A Non-equipartition Shock Wave Traveling in a Dense Circumstellar Environment around SN 2020oi

We report the discovery and panchromatic follow-up observations of the young Type Ic supernova (SN Ic) SN 2020oi 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 behavior of the supernova is similar to those of other normal SNe Ic. The event was not detected in the X-ray band but our radio observations revealed a bright mJy source ( ${L}_{\nu }\approx 1.2\times {10}^{27}\,\mathrm{erg}\,{{\rm{s}}}^{-1}\,{\mathrm{Hz}}^{-1}$ ). 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 data set 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 ∊<SUB>e</SUB>/∊<SUB>B</SUB> ≳ 200, similar to a few other cases of stripped envelope SNe. Our modeling of the radio data suggests that the shock wave driven by the SN ejecta into the circumstellar matter (CSM) is moving at $\sim 3\times {10}^{4}\,\mathrm{km}\,{{\rm{s}}}^{-1}$ . 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 $1000\,\mathrm{km}\,{{\rm{s}}}^{-1}$ . The temporal evolution of the radio emission suggests a radial CSM density structure steeper than the standard r<SUP>-2</SUP>.