The radio evolution of the ordinary Type Ic supernova SN 2002ap

We report the discovery and monitoring of radio emission from the Type Ic supernova SN 2002ap ranging in frequency from 1.43 to 22.5 GHz and in time from 4 to 50 days after the SN explosion. As in most other radio SNe, the radio spectrum of SN 2002ap shows evidence for absorption at low frequencies, usually attributed to synchrotron self-absorption (SSA) or free-free absorption. While it is difficult to discriminate between these two processes based on a goodness of fit, the unabsorbed emission in the free-free model requires an unreasonably large ejecta energy. Therefore, on physical grounds we favor the SSA model. In the SSA framework, at about day 2, the shock speed is approximate to0.3c, the energy in relativistic electrons and magnetic fields is approximate to1.5 x 10(45) ergs, and the inferred progenitor mass-loss rate is approximate to5 x 10(-7) M-. yr(-1) (assuming a 10(3) km s(-1) wind). These properties are consistent with a model in which the outer, high-velocity SN ejecta interact with the progenitor wind. The amount of relativistic ejecta in this model is small, so the presence of broad lines in the spectrum of a Type Ib/c SN as observed in SN 2002ap is not a reliable indicator of relativistic ejecta and hence gamma-ray emission.