Seven Years of Coordinated Chandra-NuSTAR Observations of SN 2014C Unfold the Extreme Mass-loss History of Its Stellar Progenitor

We present the results from our 7 yr long broadband X-ray observing campaign of SN 2014C with Chandra and NuSTAR. These coordinated observations represent the first look at the evolution of a young extragalactic SN in the 0.3-80 keV energy range in the years after core collapse. We find that the spectroscopic metamorphosis of SN 2014C from an ordinary type Ib SN into an interacting SN with copious hydrogen emission is accompanied by luminous X-rays reaching L ( x ) & AP; 5.6 x 10(40) erg s(-1) (0.3-100 keV) at & SIM;1000 days post-explosion and declining as L ( x ) & PROP; t (-1) afterwards. The broadband X-ray spectrum is of thermal origin and shows clear evidence for cooling after peak, with T(t)& AP;20keV(t/tpk)-0.5 NHint(t)& AP;3x1022(t/400days)-1.4cm-2 5 - 10(6) cm(-3)), H-rich disk-like circumstellar medium (CSM) with inner radius & SIM;2 x 10(16) cm and extending to & SIM;10(17) cm. Based on the declining NHint(t) and X-ray luminosity evolution, we infer a CSM mass of & SIM;(1.2 f-2.0 f)M & ODOT;