ABSTRACT
the values <1>1 "' 10-4 erg cm-2 . If a source located at a distance R radiates isotropically, its total energy output in gamma-ray photons is obviously W, = 47rR2 <I>"Y. This discovery was reported only in 1973 [99]. Gamma-ray bursts have been intensively investigated since then but their nature has long remained unclear. The point is that the angular resolution of gammatelescopes is not high and observations in other bands (radio wavelength, optical, and X-ray) in the direction of a gamma-ray burst were not carried out immediately. Thus, the source remained absolutely unknown. One of the probable candidates were neutron stars in the Galaxy. In this case, for comparatively close neutron stars at a distance R "' 100 pc ~ 3 x 1020 em, the energy output was W1 ;S 1038 erg. This is already very much if we recall that the total luminosity of the Sun is L0 = 3.83 x 1033 erg s-1 . However, the distribution of even weak gamma-ray bursts over the sky proved to be isotropic, which means that their sources cannot be located in the galactic disc. If they are located in the giant galactic halo so that R "' 100 kpc (this does not already contradict the data on the angular distribution of sources), then W, ;S 1044 erg. Finally, if the bursts are of cosmological origin and, for example, R "' 1000 Mpc then we already have W1 ;S 1052 erg. This value is so large that many scientists (including me) gave preference to the halo model but, in 1997, a gamma-burst was 'observed' immediately (i.e. its position was located almost instantaneously) and sources with a large redshift were discovered [100, 101]. So, for the burst GRB 971214 (the designation implies that this burst was registered on 14 December, 1997) the redshift parameter11 was z = 3.46 [102] . For the burst GRB 970508, this parameter was z ~ 0.8. The sources (already many of them are known) were observed both in the X-ray and optical bands and some of them also in the radio wavelength band. The work is in full swing, and literally a day after the previous sentences were written for the previous edition, on 23 January, 1999, a powerful burst, GRB 990123, was observed over the entire investigated gamma-ray band from 30 keV to 300 MeV, which lasted 100 s. Simultaneously with the gamma-burst, a burst of light was registered whose maximum luminosity reached L0 "' 2 x 1016 L0 "' 1050 erg s-1 . The total energy output in all the electromagnetic bands was W "' 3 x 1054 erg if the radiation was isotropic (the redshift of the event was z = 1.61). More details concerning gammarbursts are given in the review [103] (see also [245]). GRB 021004 (detected, as clear from its designation, on 4 October, 2002) proved to be very intriguing. It was accompanied by a prolonged optical afterglow. For GRB 000131, the redshift parameter was z = 4.5, which corresponds to a distance of 11 x 109 light years [154] from us. It should be emphasized that gamma-ray emission from gamma-ray bursts is most probably anisotropic. Additional strong evidence for this
