ROSAT Extended Medium-Deep Sensitivity Survey: Source Counts for 130 Fields

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.