The Late-Time Radio Emission from SN 1993J at Meter Wavelengths

This paper presents the investigations of SN 1993J using low-frequency observations with the Giant Metrewave Radio Telescope (GMRT). We analyze the light curves of SN 1993J at 1420, 610, 325, and 243 MHz during 7.5-10 yr after the explosion. The supernova has become optically thin early on in the 1420 and 610 MHz bands, while it has only recently entered the optically thin phase in the 325 MHz band. The radio light curve in the 235 MHz band is more or less flat. This indicates that the supernova is undergoing a transition from an optically thick to optically thin limit in this frequency band. In addition, we analyze the supernova radio spectra at five epochs on days 3000, 3200, 3266, 3460, and 3730 after the explosion. The spectral break in the day 3200 composite spectrum from the GMRT and Very Large Array implies that the plasma is dominated by a magnetic field, and the latter is far from being in equipartition with relativistic particles. SN 1993J is the only young supernova for which the magnetic field and the size of the radio-emitting region are determined through unrelated methods. Thus, the mechanism that controls the evolution of the radio spectra can be identified. We suggest that at all epochs, the synchrotron self-absorption mechanism is primarily responsible for the turnover in the spectra. Light-curve models based on free-free absorption in homogeneous or inhomogeneous media at high frequencies overpredict the flux densities at low frequencies. The discrepancy is increasingly larger at lower and lower frequencies. We suggest that an extra opacity, sensitively dependent on frequency, is likely to account for the difference at lower frequencies. The evolution of the magnetic field (determined from synchrotron self-absorption turnover) is roughly consistent with B ∝ t-1. The radio spectral index in the optically thin part evolves from α ~ 0.8-1.0 at a few tens of days to ~0.6 in about 10 yr.