High-Energy Spectral Complexity from Thermal Gradients in Black Hole Atmospheres

We show that Compton scattering of soft photons with energies ~100 eV in thermally stratified black hole accretion plasmas with temperatures in the range ~100 keV-1 MeV can give rise to an X-ray spectral hardening above ~10 keV. This could produce the hardening observed in the X-ray spectra of black holes, which is generally attributed to reflection or partial covering of the incident continuum source by cold optically thick matter. In addition, we show that the presence of very hot ( kT ≃ 1 MeV) cores in plasmas leads to spectra exhibiting high-energy tails similar to those observed from Galactic black hole candidates.