Extragalactic scenarios for gamma-ray bursts

The cosmological distance scale to gamma-ray bursts is indicated by the observed distribution of the bursts in angle and intensity. The redshifts of the weakest and the strongest bursts detected by the BATSE experiment are estimated to be z = 1.5 and z = 0.2, respectively, while the strongest bursts detected by the PVO in its 11 years of operation were at z = 0.05. The bursts' rate is about 10 exp -6 per year in a galaxy like ours, and a typical energy release is about 10 exp 51 ergs, assuming isotropic emission. The rate of the neutron star and black hole collisions in close binaries is somewhat higher, and those collisions release 10 exp 53 - 10 exp 54 ergs in a burstlike form, making them the primary candidates for the observed gamma-ray events. Contrary to the common belief, no observations available to date are in conflict with the cosmological distance to the bursters. A detection of gravitational lensing of some bursts, the redshift of the weakest events, and the absorption of soft X-rays by the interstellar matter in the plane of our galaxy, are the specific predictions of the cosmological scenario.