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The standard model for radio supernovae considers that the observedsynchrotron radio emission arises from the high-energy shell that results fromthe strong interaction between the expanding supernova ejecta and thecircumstellar medium. This emission is considered to be only partially absorbedby 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 SN1993J, wepresent evidence of synchrotron self-absorption. Our modeling of the radiolight curves requires a large initial magnetic field, of about 30 Gauss, andthe existence of an (initially) highly-relativistic population of electrons. Wealso show that while at early epochs the dominant absorption mechanism isexternal absorption by thermal electrons, at late epochs and long wavelengthsthe dominant absorption mechanism is synchrotron self-absorption. Consequently,the spectral turnover takes place at much shorter wavelengths than expected inthe standard model, and at long wavelengths (greater than about 90cm at currentepochs) the flux predictions depart substantially from those of the standardmodel.Comment: Added references; typos corrected. 6 pages, 4 figures, uses aa.cl

The role of synchrotron self-absorption in the late radio emission from SN 1993J