Young neutron stars spew out high energy particles that produce X-rays for several thousand years. An old neutron star can produce X-rays when matter from a nearby star falls onto its surface. When a collapsed star has more mass than three suns, it forms a black hole in space. These bizarre objects are found in rare double star systems or in the centers of galaxies where supermassive black holes have formed. This activity is due to a supermassive black hole in the center of the galaxy, or a near collision with another galaxy, or both.
Quasars are extreme examples of active galaxies.
Vast, hot, X-ray emitting gas clouds in the clusters show this process in action. Deep surveys with the Chandra X-ray Observatory should reveal the cause of this glow.