Aspherical Explosion Models for SN 1998bw/GRB 980425

Abstract

The recent discovery of the unusual supernova SN 1998bw and its apparent correlation with the γ-ray burst GRB 980425 has raised new issues concerning both γ-ray bursts and supernovae. Although the spectra of SN 1998bw resemble those of Type Ic supernovae (SNe Ic), there are distinct differences. At early times the expansion velocities inferred by the Doppler shift of (unidentified) absorption features were very high, and SN 1998bw was unusually bright and red at maximum light (Galama et al.). These distinctions make SN 1998bw a candidate of a hypernova, with explosion energies exceeding supernovae by a factor in excess of 10. We present an alternative picture that allows SN 1998bw to have an explosion energy and ejecta mass consistent with core-collapse supernovae, although at the bright end of the typical range. We specifically propose that all SNe Ic are significantly asymmetric and that SN 1998bw is a SN Ic that is distinguished principally by being viewed close to the symmetry axis. We investigate the hypothesis that SNe Ic and SN 1998bw are the results of aspherical explosions along the rotational axis of basically spherical, nondegenerate C/O cores of massive stars. Light curves for aspherical explosions are computed assuming an ellipsoidal geometry for the ejecta. Guided by the polarization observations of normal SN Ic and related events, we assume an axis ratio of 2 near maximum light. The evolution of the isodensity contours with time is discussed. We show that the light curve of SN 1998bw may be understood with an explosion energy of 2 × 1051 ergs, a total ejecta mass of 2 M☉, and a mass of 56Ni of 0.2 M☉ if it is observed at a large angle (≥60°) with respect to the plane of symmetry. In this picture, the high expansion velocities are a direct consequence of an aspherical explosion mechanism which, in turn, produces oblate isodensity contours. Prolate isodensity contours are ruled out. This interpretation suggests that the fundamental core-collapse explosion process itself is strongly asymmetric.