We present multi-wavelength observations of Supernova 2014C during the first 500 days of its evolution .
These observations represent the first solid detection of a young extragalactic stripped-envelope SN out to high-energy X-rays \sim 40 keV .
SN 2014C was the explosion of an envelope-stripped progenitor star with ordinary explosion parameters ( E _ { k } \sim 1.8 \times 10 ^ { 51 } erg and M _ { ej } \sim 1.7 { M _ { \sun } } ) .
However , over the time scale of \sim 1 yr , SN 2014C experienced a complete metamorphosis and evolved from an ordinary hydrogen-poor supernova of type Ib into a strongly interacting , hydrogen-rich supernova of type IIn , thus violating the traditional classification scheme of type-I vs. type-II SNe .
Signatures of the SN shock interacting with a dense medium are observed across the electromagnetic spectrum , from the radio to the hard X-ray band .
Coordinated observations with Swift , Chandra and NuSTAR have captured the evolution in detail and revealed the presence of a massive shell of \sim 1 { M _ { \sun } } of hydrogen-rich material at \sim 6 \times 10 ^ { 16 } cm from the explosion site .
We estimate that the shell was ejected by the progenitor star in the decades to centuries before core collapse .
This result poses significant challenges to current theories of massive star evolution , as it requires a physical mechanism responsible for the ejection of the deepest hydrogen layer of H-poor SN progenitors synchronized with the onset of stellar collapse .
Theoretical investigations point at binary interactions and/or instabilities during the last stages of nuclear burning in massive stars as potential triggers of the highly time-dependent mass loss .
We constrain these scenarios utilizing the sample of 183 SNe Ib/c with public radio observations .
Our analysis identifies SN 2014C-like signatures , consistent with strong interaction , in \sim 10 % of SNe with constraining radio data .
This fraction is somewhat larger but reasonably consistent with the expectation from the theory of recent envelope ejection due to binary evolution if the ejected material can survive in the close environment for 10 ^ { 3 } -10 ^ { 4 } yrs .
Alternatively , nuclear burning instabilities extending all the way to the core C-burning phase might also play a critical role .