Faint X-Ray Source Counts and the Origin of the X-Ray Background

We present a thorough reanalysis of the Einstein Observatory Deep Survey data in order to determine the contribution of detected, discrete sources of X-ray emission to the cosmic X-ray background. We find substantial discrepancies with previously published work on this problem. A detailed discussion of data editing and source algorithms buttresses our claim of having constructed a complete, flux-limited sample of the faintest sources detectable with the Einstein imaging proportional counter, the most sensitive X-ray instrument yet flown. We find a total of 33 sources in a survey region of ~3.3 deg^2^ down to a minimum flux threshold of 4 X 10^-14^ ergs cm^-2^ s^-1^ in the 0.3-3.5 keV band. Roughly 30% of the objects are foreground stars, leading to an extragalactic source surface density of 7 x 10^4^ sr^-1^ at this threshold. The integrated contribution from discrete sources to the number of cosmic X-ray background photons measured in this same band with the same instrument is 12%+/- 3%, substantially below previous estimates. Implications of these results for the origin of the background are discussed.