Radio emission from supernova 1993 J

Supernova 1993 J exploded in the nearby spiral galaxy M81. Its initial spectrum contained hydrogen lines and therefore it was classified as a Type II supernova. However, subsequent disappearance of hydrogen lines and appearance of helium lines made it similar to a type Ib supernova. Radio emission was detected from SN 1993 J shortly after the first peak in optical brightness. In this paper we present a model for the observed radio radiation that involves the interaction of a collisionless shock front generated by the supernova ejecta and a stellar wind produced by the progenitor supergiant. We discuss electron acceleration, magnetic field generation and interpret an observed break in the radio spectrum.