Black holes are among the most amazing and bizarre predictions of Einstein's theory of gravity. A black hole is thought to be point-like in dimension, but it is surrounded by an imaginary surface, or "edge," of finite size (its "event horizon") within which anything that ventures becomes lost forever to the rest of the universe.

Despite their reputation as implacable sinks for matter and energy, the regions around black holes are often sources of powerful emission. They can be ringed by a disk of matter, for example contributed by gas from an orbiting companion star; when infalling material interacts with that disk, radiation and matter can be ejected.