Luminous Radio Emission from the Superluminous Supernova 2017ens at 3.3 yr after Explosion

We present the results from a multiyear radio campaign of the superluminous supernova (SLSN) SN 2017ens, which yielded the earliest radio detection of an SLSN to date at the age of similar to 3.3 yr after explosion. SN 2017ens was not detected at radio frequencies in the first similar to 300 days but reached L-nu approximate to 10(28) erg s(-1) cm(-2) Hz(-1) at nu similar to 6 GHz, similar to 1250 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 (M) over dot approximate to 10(-4) M-circle dot yr(-1) at r similar to 10(17) cm from the explosion's site, for a wind speed of v(w) = 50-60 km s(-1) as measured from optical spectra. These findings are consistent with the spectroscopic metamorphosis of SN 2017ens from hydrogen poor to hydrogen rich similar to 190 days after explosion reported by Chen et al. SN 2017ens 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 similar to 0.5 M-circle dot, and low velocity of the ejection suggest that binary interactions (in the form of common-envelope evolution and subsequent envelope ejection) play a role in shaping the evolution of the stellar progenitors of SLSNe in the less than or similar to 500 yr preceding core collapse.