Faint X-ray source counts and the origin of the X-ray background

Abstract

A thorough reanalysis of the Einstein Observatory Deep Survey data is presented in order to determine the contribution of detected, discrete sources of X-ray emission to the cosmic X-ray background. Substantial discrepancies with previously published work on this problem are found. A detailed discussion of data editing and source algorithms buttresses a 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. A total of 33 sources is found in a survey region of about 3.3 sq deg down to a minimum flux threshold of 4 x 10 to the 14th ergs/sq cm/s in the 0.3-3.5 keV band. Roughly 30 percent of the objects are foreground stars, leading to an extragalactic source surface density of 70,000/sr 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 percent + or - 3 percent, substantially below previous estimates. Implications of these results for the origin of the background are discussed.