The HELLAS2XMM survey. I. The X-ray data and the log N-log S relation

We present the first results from an XMM-Newton serendipitous medium-deep survey, which covers nearly 3 deg(2). We detect a total of 1022, 495, and 100 sources, down to minimum fluxes of about 5.9 x 10(-16), 2.8 x 10(-15), and 6.2 x 10(-15) ergs cm(-2) s(-1), in the 0.5-2, 2-10, and 4.5-10 keV bands, respectively. In the soft band this is one of the largest samples available to date and surely the largest in the 2-10 keV band at our limiting X-ray flux. The measured log N-log S determinations are found to be in good agreement with previous determinations. In the 0.5-2 keV band, we detect a break at fluxes around 5 x 10(-15) ergs cm(-2) s(-1). In the harder bands, we fill in the gap at intermediate fluxes between deeper Chandra and XMM-Newton observations and shallower BeppoSAX and ASCA surveys.