Radio evolution of a Type IIb supernova SN 2016gkg

Abstract

We present extensive radio monitoring of a type IIb supernova (SN IIb), SN 2016gkg during $t \sim$ 8$-$1429 days post explosion at frequencies $\nu \sim$ 0.33$-$25 GHz. The detailed radio light curves and spectra are broadly consistent with self-absorbed synchrotron emission due to the interaction of the SN shock with the circumstellar medium. The model underpredicts the flux densities at $t \sim$ 299 days post-explosion by a factor of 2, possibly indicating a density enhancement in the CSM due to a non-uniform mass-loss from the progenitor. Assuming a wind velocity $v_{\rm w} \sim$ 200 km s$^{-1}$, we estimate the mass-loss rate to be $\dot{M} \sim$ (2.2, 3.6, 3.8, 12.6, 3.7, and 5.0) $\times$ 10$^{-6}$ $M_{\odot}$ yr$^{-1}$ during $\sim$ 8, 15, 25, 48, 87, and 115 years, respectively before the explosion. The shock wave from SN 2016gkg is expanding from $R \sim$ 0.5 $\times$ 10$^{16}$ to 7 $\times$ 10$^{16}$ cm during $t \sim$ 24$-$492 days post-explosion indicating a shock deceleration index, $m$ $\sim$ 0.8 ($R \propto t^m$), and mean shock velocity $v \sim$ 0.1c. The radio data being inconsistent with free-free absorption model and higher shock velocities are in support of a relatively compact progenitor for SN 2016gkg.