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Published online by Cambridge University Press: 07 August 2017
The most definitive information presently available on X-ray source populations comes from surveys carried out with the HEAO program (i.e., the all-sky HEAO-1 mission and the HEAO-2 Einstein Observatory) and EXOSAT. The large-sky/low-sensitivity survey (LSS) at high galactic latitudes performed with the HEAO-1 A2 experiment (Piccinotti et al. 1982) is complete and optically identified down to the level of about a millicrab. The LSS overlaps in spectral coverage (2–50 keV) with observations of the cosmic X-ray background (CXB) obtained with the same detectors (Boldt 1987). CXB surface brightness fluctuations observed in this band have been used to set constraints on the Log N–Log S relation for sources an order of magnitude fainter than those resolved in the LSS (Shafer 1983).