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Cosmic X-ray Background

  

Whether the cosmic X-ray background is a source of contamination or an interesting signal is obviously dependent on one's scientific point of view. In most observations and for all energies less than  keV it is typically the dominant background component. The cosmic X-ray background is both spatially and spectrally anisotropic with independent variations of greater than a factor of five in surface brightness in the hard and soft bands. The structure is due to both variations in emission and foreground absorption. The angular scale of the structure varies from the coarse all-sky negative correlation between galactic and the 1/4 keV background to few arc-minute scale emission by clusters of galaxies (most obvious at higher energies). Thus, there is no ``typical'' cosmic background flux or spectrum.