  SN1998bw is the most luminous radio supernova ever observed. Previous
discussions argued that its exceptional radio luminosity, 4e38 erg/s, must
originate from a highly relativistic shock which is fully decoupled from the
supernova ejecta. Here we present an alternative model in which the radio
emission 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 MeV
emit synchrotron self-absorbed radiation in the post-shock magnetic field. This
model provides an excellent fit to the observed spectra provided that the
thermal electrons are in equipartition with the ions behind the shock. The
required magnetic field is much weaker than its equipartition value and could
have been carried out by the progenitor's wind prior to the supernova
explosion. According to this model, the radio emission from SN1998bw is
unrelated to the highly relativistic blast wave that produced the gamma-ray
burst GRB980425.
