cosmic X-ray background (CXB)
X-ray image of the sky taken through the Lockman hole by the XMM-Newton Observatory, resolving some of the background into many faint individual sources. The image is color-coded, with red representing relatively low energy X-rays, photons with 500 or so times the energy of visible light. Green and blue colors correspond to increasingly energetic X-rays with up to about 10,000 times visible light energies. Notably, the faint sources tend to be green and blue, showing X-ray characteristics of huge amounts of material falling into supermassive black holes in very distant galaxies. Credit: G. Hasinger (AIP) et al., XMM-Newton, ESA.
The cosmic X-ray background (CXB) is an X-ray and gamma-ray glow that comes from all parts of the sky. First revealed in the 1970s by early X-ray satellites, its origin proved something of a mystery. Beginning in 2000, however, more powerful orbiting X-ray instruments, such as the XMM-Newton Observatory, have resolved some of this enigmatic background into many faint individual sources, their X-ray characteristics pointing to huge amounts of material falling into supermassive black holes in very distant galaxies. These results add to the growing consensus that massive black holes hold court at the center of all large galaxies and that, from across the universe, X-rays produced as matter feeds these black holes account for the bulk of the CXB.
The CXB is one of several components of the cosmic background radiation.