  The inverse-Compton X-ray emission model for supernovae has been well
established to explain the X-ray properties of many supernovae for over 30
years. However, no observational case has yet been found to connect the X-rays
with the optical lights as they should be. Here, we report the discovery of a
hard X-ray source that is associated with a Type II-b supernova. Simultaneous
emission enhancements have been found in both the X-ray and optical light
curves twenty days after the supernova explosion. While the enhanced X-rays are
likely dominated by inverse-Compton scatterings of the supernova's lights from
the Type II-b secondary peak, we propose a scenario of a high-speed supernova
ejecta colliding with a low-density pre-supernova stellar wind that produces an
optically thin and high-temperature electron gas for the Comptonization. The
inferred stellar wind mass-loss rate is consistent with that of the supernova
progenitor candidate as a yellow supergiant detected by the Hubble Space
Telescope, providing an independent proof for the progenitor. This is also new
evidence of the inverse-Compton emission during the early phase of a supernova.
