In May 2003 Very Large Telescope, VLT observed an unusual brightness in Sagittarius A*. In less than 30 minutes, a powerful flare appeared and disappeared again. It was 6 times brighter than the standard black hole emission, and it repeated itself in later observations.

The most plausible explanation was the presence of hot gas orbiting extremely close to the black hole, on the last possible orbit just before the event horizon.