The origin of the cosmic X-ray background

A high-resolution image from the Rosat X-ray satellite reveals many faint discrete sources in the 0.1-2-keV energy range. Optical spectroscopy of these sources performed at the Anglo-Australian Telescope shows that many of them are quasars, and the inferred density of quasars on the sky contributes at least 30 percent of the cosmic X-ray background at 1 kev.