The observed X-ray background is thought to result from, at the "soft" end (below 0.3 keV), Galactic X-ray emission (the "galactic" X-ray background), and, at the "hard" end (above 0.3keV), from a combination of many unresolved X-ray sources outside of the Milky Way (the "cosmic" X-ray background).
Deep surveys with X-ray telescopes, such as the Chandra X-ray Observatory, have demonstrated that around 80% of the cosmic X-ray background is due to resolved extra-galactic X-ray sources, the bulk of which are unobscured ("type-1") and obscured ("type-2") active galactic nuclei (AGN).
- T Shanks, I Georgantopoulos, GC Stewart, KA Pounds, "The origin of the cosmic X-ray background", Nature 353, 315 - 320 (26 September 1991); doi:10.1038/353315a0
- Xavier Barcons, The X-ray Background, 1992 Cambridge University Press, 324 pages ISBN 0-521-41651-5
- Audio Cain/Gay (2009) Astronomy Cast X-ray Astronomy