Abstract:
Supernovae characterized by enduring narrow optical hydrogen emission lines (SNe IIn) are believed to result primarily from the core-collapse of massive stars undergoing sustained interaction with a dense circumstellar medium (CSM). While the properties of SN IIn progenitors have relatively few direct constraints, the ongoing ejecta─CSM interaction provides unique information about late-stage stellar mass-loss preceding core collapse. We present late-time X-ray and radio observations of four ≥3000 day old SNe IIn: SN 2013L, SN 2014ab, SN 2015da, and KISS15s. The radio and X-ray emission from KISS15s indicate a mass-loss rate of Ṁ∼4×10−3M⊙yr−1 at ∼450 yr pre-SN—2 orders of magnitude below earlier optical estimates (which probed the mass loss immediately preceding the SN). We find hints of a spectral inversion in the radio spectral energy distribution of KISS15s; a possible signature of a secondary shock due to a binary system or the emergence of a pulsar wind. For SN 2013L, we obtain a mass-loss rate of Ṁ∼2×10−3M⊙yr−1 at ∼400 yr preexplosion based on the X-ray detection. For SN 2014ab and SN 2015da, we find upper limits on the mass-loss rates of Ṁ<2×10−3M⊙yr−1 explosion at ∼300 and 250 yr preexplosion, respectively. All four objects display mass-loss rates lower than estimates from earlier optical analyses by at least 1─2 orders of magnitude, necessitating a rapidly evolving progenitor process over the last centuries preexplosion. Our analysis reveals how X-ray and radio observations can elucidate progenitor evolution when these objects have faded at optical wavelengths.
