A non-equipartition shockwave traveling in a dense circumstellar environment around SN2020oi

Abstract

We report the discovery and panchromatic followup observations of the young Type Ic supernova, SN2020oi, in M100, a grand design spiral galaxy at a mere distance of $14$ Mpc. We followed up with observations at radio, X-ray and optical wavelengths from only a few days to several months after explosion. The optical behaviour of the supernova is similar to those of other normal Type Ic supernovae. The event was not detected in the X-ray band but our radio observation revealed a bright mJy source ($L_{\nu }\approx 1.2\times 10^{27}{\mathrm{erg}\mathrm{s}}^{-1}{\mathrm{Hz}}^{-1}$). Given, the relatively small number of stripped envelope SNe for which radio emission is detectable, we used this opportunity to perform a detailed analysis of the comprehensive radio dataset we obtained. The radio emitting electrons initially experience a phase of inverse Compton cooling which leads to steepening of the spectral index of the radio emission. Our analysis of the cooling frequency points to a large deviation from equipartition at the level of ${\epsilon }_e/{\epsilon }_B\gtrsim 200$, similar to a few other cases of stripped envelope SNe. Our modeling of the radio data suggests that the shockwave driven by the SN ejecta into the circumstellar matter (CSM) is moving at $\sim 3\times 10^4{\mathrm{km}\mathrm{s}}^{-1}$. Assuming a constant mass-loss from the stellar progenitor, we find that the mass-loss rate is $\dot{M}\approx 1.4\times 10^{-4}{M}_{\odot }{\mathrm{yr}}^{-1}$, for an assumed wind velocity of $1000{\mathrm{km}\mathrm{s}}^{-1}$. The temporal evolution of the radio emission suggests a radial CSM density structure steeper than the standard $r^{-2}$.

Submitted to ApJ (21 pages, 9 figures)

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