67G The Astrophysical Journal Supplement Series, 72:567-619,1990 March .5 © 1990. The American Astronomical Society. All rights reserved. Printed in U.S.A. .72. 90ApJS. THE EINSTEIN OBSERVATORY EXTENDED MEDIUM-SENSITIVITY SURVEY. 19 I. X-RAY DATA AND ANALYSIS1 I. M. Gioia,2 T. Maccacaro,2 R. E. Schild, and A. Wolter Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts J. T. Stocke Center for Astrophysics and Space Astronomy, Boulder, Colorado S. L. Morris The Observatories of the Carnegie Institute of Washington, Pasadena, California AND J. P. Henry Institute for Astronomy, Honolulu, Hawaii Received 1989 March 6; accepted 1989 August 10 ABSTRACT 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 X10“14 to — 3 X10“12 ergs cm-2 s-1 in the 0.3-3.5 keV energy band. A total area of 778 square degrees of the high Galactic latitude sky (|fr| > 20°) has been covered. The data have been analyzed using the REVI 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 -1 -14 -2 -1 sources per square degree (25,146^^20 sources sterad ) at /c = 7XlO ergs cm s (0.3-3.5 keV). The number-counts relation is best fitted by a power law of slope a = 1.48 + 0.05. Subject headings: BL Lacertae objects — galaxies: nuclei — galaxies: X-rays — stars: X-rays — X-rays: sources I. INTRODUCTION the presence of new phenomena and the existence of previ- ously unknown classes of objects (e.g., Spinrad 1987). One a) Published Medium-Sensitivity Survey complete sample which has been used for this science is the Statistical analyses of flux-limited and homogeneous sam- Einstein Observatory Medium-Sensitivity Survey (MSS). The ples of astronomical objects which can be extracted from MSS project was initiated several years ago with the aim of unbiased and fully identified surveys give important informa- carrying out a systematic investigation of the high Galactic tion on the nature and distribution of the different popula- latitude X-ray sky at fluxes intermediate between the tions. Complete samples of extragalactic objects provide the Uhuru/Ariel V limit and the Einstein Deep Survey limit. This best tool for the investigation of their evolutionary properties was accomplished by analyzing observations of serendipitous with cosmic time (Schmidt 1968; Schmidt and Green 1983). sources in the energy range 0.3-3.5 keV, detected at > 5 rms The number-flux relationship and the luminosity function of with the imaging proportional counter (IPC) on board the Galactic and extragalactic sources yield important informa- Einstein Observatory (Giacconi et al 1979). The first (Mac- tion on their respective densities and spatial distributions. cacaro etal. 1982; Stocke etal 1983) and second (Gioia etal. Subsequent studies of individual peculiar sources can reveal 1984) MSS samples addressed several of the studies outlined above, answered many questions, and opened new ones. The published MSS sample is unique in that it is a large, xThis paper uses data obtained at the Multiple Mirror Telescope Observatory (MMTO), which is operated jointly by the University of flux-limited X-ray catalog of fully identified faint sources; Arizona and the Smithsonian Institution. unbiased statistical studies of their properties are therefore 2 A1so from Istituto di Radioastronomia del CNR, Bologna, Italy. possible. In addition, at any given X-ray flux fx, the area of 567 © American Astronomical Society • Provided by the NASA Astrophysics Data System 67G .5 568 GIOIA ETAL. Vol. 72 sky searched for sources brighter than fx has been recorded. sources. The optical program, undertaken to identify all the .72. This has made possible the determination of the X-ray sources spectroscopically, is still in progress, and it will be log N{ > S)-logS curves for the extragalactic population as a reported elsewhere (Stocke et al. 1990¿z), as will the results of whole, and for AGNs (active galactic nuclei = quasars and the 6 cm VLA observations of over 500 objects (Gioia et al. 90ApJS. Seyfert galaxies), clusters of galaxies, and BL Lac objects, 1990). This paper presents the X-ray data for the sources of 19 separately (Gioia et al. 1984; Maccacaro et al 1984). Using the final EMSS sample and the optical identifications for 91% the redshift information available, the AGN X-ray luminosity of them. The number-flux relation for extragalactic X-ray function and its evolution with cosmic time have been deter- sources is derived and discussed. mined and used to make a direct estimate of the contribution The all-sky survey which will be performed by the ROSA T from AGNs to the diffuse X-ray background (Maccacaro, satellite (1990) with the Position Sensitive Proportional Gioia, and Stocke 1984). For the other two classes of extra- Counter (PSPC; Tmmper 1984), an instrument similar to the galactic objects present in X-ray surveys, clusters of galaxies Einstein IPC, will detect a very large number of X-ray sources and BL Lac objects, the statistics available allowed us to (about 100,000) down to a limiting sensitivity of about 10“13 obtain only preliminary results. However, despite the small ergs cm-2 s-1. The softer energy band (0.1-2.5 keV) at which number of BL Lac objects found in the MSS, their unusual the PSPC operates will favor the detection of Galactic objects. flux distribution suggested that, under the hypothesis of Thus ordinary stars will be the largest class of X-ray sources isotropic emission, BL Lac objects are characterized by a detected by ROSA T. At this relatively softer energy band, the cosmological evolution smaller, if at all present, than that PSPC will also be more sensitive to the effects of the Galactic required for quasars (Maccacaro etal 1984; Stocke et al absorption, which may create problems when computing, for 1982). instance, logN-logS in the 0.1-2.5 keV band (cf. Zamorani The Galactic component of the published MSS sample etal. 1988). The spectroscopic identification of all, or even constitutes - 25% of the total number of X-ray sources in most, of the all-sky survey sources will not be feasible. Even that sample. Being a serendipitous sample, the survey con- the identification of a sample of faint sources of a size tains preferentially the brightest X-ray emitters among any significantly larger than the EMSS may require several years. class of objects. Thus, it is not surprising that among the stars Thus, upon completion of the optical identification process, identified as optical counterparts to MSS sources we have the EMSS catalog will be the largest and most statistically found K and M dwarf flare stars (dKe and dMe), RS CVn complete sample of identified X-ray-selected objects available type binaries, and even a cataclysmic variable. These objects to the astronomical community for several years to come. The are already known to be strong X-ray emitters, by stellar EMSS and the EXOSAT High Galactic Latitude Survey standards, from previous observational studies (cf. Rosner, results (Giommi, Tagliaferri, and Angelini 1988) can be used Golub, and Vaiana 1985 and references therein). Pre-main- to predict the content of the ROSAT all-sky survey. sequence and O and B stars, which are also strong X-ray emitters, are severely underrepresented, since the MSS avoids II. THE SELECTION CRITERIA OF THE EMSS the IPC fields in the Galactic plane. The EMSS covers an area of 778 square degrees of the high The MSS sample contained only 112 serendipitous X-ray Galactic latitude sky, resulting from the analysis of 1435 IPC sources. It is evident that the analysis of a larger sample images. The IPC field of view is -1 square degree, but would give us a more satisfactory understanding of the behav- only a part of it has been used, because of the shadowing ior of the different classes of X-ray-selected objects. We have of the window-supporting structure. The exposure times of therefore undertaken to increase the size of the survey, ex- the IPC observations range from ~ 800 to - 40,000 s; their tending the search for sources to all appropriate IPC images limiting sensitivities range from -5x10“14 to -3X10“12 within the Einstein data bank. ergs cm“2 s“1 in the 0.3-3.5 keV IPC band.