The Supernova–Gamma-Ray Burst Connection

Abstract

Study of the polarization of supernovae has suggested that the core collapse process may be intrinsically strongly asymmetric. There is a tentative trend for supernova with smaller envelopes showing more polarization, with Type Ic having the smallest envelopes and showing the largest polarization. The recent discovery of the unusual supernova SN 1998bw and its apparent correlation with the gamma-ray burst GRB~980425 has raised new issues concerning both the gamma-ray bursts and supernovae. SN 1998bw resembled a Type Ic, but was unusually bright at maximum light in the optical and radio, and its expansion velocities were large. This makes SN 1998bw a possible candidate for a "hypernova" with explosion energies exceeding 10^52 erg. We show that the light curve of SN 1998bw can be understood as the result of viewing an aspherical explosion roughly along the symmetry axis of an exploding, non-degenerate C/O core of a massive star with a kinetic energy of 2x10^51 erg, a total ejecta mass of 2 solar masses, and a nickel-56 mass of 0.2 solar masses. In this model, the high expansion velocities are a direct consequence of the aspherical explosion which, in turn, produces oblate iso-density contours and that accounts for the polarization. It is not yet clear how either the hypernovae or these asymmetric models can produce gamma-ray bursts.