X-ray background
The observed X-ray background is thought to result from, at the "soft" end, galactic X-ray emission, the "galactic" X-ray background, and, at the "hard" end, from a combination of many unresolved X-ray sources outside of the Milky Way, the "cosmic" X-ray background.
The galactic X-ray background is produced largely by emission from hot gas in the Local Bubble within 100 parsecs of the Sun.
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 and obscured active galactic nuclei.
