2002ApJ...577L...5B

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 ~0.3c, the energy in relativistic electrons and magnetic fields is ~1.5x10⁴⁵ ergs, and the inferred progenitor mass-loss rate is ~5x10^–7 M_☉ yr^–1 (assuming a 10³ 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 γ-ray emission.