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 (~10^-13 to 10^-15 ergs cm^-2 s^-1). SEXSI, which studies sources selected from more than 2 deg², provides an essential complement to the Chandra Deep Fields, which reach depths of 5×10^-16 ergs cm^-2 s^-1 (2-10 keV) but over a total area of less than 0.2 deg². 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 logN-logS 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 logN-logS 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.