energy tails similar to those o bserved from Galactic black hole candidates.