The Einstein Two-Sigma Catalog: Silver Needles in the X-Ray Haystack

To facilitate the study of X-ray sources fainter than those contained in the Einstein Medium-Sensitivity Survey (EMSS), we have constructed a new catalog of sources and fluctuations exceeding 2 σ significance in 2520 high-latitude Einstein IPCC images. We have employed various tests to validate our source-search algorithm for both high- and low- significance sources, and to identify and remove the small number of spurious sources induced by our detection procedure. Based on the known vignetting and background characteristics of the IPC and the measured X- ray log N-log S relation, we have modeled the number of real sources expected in the catalog in order to evaluate its statistical properties below 4 σ significance. Our modeling suggests that ~13,000 sources in the catalog are real celestial X-ray sources, an increase of ~9100 over the number found in previous analyses of the same IPC images. We find that not only is the reliability of the Two-Sigma Catalog a function of source significance σ, it is a function of off-axis angle on the detector as well. The application of different signal-to-noise thresholds at different off-axis angles thus enables one to tune the reliability of the catalog. The chief motivation for studying large numbers of faint X-ray sources is to search for possible new components of the cosmic X-ray background. To select out real celestial X-ray sources in the Two-Sigma Catalog, we apply astronomical catalogs at other wavelengths as filters. For example, the cross-correlation of the Two-Sigma Catalog with catalogs from surveys of the radio and infrared sky has yielded large samples of faint X-ray sources that are ~90% reliable. Optical spectroscopy of 77 unidentified faint X-ray sources has turned up several surprises, illustrating the merits of selecting X-ray sources using a variety of methods: high- redshift quasars (one at z = 4.30), which are absent in the EMSS, X-ray- luminous (L_x_ ~ 10^43^ ergs s^-1^) radio-loud elliptical galaxies with optical spectra devoid of emission lines, and infrared-bright active galactic nuclei whose optical spectra are dominated by starburst galaxy features. Follow-up observations are scheduled to determine whether any of these types of objects represent a previously unrecognized component of the X-ray background.