Comptonization processes in galactic and extragalactic high energy sources

We review the principal radiation mechanisms which can be responsible for the production of high energy emission of Galactic and extragalactic sources. The spectral properties of Comptonization processes are studied. We present a rigorous treatment of the problem in terms of the Boltzmann kinetic equation formalism. We also overview the different exact and approximate analytical and numerical techniques which decouple the photon transport in six dimensions to that in the configuration space and in energy space separately. CGRO, RXTE, and SAX observations of Galactic black hole candidates, neutron star systems, and Seyfert nuclei show that their spectra can be fitted by thermal Comptonization models in the hard state, and two components in the soft state. In the latter case, these components show a blackbody like spectrum with the color temperature which is a fraction of keV and tens of eV for Galactic and extragalactic sources, respectively, and an extended power-law with spectral index between 1 and 2 (in the neutron star systems in their soft State this power-law component is not detected). We give arguments for an explanation of these two types of spectral states in the framework of the thermal and bulk motion Comptonization processes.