Gamma-ray Bursts and Hypernovae

Abstract

In these proceedings, I discuss recent progress in understanding the nature of cosmic gamma-ray bursts (GRB), with the focus on the apparent relation of several GRBs with an energetic subclass of stellar explosions, type Ib/c core-collapse supernovae. This relation provides the strong case that the GRB phenomenon is connected with the final stages of massive star evolution and possibly with the formation of neutron stars and black holes. I speculate that intrinsically faint, apparently spherically symmetric nearby GRB 980425 and 031203 associated with bright hypernovae SN 1998bw and SN 2003lw, respectively, can signal the formation of a neutron star in the end of gravitational collapse, while the bulk of cosmological GRBs with a universal energy release of $\sim 10^{51}$ ergs in narrow-collimated jets are produced when a black hole is formed. In the former case, the energy source of GRB is the neutron star rotational energy; in the latter case the GRB energy is due to non-stationary accretion onto the black hole.