SN 2016coi (ASASSN-16fp): An Energetic H-stripped Core-collapse Supernova from a Massive Stellar Progenitor with Large Mass Loss

Abstract

We present comprehensive observations and analysis of the energetic H-stripped SN 2016coi (a.k.a. ASASSN-16fp), spanning the γ-ray through optical and radio wavelengths, acquired within the first hours to ∼420 days post explosion. Our observational campaign confirms the identification of He in the supernova (SN) ejecta, which we interpret to be caused by a larger mixing of Ni into the outer ejecta layers. By modeling the broad bolometric light curve, we derive a large ejecta-mass-to-kinetic-energy ratio (M <SUB>ej</SUB> ∼ 4-7 M <SUB>☉</SUB>, E <SUB>k</SUB> ∼ (7-8) × 10<SUP>51</SUP> erg). The small [Ca II] λλ7291,7324 to [O I] λλ6300,6364 ratio (∼0.2) observed in our late-time optical spectra is suggestive of a large progenitor core mass at the time of collapse. We find that SN 2016coi is a luminous source of X-rays (L <SUB>X</SUB> > 10<SUP>39</SUP> erg s<SUP>-1</SUP> in the first ∼100 days post explosion) and radio emission (L <SUB>8.5 GHz</SUB> ∼ 7 × 10<SUP>27</SUP> erg s<SUP>-1</SUP> Hz<SUP>-1</SUP> at peak). These values are in line with those of relativistic SNe (2009bb, 2012ap). However, for SN 2016coi, we infer substantial pre-explosion progenitor mass loss with a rate \dot{M} ∼ (1-2) × {10}<SUP>-4</SUP> {M}<SUB>☉ </SUB> {yr}}<SUP>-1</SUP> and a sub-relativistic shock velocity v <SUB>sh</SUB> ∼ 0.15c, which is in stark contrast with relativistic SNe and similar to normal SNe. Finally, we find no evidence for a SN-associated shock breakout γ-ray pulse with energy E <SUB> γ </SUB> > 2 × 10<SUP>46</SUP> erg. While we cannot exclude the presence of a companion in a binary system, taken together, our findings are consistent with a massive single-star progenitor that experienced large mass loss in the years leading up to core collapse, but was unable to achieve complete stripping of its outer layers before explosion.