The EXOSAT High Galactic Latitude Survey

Abstract

A survey of serendipitous sources performed in the very soft X-ray band (0.05-2.0 keV) using the EXOSAT imaging telescopes is presented. The survey covers 783 square degrees of high Galactic latitude sky and includes 210 serendipitous sources which define a complete (flux-limited) sample. An extensive program of optical and radio observations together with cross-correlations with catalogs of known objects leads to the identification of 200 of the 210 detected sources. The 10 remaining objects have been preliminarily classified on the basis of their X-ray to optical flux ratio, thus making the sample essentially fully identified. Twenty-three additional serendipitous sources that did not satisfy the requirements for inclusion in the complete sample were also identified during the optical observations program. The complete sample has been used to study the log N-log S relation and the average spectral slope of active galactic nuclei (AGN). It is found that the log N-log S slope is consistent with that of the Einstein Extended Medium-Sensitivity Survey (EMSS) and with the "Euclidean" value of 1.5. The normalization of the log N-log S relation is a strong function of the assumed spectral slope of AGNs. Consistency with the results of the EMSS implies that the average (energy) slope of extragalactic sources in the soft X-ray band is very steep (α~1.5). An analysis of the association between AGN detection and Galactic N_H_ also shows that α is steep and inconsistent with the canonical value of 0.7. The survey was particularly successful in detecting nearby Galactic soft X-ray emitters and includes a significantly higher percentage of stars than the Einstein EMSS, five white dwarfs and three previously unknown AM Herculis-type systems.