Luminous Late-time Radio Emission from Supernovae Detected by the Karl G. Jansky Very Large Array Sky Survey (VLASS)

M. C. Stroh, G. Terreran, D. L. Coppejans, J. S. Bright, R. Margutti, M. F. Bietenholz, F. De Colle, L. DeMarchi, R. Barniol Duran, D. Milisavljevic, K. Murase, K. Paterson, W. L. Williams

Published 2021-06-17Version 1

We present a population of 20 radio-luminous supernovae (SNe) with emission reaching $L_{\nu}{\sim}10^{26}-10^{29}\rm{erg s^{-1} Hz^{-1}}$ in the first epoch of the Very Large Array Sky Survey (VLASS) at $2-4$ GHz. Our sample includes one long Gamma-Ray Burst, SN 2017iuk/GRB171205A, and 19 core-collapse SNe detected at $\approx (1-60)$ years after explosion. No thermonuclear explosion shows evidence for bright radio emission, and hydrogen-poor progenitors dominate the sub-sample of core-collapse events with spectroscopic classification at the time of explosion (73%). We interpret these findings into the context of the expected radio emission from the forward shock interaction with the circumstellar medium (CSM). We conclude that these observations require a departure from the single wind-like density profile (i.e., $\rho_{\rm{CSM}}\propto r^{-2}$) that is expected around massive stars and/or a departure from a spherical Newtonian shock. Viable alternatives include the shock interaction with a detached, dense shell of CSM formed by a large effective progenitor mass-loss rate $\dot M \sim (10^{-4}-10^{-1})$ M$_{\odot}$ yr$^{-1}$ (for an assumed wind velocity of $1000\,\rm{km\,s^{-1}}$); emission from an off-axis relativistic jet entering our line of sight; or the emergence of emission from a newly-born pulsar-wind nebula. The relativistic SN\,2012ap that is detected 5.7 and 8.5 years after explosion with $L_{\nu}{\sim}10^{28}$ erg s$^{-1}$ Hz$^{-1}$ might constitute the first detections of an off-axis jet+cocoon system in a massive star. Future multi-wavelength observations will distinguish among these scenarios. Our VLASS source catalogs, which were used to perform the VLASS cross matching, are publicly available at https://doi.org/10.5281/zenodo.4895112.