Stellar disruption events support the existence of the black hole event horizon

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 M_⊙. Instead of an event horizon, if the BH candidate has a hard surface, when a star falls on to 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 3π survey if supermassive BH candidates have a hard surface at radius larger than (1 + 10^-4.4) times the Schwarzschild radius. Future observations by Large Synoptic Survey Telescope should be able to improve the limit to 1 + 10^-6.