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 ϕ = 127 days, post-explosion strong Hα 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 ϕ = 947-2494 days post-explosion. We address the evolution of the broadened Hα 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 Hα FWHM velocity width of ~2000 km s^-1 that is significantly lower than that of other broadened atomic transitions (~3000-7000 km s^-1) present in our spectra ([O I] λ6300; [O III] λ λ4959, 5007; He I λ7065; [Ca II] λ λ7291, 7324). The late radio data demand a fast forward shock (~10,000 km s^-1 at ϕ = 1700 days) in rarified matter that contrasts with the modest velocity of the Hα. 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 Hα emission arises in the boundary layer between the ejecta and torus. We also consider the possible roles of a pulsar and a binary companion.