TP-25

A Two-Step Model for Gamma-ray Bursts Assocoated with Supernovae

K.S.Cheng and Z.G.Dai

We propose a two-step mdoel for GRBs associated with supernovae. In the first step, the core collapse of a star with mass larger than 19 solar masses leasds to massive neutron star and a normal supernova,and the subsequently hypercritical accretion of the neutron star from the supernova ejecta may give rise to a jet through neutrino annihilation along the stellar rotation axis. However, because of too much surrounding material, this jet rapidly enters a nonrelativistic phase and evolves to a large bubble. In the second step, the neutron star promptly implodes to a rapidly rotating black hole surrounded by a torus once the mass of the star to the maximum mass and meanwhile its rotation frequency increases to the upper limit due to the accreted angular momentum. The gravitational binding energy of the torus may be dissipated by a magnetized relativistic wind, which may then be absorded by the supernova ejecta, thus producing an energetic hypernova. The rotational energy of the black hole may be extracted by the BZ process, leading to another jet. Because the first jet has pushed out of its front matter and left an exit, the second jet is relatively free of baryons and thus may be accelerated to an ultrarelativistic phase. We expect that the second jet generates a GRB and its afterglow.