Identification of a complete sample of northern ROSAT All-Sky Survey X-ray sources - II. The optical observations

We describe a program for the optical identi fication of a re presen tative sample of RASS sources contained in six study areas north of , with including one region near the North Galactic pole (NGP) and one near the North Ecliptic pole (NEP) and present some pre liminary results. A full identi fication has been carried out for a count-rate and area-limited complete sub sample comprising 674 sources. The ratio of "extragalactic” ( e.g. AGN, clusters of galaxies) to "stellar” (e.g. coronal emitters, active binaries) counterparts was found to depend on the galactic neutral hydrogen column density, N H . In the area near the NGP (low N H ) "extra galactic” counter parts dominate, while in the area with the highest N H most X-ray sources are identi fied with stellar counter parts.