SN 1998bw/GRB 980425 and radio supernovae

The unusual supernova SN 1998bw, which is thought to be related to the gamm a-ray burster GRB 980425, is a possible link between these two classes of o bjects. Analyzing the extensive radio emission data available for SN 1998bw , we are able to describe its time evolution within the well-established fr amework available for the analysis of radio emission from supernovae. This then allows description of a number of physical properties of the object. T he radio emission can be best explained as the interaction of a mildly rela tivistic (Gamma similar to 1.6) shock with a dense, preexplosion stellar wi nd-established circumstellar medium that is highly structured both azimutha lly, in clumps or filaments, and radially, with two observed density enhanc ements separated by similar to3 x 10(17) cm. With assumptions as to preexpl osion stellar wind conditions, it is possible to estimate that the progenit or to SN 1998bw had a mass-loss rate of similar to2.6 x 10(-5) M. yr(-1) wi th at least two similar to 40% density increases, the most recent extending from similar to 1600 to 4700 yr before explosion and the oldest known havi ng occurred, possibly with comparable length, similar to 12,000 yr before e xplosion. Because of its unusual characteristics for a Type Ib/c supernova, the relation of SN 1998bw to GRB 980425 is strengthened, consequently impr oving our understanding of these poorly understood objects.