An XMM-Newton view of the young open cluster NGC 6231

I. The catalogue

¹ Institut d'Astrophysique et de Géophysique, University of Liège, Allée du 6 Août 17, Bât. B5c, 4000 Liège, Belgium e-mail: [sana;gosset;rauw;vreux]@astro.ulg.ac.be
² Department of Astronomy and Space Science, Sejong University, Kunja-dong 98, Kwangjin-gu, Seoul 143-747, Korea e-mail: sungh@sejong.ac.kr

Received: 11 April 2005
Accepted: 22 March 2006

Abstract

This paper is the first of a series dedicated to the X-ray properties of the young open cluster NGC 6231. Our data set relies on an XMM-Newton campaign of a nominal duration of 180 ks and reveals that NGC 6231  is very rich in the X-ray domain too. Indeed, 610 X-ray sources are detected in the present field of view, centered on the cluster core. The limiting sensitivity of our survey is approximately 6  10^-15 erg cm^-2 s^-1 but clearly depends on the location in the field of view and on the source spectrum. Using different existing catalogues, over 85% of the X-ray sources could be associated with at least one optical and/or infrared counterpart within a limited cross-correlation radius of 3´´ at maximum. The surface density distribution of the X-ray sources presents a slight N-S elongation. Once corrected for the spatial sensitivity variation of the EPIC instruments, the radial profile of the source surface density is well described by a King profile with a central density of about 8 sources per arcmin² and a core radius close to 3.1 arcmin. The distribution of the X-ray sources seems closely related to the optical source distribution. The expected number of foreground and background sources should represent about 9% of the detected sources, thus strongly suggesting that most of the observed X-ray emitters are physically belonging to NGC 6231. Finally, beside a few bright but soft objects – corresponding to the early-type stars of the cluster – most of the sources are relatively faint (~5  10^-15 erg cm^-2 s^-1) with an energy distribution peaked around 1.0–2.0 keV.