The role of synchrotron self-absorption in the late radio emission from SN1993J

The standard model for radio supernovae considers that the observed synchro tron radio emission arises from the high-energy shell that results from the strong interaction between the expanding supernova ejecta and the circumst ellar medium. This emission is considered to be only partially absorbed by ionized thermal electrons in the circumstellar wind of the progenitor star. Based on a study of the radio light curves of the type II supernova SN1993 J, we present evidence of synchrotron self-absorption. Our modeling of the radio light curves requires a large initial magnetic field, of about 30 Gau ss, and the existence of an (initially) highly-relativistic population of e lectrons. We also show that while at early epochs the dominant absorption m echanism is external absorption by thermal electrons, at late epochs and lo ng wavelengths the dominant absorption mechanism is synchrotron self-absorp tion. Consequently, the spectral turnover takes place at much shorter wavel engths than expected in the standard model, and at long wavelengths (greate r than or equal to 90 cm at current epochs) the flux predictions depart sub stantially from those of the standard model.