A non-equipartition shockwave traveling in a dense circumstellar environment around SN 2020oi

We report the discovery and panchromatic followup observations of the young Type Ic supernova, 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 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_ν ≈ 1.2 × 10²⁷erg s⁻¹Hz⁻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 (formula presented) & 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 ∼ 3×10⁴ km s⁻¹. Assuming a constant mass-loss from the stellar progenitor, we find that the mass-loss rate is (formula presented) for an assumed wind velocity of 1000 km s⁻¹. The temporal evolution of the radio emission suggests a radial CSM density structure steeper than the standard r⁻².