SN 1998bw/GRB 980425 and Radio Supernovae

Abstract

The unusual supernova SN 1998bw, which is thought to be related to the gamma-ray burster GRB 980425, is a possible link between these two classes of objects. Analyzing the extensive radio emission data available for SN 1998bw, we are able to describe its time evolution within the well-established framework available for the analysis of radio emission from supernovae. This then allows description of a number of physical properties of the object. The radio emission can be best explained as the interaction of a mildly relativistic (F ∼ 1.6) shock with a dense, preexplosion stellar wind-established circumstellar medium that is highly structured both azimuthally, in clumps or filaments, and radially, with two observed density enhancements separated by ∼3 × 1017 cm. With assumptions as to preexplosion stellar wind conditions, it is possible to estimate that the progenitor to SN 1998bw had a mass-loss rate of &2.6 × 10-5 M⊙ yr-1 with at least two ∼40% density increases, the most recent extending from ∼1600 to 4700 yr before explosion and the oldest known having occurred, possibly with comparable length, ∼ 12,000 yr before explosion. Because of its unusual characteristics for a Type Ib/c supernova, the relation of SN 1998bw to GRB 980425 is strengthened, consequently improving our understanding of these poorly understood objects. © 2001. The American Astronomical Society. All rights reserved.