X-ray BackgroundThe existence of a cosmic X-ray background (CXRB) was among the very first discoveries of X-ray Astronomy. Its spectrum was well measured by the HEAO-1 mission and fits remarkably well a thermal bremsstrahlung model at a temperature of ~30 keV. At galactic latitudes >20° the CXRB is remarkably isotropic, suggesting for a truly cosmological origin. Most of the energy density in the CXRB resides at ~30 keV, an energy which is not accessible to sensitive enough instrumentation in the current era. Below 1 keV the Galaxy (and probably a local hot bubble) dominates the X-ray intensity, and the extragalactic component is shielded by the interstellar medium, making it difficult to measure. In spite of this, deep ROSAT observations resolved ~70% of the CXRB in the 1-2 keV range, while Chandra has resolved practically 90% of this and ~70% of the 2-7 keV CXRB. A more recent analysis [Ref 20] based on combined surveys performed with ROSAT, Chandra and XMM-Newton showed that, with the present data, even 94.3 (+7.0 / -6.7)% and 88.8 (+7.8 / -6.6)% of the soft (0.5-2 keV) and hard (2-10 keV) CXRB can be ascribed to discrete source emission.The lack of spectral distortions of the Cosmic Microwave Background ruled out the presence of substantial amounts of intergalactic gas at the very high temperatures needed to produce the CXRB through thermal bremsstrahlung. Finding the sources that produce the CXRB, their astrophysical nature, their cosmic evolution, spatial clustering and so on has been since then the main objective of CXRB studies. The most popular models for the CXRB make use of the unified model for AGN, and assume a mixture of unabsorbed (presumably type 1) and absorbed (presumably type 2) objects as a function of redshift z. Medium sensitivity and deep surveys carried out with ROSAT revealed that at soft X-ray energies the X-ray sky is dominated by AGNs, most of which are type 1 Seyferts and QSOs. A small fraction of the AGN were type 2 and other narrow emission line galaxies (e.g., starburst galaxies). However, due to its bandpass limited to soft X-ray photons ROSAT missed most of the sources absorbed by H columns in excess of 1021 cm-2. [Ref 4] |