Single black holes as parent bodies of galactic and extragalactic gamma-ray bursts

There are several pieces of evidence that the sample of gamma-ray bursts (GRBs) is inhomogeneous and that these objects can be situated in the Galaxy as well as at cosmological distances. We consider single black holes (BHs) of 10 - 100 M-. as parent objects for GRBs of both types. The origin of GRBs is connected to the release of energy generated by magnetic field inhomogeneities together with interstellar plasma accretion onto a regular magnetosphere around a BH. The percolation nature of the energy release for this kind of instability yields a universal power law for the GRB energy distribution, that is E-beta (with beta = 1.7 - 1.8), which is the same as that for the flares observed in the Sun, in flare stars and in X-ray sources. The number of GRBs per galaxy is about 10(3) for both galactic (with luminosities up to 10(38) erg s(-1)) and cosmological (with luminosities up to 10(52) erg s(-1)) populations. We also discuss the possibility of detecting galactic BHs as parent bodies of GRBs in quiescent phase, as stars of magnitude 20 to 29 with continuum spectra and fast variability.