A New Model for the Radio Emission from SN 1994I and an Associated Search for Radio Transients in M51

TL;DR

This paper revises the radio emission model for supernova SN 1994I, estimates the progenitor's mass loss rate, and conducts a pilot search for radio transients in M51, placing upper limits on transient rates using archival data.

Contribution

It introduces a new synchrotron self-absorption model for SN 1994I and demonstrates the use of archival radio data to constrain transient rates in nearby galaxies.

Findings

The revised model fits the radio data well and estimates the progenitor's mass loss rate.

No new radio transients were detected in the archival data.

An upper limit of 17 deg$^{-2}$ for transient source density above 0.5 mJy was established.

Abstract

We revisit the exquisite archival radio data for the Type Ic supernova SN 1994I and present a revised model for the 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 $\dot{M}=3.0 \times 10^{-5} M_{\odot}$ 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 \cite{weil11}, but our $\dot{M}$ 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 $2\sigma$ upper limit of 17 deg$^{-2}$ for the source density of radio transients above 0.5 mJy ($L\gtrsim4\times10^{25}$ erg s$^{-1}$ Hz$^{-1}$ at the distance of M51). This study highlights the feasibility of utilizing archival high-cadence radio studies of supernova host galaxies to place constraints on the radio transient rate as a function of luminosity in the local Universe.