Fading Echoes of Interaction: Probing Centuries of Preexplosion Mass Loss in Four Type IIn Supernovae

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 M ̇ ∼ 4 × 1 0 − 3 M ⊙ 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 M ̇ ∼ 2 × 1 0 − 3 M ⊙ 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 M ̇ < 2 × 1 0 − 3 M ⊙ 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.