THE EINSTEIN 2-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 sigma significan ce in 2520 high-latitude Einstein IPCC images. We have employed variou s 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 know n vignetting and background characteristics of the IPC and the measure d X-ray log N-log S relation, we have modeled the number of real sourc es expected in the catalog in order to evaluate its statistical proper ties below 4 sigma significance. Our modeling suggests that similar to 13,000 sources in the catalog are real celestial X-ray sources, an in crease of similar to 9100 over the number found in previous analyses o f the same IPC images. We find that not only is the reliability of the Two-Sigma Catalog a function of source significance sigma, it is a fu nction of off-axis angle on the detector as well. The application of d ifferent signal-to-noise thresholds at different off-axis angles thus enables one to tune the reliability of the catalog. The chief motivati on 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 astron omical catalogs at other wavelengths as filters. For example, the cros s-correlation of the Two-Sigma Catalog with catalogs from surveys of t he radio and infrared sky has yielded large samples of faint X-ray sou rces that are similar to 90% reliable. Optical spectroscopy of 77 unid entified faint X-ray sources has turned up several surprises, illustra ting 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) approximate to 10(43) ergs s(-1)) radio-loud el liptical galaxies with optical spectra devoid of emission lines, and i nfrared-bright active galactic nuclei whose optical spectra are domina ted by starburst galaxy features. Follow-up observations are scheduled to determine whether any of these types of objects represent a previo usly unrecognized component of the X-ray background.