Astron. Astrophys. 324, L17-L20 (1997)

1. Introduction

The idea of cosmological origin of gamma ray bursts (Prilutski & Usov 1975; Usov & Chibisov 1975) is well supported by their observed isotropy on the sky and non-uniform spatial distribution Meegan et al. 1992). The enormous luminosity needed for explanation of gamma ray bursts if their origin is cosmological imply that the best candidates for their sources are the mergers of binary neutron stars and/or neutron stars with black holes at redshifts (), (Blinnikov, et al. 1984; Paczyski 1991, 1992). Lipunov et al (1995) showed that the cosmological origin of gamma ray bursts is consistent with the observed BATSE gamma ray bursts - distribution, but the beaming of the radiation is required for the balancing of the observed and predicted event rates.

The cosmological models for gamma ray bursts have not yet been proven; moreover, they come across severe problems, one of them being the problem of the efficiency of transforming the gravitational binding energy into gamma-rays. The explanation of the observed profiles of the bursts remain another unsolved problem.

Recently Shaviv and Dar (1995, 1996a, b) proposed a model of the gamma ray bursts origin which was the first one to reproduce the observed burst profiles, and also solved the problem of energy transformation into gamma rays. In this model, a gamma ray burst is produced by reemission of the soft field photon (  eV) by the atoms of a relativistic jet (or expanding shell) (with ), which transforms these photons to those of the energies  MeV.

It is suggested that such a jet or shell can be generated during a binary neutron star merger. In a dense stellar region such as a globular cluster core or a galaxy center, the particles of the jet would pass close to several stars. The light photons filling the vicinity of the stars in the comoving frame of the jet would have energy  keV. Shaviv and Dar show that these photons could be absorbed by photoionization or photoexcitation of heavy atoms (like iron) that may be present in the jet, the cross section of such absorption being much higher than that of Compton scattering. When reemitted, in the rest frame of the observer the photons will have energy of the order of  Mev and will be beamed into the angle. Thus, when passing by a star, the jet would produce a burst of radiation.

These photons generated closer to the jet source should come to the observer earlier than those generated far from it because the jet expansion velocity is smaller than the velocity of light. Therefore, each of the stars on the way of the jet would produce a peak in the burst profile. Below we will determine the shape of this peak. In general, the observed burst profile would map the distribution of the soft photon density on the way of the jet: , where R is the distance from the source to the current point, t is the total time taken the signal to pass from the jet source to the observer: , where D is the distance from the source to the observer.

The above model explains many of the observed features of gamma ray bursts, including the profiles.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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