A NEW MODEL FOR THE RADIO EMISSION FROM SN 1994I AND AN ASSOCIATED SEARCH FOR RADIO TRANSIENTS IN M51

ABSTRACT We revisit the exquisite archival radio data for the Type Ic supernova SN 1994I and present a revised model for the supernova (SN) radio emission and a pilot study that aims to constrain the rate of C-band radio transients within the face-on host galaxy, M51 (NGC 5194). We find that the temporal and spectral evolution of the SN 1994I radio emission are well fit by a synchrotron self-absorption model and use this to estimate physical parameters. We compute a pre-explosion mass loss rate of yr−1 for the progenitor, consistent with those observed from galactic Wolf-Rayet stars. Our model makes different assumptions for the dynamical model for the shockwave interaction than the model previously published by Weiler et al., but our is consistent with theirs to within errors and assumptions. Drawing from a subset of the archival radio observations from the Very Large Array collected for the monitoring of SN 1994I, we conduct a pilot study to search for previously unidentified transients. Data were primarily taken at a frequency of 4.9 GHz and are logarithmic in cadence, enabling sensitivity to transients with variability timescales ranging from days to months. We find no new transient detections in 31 epochs of data, allowing us to place a upper limit of 17 deg−2 for the source density of radio transients above 0.5 mJy ( erg s−1 Hz−1 at the distance of M51). This study highlights the feasibility of utilizing archival high-cadence radio studies of SN host galaxies to place constraints on the radio transient rate as a function of luminosity in the local universe.