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 can not 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 _ { \odot } .
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 3 \pi 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 } .