Circumstellar Emission from Type Ib and Ic Supernovae
  The presumed Wolf-Rayet star progenitors of Type Ib/c supernovae have fast, low density winds and the shock waves generated by the supernova interaction with the wind are not expected to be radiative at typical times of observation. The injected energy spectrum of radio emitting electrons typically has an observed index p=3, which is suggestive of acceleration in cosmic ray dominated shocks. The early, absorbed part of the radio light curves can be attributed to synchrotron self-absorption, which leads to constraints on the magnetic field in the emitting region and on the circumstellar density. The range of circumstellar densities inferred from the radio emission is somewhat broader than that for Galactic Wolf-Rayet stars, if similar efficiencies of synchrotron emission are assumed in the extragalactic supernovae. For the observed and expected ranges of circumstellar densities to roughly overlap, a high efficiency of magnetic field production in the shocked region is required (epsilon_B ~ 0.1). For the expected densities around a Wolf-Rayet star, a nonthermal mechanism is generally required to explain the observed X-ray luminosities of Type Ib/c supernovae. Although the inverse Compton mechanism can explain the observed X-ray emission from SN 2002ap if the wind parameters are taken from the radio model, the mechanism is not promising for other supernovae unless the postshock magnetic energy density is much smaller than the electron energy density. In some cases another mechanism is definitely needed and we suggest that it is X-ray synchrotron emission in a case where the shock wave is cosmic ray dominated so that the electron energy spectrum flattens at high energy. More comprehensive X-ray observations of a Type Ib/c supernova are needed to determine whether this suggestion is correct. 
Roger A. Chevalier, Claes Fransson