Luminous Radio Emission from the Superluminous Supernova 2017ens at 3.3 years after explosion

Abstract

We present the results from a multi-year radio campaign of the superluminous supernova (SLSN) 2017ens, which yielded the earliest radio detection of a SLSN to date at the age of $\sim$3.3 years after explosion. SN2017ens was not detected at radio frequencies in the first $\sim$300\,d of evolution but reached $L_{\nu}\approx 10^{28}\,\rm{erg\,s^{-1}\,cm^{-2}}$ at $\nu\sim 6$ GHz, $\sim1250$ days post-explosion. Interpreting the radio observations in the context of synchrotron radiation from the supernova shock interaction with the circumstellar medium (CSM), we infer an effective mass-loss rate of $\approx 10^{-4}\,\rm{M_{\odot}yr^{-1}}$ at $r\sim 10^{17}$ cm from the explosion's site, for a wind speed of $v_w=50-60\,\rm{km\,s^{-1}}$ measured from optical spectra. These findings are consistent with the spectroscopic metamorphosis of SN2017ens from hydrogen-poor to hydrogen-rich $\sim190$ d after explosion reported by Chen et al., 2018. SN2017ens is thus an addition to the sample of hydrogen-poor massive progenitors that explode shortly after having lost their hydrogen envelope. The inferred circumstellar densities, implying a CSM mass up to $\sim0.5\,\rm{M_{\odot}}$, and low velocity of the ejection point at binary interactions (in the form of common envelope evolution and subsequent envelope ejection) playing a role in shaping the evolution of the stellar progenitors of SLSNe in the $\lesssim 500$ yr preceding core collapse.