A "Hypernova" Model for SN 1998bw and $\gamma$-Ray Burst of 25 April 1998

The supernova (SN) 1998bw sets an important precedent because of its possible link to the gamma-ray burst (GRB980425), suggested by their coincidence in position and time of occurrence$^{1,2}$. The observed spectral features indicate that SN 1998bw is likely to be a type Ic supernova, originating from a carbon+oxygen (C+O) star progenitor. However, the observed peak luminosity, $L of previously known type Ib/Ic supernovae. This implies the production of present a `hypernova' model for SN 1998bw and discuss its possible connection to GRB980425$^{1}$. The model is an exploding C+O star with a mass of $\sim 12-15 M_ødot$, which is characterized by an explosion energy of $\sim 2-5 evolution calculated for this model are in good agreement with observations of SN 1998bw$^{2}$. Models with smaller mass and energy are not consistent with the observations. The extremely large explosion energy could be explained by a rapidly rotating black hole with a strong magnetic field. The massive progenitor star (initially $\sim 40 M_\odot$) must have had an unusually large angular momentum or a strong magnetic field owing possibly to binary interaction. At the shock breakout in this energetic explosion, the surface layer is easily accelerated to relativistic speeds and the consequent interaction with the circumstellar material produces an external shock that could give rise to the gamma-ray burst