Observations of faint, hard-band X-ray sources in the field of CRSS J0030.5+2618 with the Chandra X-Ray Observatory and the Hobby-Eberly Telescope

We present results from a study of 2-8 keV X-ray sources detected by the Ad vanced CCD Imaging Spectrometer on the Chandra X-Ray Observatory in the fie ld of the z = 0.516 cluster CRSS J0030.5 + 2618. In our 63.5 arcmin(2) sear ch area, we detect 10 sources with 2-8 keV fluxes down to approximate to 4 x 10(-15) ergs cm(-2) s(-1); our lowest flux sources are approximate to 10 times fainter than those previously available for study in this band. Our d erived source density is about an order of magnitude larger than previous s ource counts above 2 keV, although this density may be enhanced somewhat be cause of the presence of the cluster. We detail our methods for source dete ction and characterization, and we show that the resulting source list and parameters are robust. We have used the Marcario Low-Resolution Spectrograp h on the Hobby-Eberly Telescope to obtain optical spectra for several of ou r sources; by combining these spectra with archival data, we find that the sources appear to be active galaxies, often with narrow permitted lines, re d optical continua, of hard X-ray spectra. Four of the X-ray sources are un detected to R = 21.7; if they reside in L* galaxies they must have z > 0.55 -0.75 and hard X-ray luminosities of L2-8 greater than or similar to 4 x 10 (42) 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 con straints on any large, completely new population of X-ray sources that appe ars above 2-3 keV.