2000AJ....119.2349B

We present results from a study of 2-8 keV X-ray sources detected by the Advanced CCD Imaging Spectrometer on the Chandra X-Ray Observatory in the field of the z=0.516 cluster CRSS J0030.5+2618. In our 63.5 arcmin² search area, we detect 10 sources with 2-8 keV fluxes down to ~4x10^–15 ergs.cm^–2.s^–1; our lowest flux sources are ~10 times fainter than those previously available for study in this band. Our derived source density is about an order of magnitude larger than previous source counts above 2 keV, although this density may be enhanced somewhat because of the presence of the cluster. We detail our methods for source detection and characterization, and we show that the resulting source list and parameters are robust. We have used the Marcario Low-Resolution Spectrograph on the Hobby-Eberly Telescope to obtain optical spectra for several of our sources; by combining these spectra with archival data, we find that the sources appear to be active galaxies, often with narrow permitted lines, red optical continua, or hard X-ray spectra. Four of the X-ray sources are undetected to R=21.7; if they reside in L^* galaxies they must have z>0.55-0.75 and hard X-ray luminosities of L_2–8≳4x10⁴² ergs.s^–1. We detect all but one of our 2-8 keV sources in the 0.2-2 keV band as well. This result extends to significantly lower fluxes the constraints on any large, completely new population of X-ray sources that appears above 2-3 keV.