Observations of GRO and UHEGR show that a number of Galactic and extragalactic accreting systems release most of their energy in the ultrarelativistic energy range (more than GeV). This result contradicts one of the principal conclusions of the standard models of accretion about a predominantly thermal character of energy release. This contradiction is caused by ignoring in the standard approach the processes of generation and amplification of magnetic field in the case of accretion onto a magnetized gravitating center. A new approach taking into account the processes mentioned above is applied to disk accretion onto a nonmagnetized gravitating center, as well as onto a magnetosphere. It is shown that in both cases the accretion is strictly controlled by the magnetic field, which leads to new conditions of equilibrium and stability and turns on nonthermal processes of energy release. The resulting configuration of the magnetic field, in which the main energy release takes place in both cases, is called “Z-pinch,” and is formed in the polar region of an accreting object. Effective particle acceleration occurs in it owing to the chain of MHD and resistive plasma instabilities, resulting in current discontinuity with the formation of “double layers” and generation of electric fields close to the Dreicer limit in them. The maximum energies of the accelerated particles are limited by the value 10 EeV, that coincides with the results of UHEGR observations.

Subject headings: acceleration of particles — accretion, accretion disks — gamma rays: theory — MHD