Stellar Black Holes
As this gas forms a flattened disk, it swirls toward the companion. Friction caused by collisions between the particles in the gas heats them to extreme temperatures and they produce X-rays that flicker or vary in intensity within a second.
Many bright X-ray binary sources have been discovered in our galaxy and nearby galaxies. In about ten of these systems, the rapid orbital velocity of the visible star indicates that the unseen companion is a black hole. (The figure at left is an X-ray image of the black hole candidate XTE J1118+480.) The X-rays in these objects are produced by particles very close to the event horizon. In less than a second after they give off their X-rays, they disappear beyond the event horizon.
However, not all the matter in the disk around a black hole is doomed to
fall into the black hole. In many black hole systems, some of the gas escapes as a hot wind that is blown away from the disk at high speeds. Even more dramatic are the high-energy jets that radio and X-ray observations show exploding away from some stellar black holes. These jets can move at nearly the speed of light in tight beams and travel several light years before slowing down and fading away.
Chandra image of the
black hole candidate
(central bright point).
The spikes and rays
extending from the
bright point are
J.McClintock & M.Garcia)
Do black holes grow when matter falls into them? Yes, the mass of the black hole increases by an amount equal to the amount of mass it captures. The radius of the event horizon also increases by about 3 kilometers for every solar mass that it swallows. A black hole in the center of a galaxy, where stars are densely packed, may grow to the mass of a billion Suns and become what is known as a supermassive black hole. Recently Chandra has found evidence that black holes with masses of about a thousand Suns can be formed in dense star clusters by processes that are not yet understood.