A NEW MECHANISM FOR GAMMA-RAY BURSTS IN SN TYPE-I EXPLOSIONS .1. WEAKMAGNETIC-FIELD

We propose a new mechanism for high-energy gamma-ray bursts in superno va type I (SN I) explosions. From their observational features, they a re a new type of bursts, different from others observed. A presupernov a 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 ac cretion shock in the vicinity of the white dwarf at a distance of the order of the accretion radius. The gas inside the accretion radius con stitutes the main fraction of the target for gamma-ray production. The supernova explosion and the shock propagation in the white dwarf resu lt in the hydrodynamical acceleration of the outer layers of the star. It proceeds in two stages: the first stage is caused by the shock pro pagating in the outer layers of the star, and the second stage is conn ected with the adiabatic expansion of the ejected shell into low-densi ty medium around the white dwarf. The spectrum of accelerated particle s 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) a nd 10(47). The typical duration of the burst is similar to 1-3 s for s imilar to 100 MeV photons and 10(-3) s for similar to 1 GeV photons. T hus, the bursts can be detected at distances less than 1 Mpc, with fre quency less or equal to that of SN I. The gamma-ray burst might have o ne or two precursors. The first one is produced during the shock break out, when the shock approaches the star surface and crosses it. This b urst is produced by the heated gas behind the shock; the radiation is blueshifted because of the relativistic motion of the shell. The secon d burst might be produced under the appropriate choice of the paramete rs at the stage of the adiabatic expansion of the shell of the acceler ated matter, when the shell becomes transparent for radiation. Our cal culations are valid in the case of a weak magnetic field. The case of strong magnetic field will be considered in Paper II (in preparation).