Six years of luminous X-ray emission from the strongly interacting type-Ib SN 2014C captured by Chandra and NuSTAR

We present the first coordinated soft and hard 0.3-80 keV X-ray campaign of the extragalactic supernova SN 2014C in the first $sim$2307 d of its evolution. SN 2014C initially appeared to be an ordinary type Ib explosion but evolved into a strongly-interacting hydrogen-rich type IIn SN over $sim1 rm{yr}$. We observed signatures of interaction with a dense medium across the X-ray spectrum, which revealed the presence of a $sim 1-2 rm{M}_{odot}$ shell of material at $sim6times10^{16} rm{cm}$ from the progenitor. This finding challenges current understanding of hydrogen-poor core-collapse progenitor evolution. Potential scenarios to interpret these observations include (i) the ejection of the hydrogen envelope by the progenitor star in the centuries prior to the explosion; (ii) interaction of the fast Wolf-Rayet (WR) star wind with the slow, dense wind of the Red Super Giant (RSG) phase, with an anomalously short WR phase.