A Subrelativistic Shock Model for the Radio Emission of SN 1998bw

SN1998bw is the most luminous radio supernova ever observed. Previousdiscussions argued that its exceptional radio luminosity, 4e38 erg/s, mustoriginate from a highly relativistic shock which is fully decoupled from thesupernova ejecta. Here we present an alternative model in which the radioemission originates from a sub-relativistic shock with a velocity of 0.3c,generated in the surrounding gas by the expanding ejecta. In this model,thermal electrons heated by the shock to a relativistic temperature of 60 MeVemit synchrotron self-absorbed radiation in the post-shock magnetic field. Thismodel provides an excellent fit to the observed spectra provided that thethermal electrons are in equipartition with the ions behind the shock. Therequired magnetic field is much weaker than its equipartition value and couldhave been carried out by the progenitor's wind prior to the supernovaexplosion. According to this model, the radio emission from SN1998bw isunrelated to the highly relativistic blast wave that produced the gamma-rayburst GRB980425.Comment: 12 pages, 2 figures, submitted to ApJ Letter