The dense and non-homogeneous circumstellar medium revealed in radio wavelengths around the Type Ib SN 2019oys

TL;DR

This study presents broadband radio observations of SN 2019oys, revealing a dense, non-homogeneous circumstellar medium through spectral modeling, indicating a high progenitor mass-loss rate and complex CSM structure.

Contribution

It provides detailed modeling of radio spectra to characterize the shockwave, CSM density, and mass-loss history of the progenitor, highlighting non-homogeneous CSM around a Type Ib supernova.

Findings

Detected strong synchrotron emission with free-free absorption signs.

Estimated shock velocity of 14,000 km/s and high mass-loss rate.

Identified non-homogeneous CSM structure from late-time spectra.

Abstract

We present here broadband radio observations of the CSM interacting SN2019oys. SN2019oys was first detected in the optical and was classified as a Type Ib SN. Then, about $\sim 100$ days after discovery, it showed an optical rebrightening and a spectral transition to a spectrum dominated by strong narrow emission lines, which suggests strong interaction with a distant, dense, CSM shell. We modeled the broadband, multi-epoch, radio spectra, covering 2.2 to 36 GHz and spanning from 22 to 1425 days after optical discovery, as a synchrotron emitting source. Using this modeling we characterized the shockwave and the mass-loss rate of the progenitor. Our broadband radio observations show strong synchrotron emission. This emission, as observed 201 and 221 days after optical discovery, exhibits signs of free-free absorption from the material in front of the shock traveling in the CSM. In addition, the steep power law of the optically thin regime points towards synchrotron cooling of the radiating electrons. Analyzing these spectra in the context of the SN-CSM interaction model gives a shock velocity of 14,000 $\rm km \, s^{-1}$, and an electron number density of $2.6 \times 10^5 \, \rm cm^{-3}$ at a distance of $2.6 \times 10^{16}$ cm. This translates to a high mass-loss rate from the progenitor massive star of $6.7 \times 10^{-4} \, \rm M_{\odot} yr^{-1}$ for an assumed wind of 100 $\rm km s^{-1}$ (assuming constant mass-loss rate in steady winds). The late-time radio spectra, 392 and 557 days after optical discovery, are showing broad spectral peaks. We show that this can be explained by introducing a non-homogeneous CSM structure.