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 curves 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 with the hydrodynamic model of the explosion of a 14 M_solar C+O star, the core of a star with initial mass 40 M_solar, assuming that the explosion was very energetic (kinetic energy E_K=5×10⁵² ergs) and that 0.4 M_solar of ⁵⁶Ni were synthesized. At late times, however, the observed light-curve tail declines more slowly than this energetic model and is in better agreement with a less energetic (E_K=7×10⁵¹ ergs) one. This shift to a less energetic model may imply that the inner part of the ejecta has higher density and lower velocities than the model with E_K=5×10⁵² ergs, so that γ-rays deposit more efficiently. An aspherical explosion can produce such a structure of the ejecta. We also study detailed nucleosynthesis calculations for hyperenergetic supernova explosions and compare the yields with those of normal supernovae.