GAMMA-RAY BURSTS FROM RELATIVISTIC BEAMS IN NEUTRON-STAR MERGERS

We show that following the coalescence of two neutron stars two opposi te relativistic beams are formed along the system axis. They are feede d in energy by nu<(nu)over bar> annihilation and can reach relativisti c factors Gamma similar to 10(2)-10(3). These high Gamma values are po ssible because (i) the central core of the merger is cooler than the s urrounding disk and (ii) most of the energy is released beyond the son ic point in the beam. Results obtained for different geometries of the disk neutrinosphere are presented and compared. The interaction of th e ejected matter with the interstellar medium will produce a gamma-ray burst if the kinetic energy (which can exceed 10(51) erg) can be effi ciently radiated by non thermal processes in shocks (Rees & Meszaros 1 992). We compute the burst duration and estimate the strength of its t hermal precursor as a function of Gamma. Finally, we briefly discuss c onstraints on the model imposed by the burst statistics.