Synchrotron aging and the radio spectrum of SN 1993J

We combine the Giant Metrewave Radio Telescope low-frequency radio observations of SN 1993J with the Very Large Array high-frequency radio data to get a near-simultaneous spectrum around day 3200 since explosion. The low-frequency measurements of the supernova determine the turnover frequency and flux scale of the composite spectrum and help reveal a steepening in the spectral index, Deltaalpha similar to 0.6, in the optically thin part of the spectrum. This is the first observational evidence of a break in the radio spectrum of a young supernova. We associate this break with the phenomenon of synchrotron aging of radiating electrons. From the break in the spectrum we calculate the magnetic field in the shocked region independent of the equipartition assumption between energy density of relativistic particles and magnetic energy density. We determine the ratio of these two energy densities and find that this ratio is in the range 8.5 x 10(-6) to 5 x 10(-4). We also predict the nature of the evolution of the synchrotron break frequency with time, with competing effects due to diffusive Fermi acceleration and adiabatic expansion of the radiative electron plasma.