The radio spectra of SN 2020oi: Effects of radiative cooling on the deduced source properties

TL;DR

This paper investigates how radiative cooling affects the interpretation of radio spectra in supernova SN 2020oi, emphasizing the importance of including cooling in spectral modeling to accurately determine source properties.

Contribution

It demonstrates that incorporating cooling directly into spectral fits aligns observations with equipartition and constant mass-loss, challenging previous claims that neglected this effect.

Findings

Inverse Compton cooling is evident in early spectra.

Including cooling in models yields consistent source properties.

Relativistic electron energies are likely too low for Fermi acceleration.

Abstract

Observations of radiative cooling in a synchrotron source offer a possibility to further constrain its properties. Inverse Compton cooling is indicated in the radio spectra during the early phases of SN\,2020oi. It is shown that contrary to previous claims, observations are consistent with equipartition between relativistic electrons and magnetic field as well as a constant mass-loss rate of the progenitor star prior to the supernova explosion. The reason for this difference is the need to include cooling directly in the fitting procedure rather than estimating its effects afterward. It is emphasized that the inferred properties of the supernova ejecta are sensitive to the time evolution of the synchrotron self-absorption frequency; hence, great care should be taken when modeling spectra for which cooling and/or inhomogeneities are indicated. Furthermore, it is noted that the energies of the relativistic electrons in the radio emission regions in supernovae are likely too low for first-order Fermi acceleration to be effective.