  Massive stars end their short lives in spectacular explosions, supernovae,
that synthesize new elements and drive galaxy evolution. Throughout history
supernovae were discovered chiefly through their delayed optical light,
preventing observations in the first moments (hours to days) following the
explosion. As a result, the progenitors of some supernovae and the events
leading up to their violent demise remain intensely debated. Here we report the
serendipitous discovery of a supernova at the time of explosion, marked by an
extremely luminous X-ray outburst. We attribute the outburst to the break-out
of the supernova shock-wave from the progenitor, and show that the inferred
rate of such events agrees with that of all core-collapse supernovae. We
forecast that future wide-field X-ray surveys will catch hundreds of supernovae
each year in the act of explosion, and thereby enable crucial neutrino and
gravitational wave detections that may ultimately unravel the explosion
mechanism.
