Light curve and spectral models for the hypernova sn 1998bw associated with GRB 980425

A refined model for the unusual Type Ic supernova 1998bw, discovered as the optical counterpart to GRB 980425, is presented, and synthetic light curve s and spectra are compared with the observations. The first 30 days of the light curve and the broad-line features of the spectra can be reproduced wi th the hydrodynamic model of the explosion of a 14 M-circle dot C + O star, the core of a star with initial mass 40 M-circle dot, assuming that the ex plosion was very energetic (kinetic energy ergs) and that 0.4 M-circle dot of Ni-56 were synthesized. At late times, however, the observed light-curve tail declines more slowly than this energetic model and is in better agree ment with a less energetic (E-K = 7 x 10(51) ergs) one. This shift to a les s energetic model may imply that the inner part of the ejecta has higher de nsity and lower velocities than the model with E-K = 5 x 10(52) ergs, so th at gamma -rays deposit more efficiently. An aspherical explosion can produc e such a structure of the ejecta. We also study detailed nucleosynthesis ca lculations for hyperenergetic supernova explosions and compare the yields w ith those of normal supernovae.