A HYPERNOVA MODEL FOR THE SUPERNOVA ASSOCIATED WITH THE GAMMA-RAY BURST OF 25 APRIL 1998

The discovery of the unusual supernova SN1998bw, and its possible asso ciation with the gamma-ray burst GRB980425(1-3), provide new insights into the explosion mechanism of very massive stars and the origin of s ome classes of gamma-ray bursts. Optical spectra indicate that SN1998b w is a type Ic supernova(3,4), but its peak luminosity is unusually hi gh compared with typical type Ic supemovae(3). Here we report our find ings that the optical spectra and the light curve of SN1998bw can be w ell reproduced by an extremely energetic explosion of a massive star c omposed mainly of carbon and oxygen (having lost its hydrogen and heli um envelopes). The kinetic energy of the ejecta is as large as (2-5) x 10(52) erg; more than ten times that of previously observed supernova e, This type of supernova could therefore be termed 'hypernova'. The e xtremely large energy suggests the existence of a new mechanism of mas sive star explosion that can also produce the relativistic shocks nece ssary to generate the observed gamma-rays.