Some X-ray novae are thought to contain neutron stars, while others are thought to contain black holes. But when an X-ray nova contains a black hole, the energy spiralling inward theoretically would disappear suddenly, without the final burst of energy. Researchers said Thursday they have found at least four examples of energy streams reacting as expected around a black hole. This is the strongest evidence yet, said astronomer Ramesh Narayan, for the presence of an event horizon, the threshold that is a key part of a black hole. An event horizon is the point at which matter and energy being sucked into a black hole disappear.
Stuff can fall in, but nothing can get out.
Ironically, proving the existence of an event horizon required the astronomers to demonstrate the absence of an energy signal. If the energy was seen, then the object was not a black hole. If the energy simply disappeared, then the most likely explanation is that it was sucked into the event horizon of a black hole. By detecting very little energy from these black hole candidates, we have new proof that event horizons exist. Black holes really are black.