  Many black hole (BH) candidates have been discovered in X-ray binaries and in
the nuclei of galaxies. The prediction of Einstein's general relativity is that
BHs have an event horizon --- a one-way membrane through which particles fall
into the BH but cannot exit. However, except for the very few nearby
supermassive BH candidates, our telescopes are unable to resolve and provide a
direct proof of the event horizon. Here, we propose a novel observation that
supports the existence of event horizons around supermassive BH candidates
heavier than 10^{7.5} solar masses. Instead of an event horizon, if the BH
candidate has a hard surface, when a star falls onto the surface, the shocked
baryonic gas will form a radiation pressure supported envelope that shines at
the Eddington luminosity for an extended period of time from months to years.
We show that such emission has already been ruled out by the Pan-STARRS1 3pi
survey if supermassive BH candidates have a hard surface at radius larger than
1 + 10^{-4.4} times the Schwarzschild radius. Future observations by LSST
should be able to improve the limit to 1 + 10^{-6}.
