The late-time radio spectrum of SN 1993J

We present VLA radio continuum measurements of SN 1993J in M 81 at the frequencies of 0.32 (P-band), 1. 3 and 1.7 (L-band), 4.9 (C-band), 8.5 (X-band), and 14.9 (U-band) GHz carried out on December 17 and 21, 2000, about 2820 days after the supernova explosion. We find that a power-law spectrum, free-free absorbed by an homogeneous, or clumpy, distribution of ionized gas yields the best fit to the radio data. A combined homogeneous-clumpy model is not favored, but neither totally excluded. This result contrasts with the modeling of the early (t less than or similar to 230 days) radio emission from SN 1993J, where a mixture of homogeneous and clumpy absorbers appeared to be necessary to adequately describe the behavior of the light curves. The radio spectrum of supernova SN 1993J between 0.32 and 14.9 GHz is well characterized alpha = 0.67 +/- 0.02 (S-nu infinity nu(alpha)), typical of an optically thin radio supernova. A fit to the radio spectra of SN 1993J from similar to70 up to 2820 days shows that the observed spectral index of SN 1993J has been slowly evolving since t similar to 1000 days, with the observed spectral index changing from alpha approximate to -1 to alpha = -0.67. This spectral evolution seems to suggest that, in addition to the radiative (synchrotron) losses, adiabatic cooling and ionization (Coulomb) losses at the lowest frequencies might be contributing significantly to the integrated electron spectrum.