Radio Emission from Supernovae

I consider radio emission from the remarkable SN1998bw. Ni-56 and Co-56 decays produce a gamma-ray flux whose Compton-scattered electrons naturally explain the observed mildly relativistic exapnsion of the radio source and its double- peaked history. Such models require a surrounding plasma, perhaps produced by the supernova progenitor, whose interaction with the nonrelativistic debris may account for the observed X-ray source. The radio spectrum appears to be self- absorbed. This interpretation determines the brightness temperature, and hence the energy of the radiating electrons, implying a surprisingly large magnetic field. Attempts to avoid this conclusion by interpreting the spectrum as the result of inverse bremsstrahlung absorption do not lead to significantly lower fields. The large inferred field may have several explanations: radiation from a central pulsar, a turbulent hydrodynamic dynamo or an aspherical Compton current, but a frozen-in field from the supernova progenitor is not adequate. The electron-ion and particle-field equipartition problems are discussed. Compton electrons also explain the inferred expansion speed of SN1987A's spots.