SN 1993J: The Early Radio Emission and Evidence for a Changing Presupernova Mass-Loss Rate

We present daily-to-weekly monitoring of the first 8 months of radio emission from supernova (SN) 1993J using the Very Large Array at five wavelengths, providing the most detailed radio light curves ever established for an SN. While the radio emission evolves regularly in both time and frequency, the usual form of the SN shock/circumstellar medium interaction model does not adequately describe the observations. In particular, for a spherically symmetric geometry, the circumstellar density profile is flatter (ρ_CSM_ is proportional to r^-1.5^) than that (ρ_CSM_ is proportional to r^-2^) generally assumed for a constant mass-loss rate, constant-velocity stellar wind. This is interpreted as a decrease in the pre-SN mass-loss rate (or increase in wind velocity) immediately prior to explosion. Additionally, the rate of increase in the early radio emission at each frequency cannot simply he described as due to decreasing external absorption by a uniform medium, but requires the presence of higher density "clumps" or "filaments" embedded in the stellar wind.