X-ray emission covering the entire sky, discovered in the 1960s. For 35 years it was uncertain whether this background consists of a large number of point sources, or emission from hot intergalactic gas. The ROSAT and Chandra observatories have shown that over 75% of this background can be resolved into distinct sources. About half of these individual sources are active galactic nuclei and quasars. The other half are X-ray binaries in other galaxies.
Subjects: Astronomy and Astrophysics.