Seven Years of SN 2014C: A Multiwavelength Synthesis of an Extraordinary Supernova

SN 2014C was originally classified as a Type Ib supernova, but at phase phi = 127 days, post-explosion strong H alpha emission was observed. SN 2014C has since been observed in radio, infrared, optical and X-ray bands. Here we present new optical spectroscopic and photometric data spanning phi = 947-2494 days post-explosion. We address the evolution of the broadened H alpha emission line, as well as broad [O iii] emission and other lines. We also conduct a parallel analysis of all publicly available multiwavelength data. From our spectra, we find a nearly constant H alpha FWHM velocity width of similar to 2000 km s(-1) that is significantly lower than that of other broadened atomic transitions (similar to 3000-7000 km s(-1)) present in our spectra ([O i] lambda 6300; [O iii] lambda lambda 4959, 5007; He i lambda 7065; [Ca ii] lambda lambda 7291, 7324). The late radio data demand a fast forward shock (similar to 10,000 km s(-1) at phi = 1700 days) in rarified matter that contrasts with the modest velocity of the H alpha. We propose that the infrared flux originates from a toroidal-like structure of hydrogen surrounding the progenitor system, while later emission at other wavelengths (radio, X-ray) likely originates predominantly from the reverse shock in the ejecta and the forward shock in the quasi-spherical progenitor He-wind. We propose that the H alpha emission arises in the boundary layer between the ejecta and torus. We also consider the possible roles of a pulsar and a binary companion.