Cosmic X-ray Background
Recent progress in observational study of the cosmic X-ray background (CXB) is reviewed mainly based on results from the large area counter on board Ginga. According to the Ginga fluctuation analysis of the sky background (Hayashida 1990), the log N – log S relation of the form N ∝ S -3/2 in the 2-10 keV band seems to hold down to a flux level as low as 10-13 erg cm-2 s-1, and the integrated flux of all discrete sources above 10-13 erg cm-2 s-1 amounts to about 30% of the CXB. The angular auto-correlation function of the CXB obtained with Ginga (Kondo 1991) shows that the contribution of the sources having a correlation length of 5 (H/100 km s-1 Mpc-1)-1 Mpc is at most 50% of the CXB (95% confidence limit). This upper limit for the correlation length suggests that more than 50% of the CXB do not come from members of clusters of galaxies within z < 1. Some possibilities for the CXB origin are discussed.
KeywordsSpectral Index Active Galactic Nucleus Luminosity Function Discrete Source Flux Level
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