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 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 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☉, E_k ∼ 7 − 8 × 10⁵¹ 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_X > 10³⁹ erg s⁻¹ in the first ∼ 100 days post explosion) and radio emission (L_{8.5 GHz} ∼ 7 × 10²⁷ erg s⁻¹Hz⁻¹ at peak). These values are in line with those of relativistic SNe (2009bb, 2012ap). However, for SN 2016coi we infer substantial