Why does accretion of matter onto black holes make them luminous?
As the matter falls into the black hole, and before it enters the event horizon, it turns gravitational potential energy into kinetic energy and so picks up a lot of speed. Some of this energy is then converted into light and other electromagnetic energy. But the most luminous part of black holes are the jets of accelerated matter that are emitted from the polar regions. These are probably generated because of the very strong rotating magnetic field that is usually associated with Black Holes. You should also look at: http://imagine.gsfc.nasa.gov/docs/science/know_l2/black_holes.html for more information. Thanks for your question.
