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Radiative mechanisms in accreting black holes
Accreting black holes are known to show two spectral states. In the hard state, the photon index is about =1.7 and most of the energy is emitted at about 100 keV by thermal Comptonization. In the soft state, the optically thick disk dominates at about 1-10 keV and there is a long power-law tail extending up to 10 MeV. This tail can be a result of Compton scattering by non-thermal electrons. Why the electron distributions are so much different in the two states is a mystery. Why the spectral index seems to saturate at 1.7 in the hard state is also unknown. We will discuss the physical processes that control the spectral shape. By solving for the first time the relativistic kinetic equations for electrons and photons simultaneously accounting for Compton scattering, synchrotron emission, absorption and particle thermalization, we show how the resulting spectrum in the two spectral states can be reproduced.