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

We present the investigations of SN 1993J using low frequency observationswith the Giant Meterwave Radio Telescope. We analyze the light curves of SN1993J at 1420, 610, 325 and 243 MHz during $7.5-10$ years since explosion.Thesupernova has become optically thin early on in the 1420 MHz and 610 MHz bandswhile it has only recently entered the optically thin phase in the 325 MHzband. The radio light curve in the 235 MHz band is more or less flat. Thisindicates that the supernova is undergoing a transition from an optically thickto optically thin limit in this frequency band. In addition, we analyze the SNradio spectra at five epochs on day 3000, 3200, 3266, 3460 and 3730 sinceexplosion. Day 3200 spectrum shows a synchrotron cooling break. SN 1993J is theonly young supernova for which the magnetic field and the size of the radioemitting region are determined through unrelated methods. Thus the mechanismthat controls the evolution of the radio spectra can be identified. We suggestthat at all epochs, the synchrotron self absorption mechanism is primarilyresponsible for the turn-over in the spectra. Light curve models based on freefree absorption in homogeneous or inhomogeneous media at high frequenciesoverpredict the flux densities at low frequencies. The discrepancy isincreasingly larger at lower and lower frequencies. We suggest that an extraopacity, sensitively dependent on frequency, is likely to account for thedifference at lower frequencies. The evolution of the magnetic field(determined from synchrotron self absorption turn-over) is roughly consistentwith $B \propto t^{-1}$. Radio spectral index in the optically thin partevolves from $\alpha \sim 0.8-1.0$ at few tens of days to $\sim 0.6$ in about10 years.Comment: 37 pages, 9 figures in LaTex; scheduled for ApJ 10 September 2004, v612 issue; send comments to: poonam@tifr.res.i