The Late Radio Spectrum of SN 1993

We present VLA radio continuum measurements of SN1993J in M81 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, but not a mixture of both– distribution of ionized gas, yields the best fit to the radio data. This result contrasts with the modeling of the early (t ∼ < 230 days) radio emission from SN1993J, where a mixture of homogeneous and clumpy absorbers appeared to be necessary to adequately describe the behavior of the light curves, and thus gives support to the suggestion that a change in the mass-loss properties of the progenitor star took place in the last stages of its evolution. The radio spectrum of supernova SN1993J between 0.32 and 14.9 GHz is well characterized by α = −0.67 ± 0.02 (Sν ∝ ν α), typical of an optically thin radio supernova. A fit to the radio spectra of SN1993J from ∼ 70 up to 2820 days shows that the observed spectral index of SN1993J has been slowly evolving since t ∼1000 days on, with the observed spectral index changing from α ≈ −1 to α = −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.