Quasars and Active Galaxies
The most luminous beacons in the the universe are quasars. They are powered by the release of gravitational energy as matter falls onto supermassive black holes at the centers of galaxies. This energy release is far more efficient than the nuclear fusion reactions that power stars, so that quasars can be seen all the way across the observable universe. Quasars, and their somewhat less powerful cousins 'Active Galactic Nuclei' spread the light they emit right across the electromagnetic spectrum, and the X-rays they emit seem to come from the very most central regions, just a few times the size of the event horizon. Astronomers in the CfA High Energy Division study X-rays from many types of quasars from low to high power and from nearby to very distant.
Tom Aldcroft, Martin Elvis, Antonella Fruscione, Josh Grindlay, Paul Green, Dan Harris, Julia Lee, Dan Schwartz, Aneta Siemiginowksa, Harvey Tananbaum, Belinda Wilkes