"X-rays from Extra-galactic Black Holes"
Most candidate black holes reside in the centers of active
galaxies. The holes themselves are inherently dark, but they apparently
accrete any surrounding matter, giving off huge luminosities in the process.
Optical and radio observations have begun to zoom in on the central regions
of these galaxies, but we are still a long way from direct observation
of the event horizon. Compelling evidence for the presence of a black hole
has, however, been obtained from X-ray spectroscopic data. These show matter
orbiting with extremely high velocities, and the characteristic red-shift
expected due to time dilation in the intense gravitational field. The observations
can be used to determine both the properties of the black holes, and the
processes fueling galaxy activity.