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We present an analysis of serendipitous sources detected in 130 high Galactic latitude, long-exposure ROSA T PSPC observations. By application of an efficient source detection technique and analyzing a larger number of fields compared to previous studies, we have sufficiently increased the sky coverage of the survey and are able to investigate the X-ray source population over almost three decades of intensity, from approximately 1.2 × 10-15 to approximately 7.0 × 10-13 ergs s-1 cm-2 in the 0.5-2.0 keV energy band. Using the 2678 detected sources, we refine the extragalactic log N-log S and find that the differential number-flux relation is well described by a broken power law which flattens from α = 2.55±0.08 above a flux of 2.2±0.2 x 10-14 s-1 cm-2 to 1.86±0.06 at fainter fluxes, which is in a good agreement with previous studies. At our limiting flux of 1.2 × 10-15 ergs s-1 cm-2, the detected (point) sources comprise 66±7% (68% confidence limits) of the X-ray background in the 1-2 keV band.
Vikhlinin et al. (Sun,) studied this question.