2001ApJ...562..670W

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 (Γ∼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 ∼3x10¹⁷ 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 ∼2.6x10^–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.