Identification of a complete sample of northern ROSAT All-Sky Survey X-ray sources

II. The optical observations

¹ Landessternwarte Königstuhl (LSW), D-69117 Heidelberg, Germany
² Max–Planck–Institut für Extraterrestrische Physik (MPE), Giessenbachstrasse, Postfach 1603, D-85740 Garching, Germany
³ Universitätssternwarte München, Scheinerstr. 1, D-81679 München, Germany
⁴ Instituto de Astronomia-UNAM, Apdo Postal 70-264, 04510 México D.F., Mexico
⁵ Instituto Nacional de Astrofisica, Optica y Electronica (INAOE), A. Postal 51 y 216 Z.P., 72000 Puebla, Mexico
⁶ Observatoire de Strasbourg, 11 rue de l'Université, F-67000 Strasbourg, France

Send offprint request to: F.-J. Zickgraf

Received: 29 August 1996
Accepted: 5 September 1996

Abstract

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.