Cosmological origin

From a geometrical point of view, to place the GRB emitters at cosmological distances is the most suitable way to match the observed properties of isotropy and apparent inhomogeneity. The former would be an automatic result of the large distance to the emitters, while the latter could come from redshift effects and/or intrinsic source evolution. The main argument against this model is the lack of unambiguous observational evidences, like time dilation effects ([Norris et al. 1994]) or gravitational lensing.

If the mean emission distance is at redshift $z \sim 1$(that is about 10¹² times the radius of our Solar System) then the implied intrinsic luminosity would be of the order of 10⁵¹  erg  s^-1. This can only be obtained through catastrophic events, like the merging of two compact components of a binary system such as a neutron star-neutron star or a neutron star-black hole binary ([Paczynski 1986]). The needed rate for taking into account the observed distribution of GRBs is about 10^-6 per galaxy per year ([Fishman et al. 1995]).

Given the destructive nature of the emission process, an experimental proof against the cosmological model would be the detection of a repeating source of GRBs. This has been actually detected in three cases so far, but they have shown peculiar spectral characteristics compared to the GRBs and have been therefore reclassified as Soft Gamma-ray Repeaters. Their counterparts seem to be hosted in supernova remnants.

In case such a catastrophic event such as the merging of two compact stars takes place, it is hard to think to an evolution other than that described by the fireball model ([Cavallo et al. 1978,Rees et al. 1992]). In its basic version this is a relativistic expanding shell of electron-positron pairs deriving from the annihilation of neutrinos and anti-neutrinos carrying out most of the energy of the initial explosion. The expanding shell, possibly enriched with a few highly energetic protons, collides with the interstellar medium provoking a shock front that accelerates the electrons, causing them to emit (for example, synchrotron radiation). The Lorentz factor of the expanding shell can account for the blue-shift of the X-rays into the gamma-ray band typical of the GRBs. The morphological/temporal diversity can be accounted for by the possible diversity in the environmental conditions and/or by the existence of more than one shell and therefore from the interaction of one with the other ([Fenimore et al. 1996]).