X-ray background
A galactic X-ray background was discovered very early in the history of X-ray astronomy in 1962 by Riccardo Giacconi and colleagues . The radiation is isotropic at energies above 2 k eV and there is no correlation to galactic sources . The X-ray brightness of a volume is proportional to the mass of the stars it contains .
It is assumed that the X-ray background is not a real background, but rather arises from the superposition of many individual sources. In the energy range from 1 to 10 keV, ROSAT , XMM-Newton and Chandra were able to resolve about 80–90% of the X-ray background in individual sources - predominantly active galactic nuclei - this is still pending for higher energies.