A NEW MECHANISM FOR GAMMA-RAY BURSTS IN SN TYPE I EXPLOSIONS. I. WEAK MAGNETIC FIELD

摘要

We propose a new mechanism for high-energy gamma-ray bursts in supernova type I (SN I) explosions. From their observational features, they are a new type of bursts, different from others observed. A presupernova is assumed to be a binary system made up of a red giant and a white dwarf with a wind accretion. The accretion flow is terminated by an accretion shock in the vicinity of the white dwarf at a distance of the order of the accretion radius. The gas inside the accretion radius constitutes the main fraction of the target for gamma-ray production. The supernova explosion and the shock propagation in the white dwarf result in the hydrodynamical acceleration of the outer layers of the star. It proceeds in two stages: the first stage is caused by the shock propagating in the outer layers of the star, and the second stage is connected with the adiabatic expansion of the ejected shell into low-density medium around the white dwarf. The spectrum of accelerated particles is steep, and the maximum energy does not exceed 1000 GeV. The gamma-ray burst is produced by the interaction of the accelerated particles with the gas in the binary system. Most of the photons have energies about 100 MeV. The total number of emitted photons is between 10~(46) and 10~(47). The typical duration of the burst is ～1-3 s for ～100 MeV photons and 10~(-3) s for ～1 GeV photons. Thus, the bursts can be detected at distances less than 1 Mpc, with frequency less or equal to that of SN I. The gamma-ray burst might have one or two precursors. The first one is produced during the shock breakout, when the shock approaches the star surface and crosses it. This burst is produced by the heated gas behind the shock; the radiation is blueshifted because of the relativistic motion of the shell. The second burst might be produced under the appropriate choice of the parameters at the stage of the adiabatic expansion of the shell of the accelerated matter, when the shell becomes transparent for radiation. Our calculations are valid in the case of a weak magnetic field. The case of strong magnetic field will be considered in Paper II (in preparation).