The Serendipitous Extragalactic X-Ray Source Identification program. I. Characteristics of the hard X-ray sample

The Serendipitous Extragalactic X-Ray Source Identification (SEXSI) program is designed to extend greatly the sample of identified extragalactic hard X-ray (2 - 10 keV) sources at intermediate fluxes (similar to10(-13) to 10(-15) ergs cm(-2) s(-1)). SEXSI, which studies sources selected from more than 2 deg(2), provides an essential complement to the Chandra Deep Fields, which reach depths of 5 x 10(-16) ergs cm(-2) s(-1) (2 - 10 keV) but over a total area of less than 0.2 deg(2). In this paper we describe the characteristics of the survey and our X-ray data analysis methodology. We present the cumulative flux distribution for the X-ray sample of 1034 hard sources and discuss the distribution of spectral hardness ratios. Our log N-log S in this intermediate flux range connects to those found in the Deep Fields, and by combining the data sets, we constrain the hard X-ray population over the flux range in which the differential number counts change slope and from which the bulk of the 2 - 10 keV X-ray background arises. We further investigate the log N-log S distribution separately for soft and hard sources in our sample, finding that while a clear change in slope is seen for the softer sample, the hardest sources are well described by a single power law down to the faintest fluxes, consistent with the notion that they lie at lower average redshift.