Abstract
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.