THE EINSTEIN-OBSERVATORY EXTENDED MEDIUM-SENSITIVITY SURVEY .1. X-RAY DATA AND ANALYSIS

This paper presents the results of the analysis of the X-ray data and the optical identification for the Einstein Observatory Extended Medium-Sensitivity Survey (EMSS). The survey consists of 835 serendipitous sources detected at or above 4 times the rms level in 1435 imaging proportional counter (IPC) fields with centers located away from the Galactic plane. Their limiting sensitivities range from ∼5 × 10-14 to ∼ 3 × 10-12 ergs cm-12 s-1 in the 0.3-3.5 keV energy band. A total area of 778 square degrees of the high Galactic latitude sky (|b| > 20°) has been covered. The data have been analyzed using the REV1 processing system implemented a few years ago at the Center for Astrophysics. This system makes use, for each IPC image, of a background map, a kind of "flat field" which takes into account the nonuniformities of the detector. The selection criteria for the fields and the sources are described, as well as the detection algorithms used to determine the existence and location of the sources. The resulting EMSS catalog of X-ray sources is a flux-limited and homogeneous sample of astronomical objects that can be used for statistical studies. An optical program to identify the 835 sources spectroscopically is underway. As of 1989 June, 91% of the sample sources has been identified. A classification has been assigned to the remaining sources based on the ratio of X-ray to visual flux as described in the paper. In agreement with previous results from the Medium-Sensitivity Survey samples, active galactic nuclei dominate the extragalactic component, followed by clusters of galaxies, BL Lacertae objects and "normal" galaxies. About 27% of the EMSS sources are Galactic. The revised estimate of the surface density of extragalactic X-ray sources is 7.7-0.5+0.6 sources per square degree (25,146-1853+1620 sources sterad-1) at fx = 7 × 10-14 ergs cm-2 s-1 (0.3-3.5 keV). The number-counts relation is best fitted by a power law of slope α = 1.48 ± 0.05.