The Einstein Observatory Extended Medium-Sensitivity Survey. II - The optical identifications

The optical identifications of the Einstein Extended Medium Sensitivity Survey (EMSS) are presented. At present over 96% of the 835 X-ray sources in the survey have been successfully identified in the following proportions: active galactic nuclei (QSOs, quasars, and Seyferts), 51. 1%; BL Lacertae objects, 4. 3%; clusters of galaxies, 12. 2%; normal galaxies, 2. 1%; cooling flow galaxies, 0. 6%; Galactic stars, 25. 8%; and unidentified, 3. 9%. All of the EMSS sources are identified with previously known classes of X-ray emitters except for five sources now identified with cooling flow galaxies whose novel attributes are described. The Galactic stars are primarily normal dwarf stars but also include RS CVn and W UMa binary systems, two FK Comae stars, five cataclysmic variables including new examples of AM Herculis stars, one white dwarf, and one X-ray binary candidate. The extragalactic sources are predominantly AGNs, which range from low-z, low-luminosity (z 2) but also include distant clusters of galaxies (0. 05 lambda_ = 8 x 10^-14^ ergs s^-1^ cm^-2^ in the 0. 3-3. 5 keV band) ; |b| >= 20ᵈeg^) is possible at an extremely high confidence level. Thus this sample can be used for detailed statistical investigations (e. g. , luminosity functions, determination of cosmological evolution and X-ray background contribution). Our results also suggest that, although time-consuming and difficult, the complete identification of high Galactic latitude ROSAT all-sky survey sources is possible with present ground-based telescopes and instrumentation.