An Etymological Dictionary of Astronomy and Astrophysics

A diffuse background radiation in X-ray wavelengths which has several origins. At very low energies it is due to hot gas in the → Local Bubble. In the → soft X-ray energy band it comes from active galaxies at moderate redshifts. In → hard X-ray range the background is thought to be due to integrated emission from many → quasars at various redshifts.

See also: → X-ray; → background.