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Astron. Astrophys. 350, 743-752 (1999)


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

IV. Statistical analysis

J. Krautter 1, F.-J. Zickgraf 2, I. Appenzeller 1, I. Thiering 1,3, W. Voges 4, C. Chavarria 5, R. Kneer 1, R. Mujica 6, M.W. Pakull 2, A. Serrano 6 and B. Ziegler 7

1 Landessternwarte Königstuhl, D-69117 Heidelberg, Germany
2 Observatoire Astronomique de Strasbourg, 11, rue de l'Université, 67000 Strasbourg, France
3 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
4 Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, Postfach 1603, 85740 Garching, Germany
5 Instituto de Astronomia-UNAM, Apdo. Postal 70-264, 04510 México D.F., Mexico
6 Instituto Nacional de Astrofisica, Optica y Electronica (INAOE), A. Postalo 51 y 216 Z.P., 72000 Puebla, México
7 Department of Physics, University of Durham, South Rd, Durham DH1 3LE, UK

Received 4 January 1999 / Accepted 26 August 1999

Abstract

We present the statistical evaluation of a count rate and area limited complete sample of the ROSAT All-Sky Survey (RASS) comprising 674 sources. The RASS sources are located in six study areas outside the galactic plane ([FORMULA]) and north of [FORMULA]. The total sample contains 274 (40.7%) stars, 26 (3.9%) galaxies, 284 (42.1%) AGN, 78 (11.6%) clusters of galaxies and 12 (1.8%) unidentified sources. These percentages vary considerably between the individual study areas due to different mean hydrogen column densities [FORMULA]. For the accuracy of the RASS positions, i.e., the distance between optical source and X-ray position, a 90% error circle of about 30" has been found. Hardness and X-ray-to-optical flux ratios show in part systematic differences between the different object classes. However, since the corresponding parameter spaces overlap significantly for the different classes, an unambiguous identification on the basis of the X-ray data alone is not possible. The majority of the AGN in our sample are found at rather low redshifts with the median value of the whole sample being [FORMULA]. The median values for the individual fields vary between [FORMULA] and [FORMULA]. The slope of the [FORMULA] distributions for our AGN ([FORMULA]) is in good agreement with the euclidian slope of -1.5. The BL Lacs (with a slope of [FORMULA]) exhibit a much flatter distribution.

Key words: surveys – X-rays: general – X-rays: galaxies – X-rays: stars – galaxies: active – galaxies: clusters: general

Present address: Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany

Present address: Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Send offprint requests to: J. Krautter

© European Southern Observatory (ESO) 1999

Online publication: October 14, 1999

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