Identification of a Complete Sample of Northern Rosat All-Sky Survey X-Ray Sources

Abstract

We present results of the optical identification of a spatially complete, flux limited sample of about 700 ROSAT All-Sky X-ray sources contained in 6 study areas north of δ = −9° with |b^II |> 20° (including one region near the North Galactic pole (NGP), another one near the North Ecliptic pole (NEP)). Countrate limits are 0.01 cts s^–1 near the NEP and 0.03 cts s^–1 for the other areas. The optical observations were performed at the 2.15-m telescope of the Guillermo Haro Observatory, Mexico, using the Landessterwarte Faint Object Spectrograph Camera which allows to carry out direct CCD imaging and multi-object spectroscopy. The limiting magnitude is about 19^m for spectroscopy and about 23^m for B and R direct imaging. Our analysis shows a dependency of the ratio of ‘extragalactic’ (e.g., AGN, cluster of galaxies) to ‘stellar’ (e.g., coronal emitters, active binaries) counterparts on N _H. In the area near the NGP (low N _H) ‘extragalactic’ counterparts dominate, while in the area with the highest N _H ‘stellar’ counterparts dominate.