Recent observations of the EGRET-telescope aboard the Compton Gamma Ray Observatory (CGRO), results from Čerenkov shower telescopes, and fast variability studies have demonstrated the dominant role of nonther mal processes for some accreting relativistic objects. This is perhaps an unex pected result, because the basic assumption of the standard accretion models (Shakura & Sunyaev 1973, Sunyaev & Titarchuk 1980, Galeev et al. 1979) is the thermal nature of all processes that determine the energy output. At the same time, the recent observations argue that the source of the emission is not the thermal plasma as a whole, but a very small fraction of charged particles accelerated to very high energy (ε/mc2 > (30-1000) with a “universal” energy spectrum nε ∣ ε−α. These observational results suggest a revison of the global approach to the accretion process to find any new (or forgotten “old”) energy conversion mechanisms with predominately nonthermal output (Lynden-Bell 1969, Ikhsanov & Pustil’nik 1994).

The nature of the energy source and mechanisms of energy conversion to either plasma heating (“thermal” mode) or to acceleration of charged particles (“nonthermal” mode) is the main question for accretion theories.