SN 2016coi (ASASSN-16fp): an energetic H-stripped core-collapse supernova from a massive stellar progenitor with large mass loss

We present comprehensive observations and analysis of the energetic H-stripped SN 2016coi (a.k.a. ASASSN-16fp), spanning the gamma-ray through optical and radio wavelengths, acquired within the first hours to ~420 days post explosion. Our campaign confirms the identification of He in the SN ejecta, which we interpret to be caused by a larger mixing of Ni into the outer ejecta layers. From the modeling of the broad bolometric light curve we derive a large ejecta mass to kinetic energy ratio (M_ej~4-7 M_sun, E_k~ 7-8x10^51 erg). The small [CaII] lambda lambda 7291,7324 to [OI] lambda lambda 6300,6364 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_X>10^39 erg s^-1 in the first ~100 days post explosion) and radio emission (L_8.5 GHz ~ 7x10^27 erg s^-1 Hz^-1 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 rate dot(M)~ (1-2)x 10^-4 M_sun yr^-1 and a sub-relativistic shock velocity v_sh~0.15c, in stark contrast with relativistic SNe and similar to normal SNe. Finally, we find no evidence for a SN-associated shock breakout gamma-ray pulse with energy E_gamma>2x10^46 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.