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

Abstract

SN 1998bw is the most luminous radio supernova ever observed. Previous discussions argued that its exceptional radio luminosity, ~4 × 1038 ergs s-1, must originate from a highly relativistic shock that is fully decoupled from the supernova ejecta. Here we present an alternative model in which the radio emission originates from a subrelativistic shock with a velocity 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 postshock 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. Our model demonstrates that the radio emission from SN 1998bw is not necessarily related to the highly relativistic blast wave that produced the γ-ray burst GRB 980425.