P105	April 2006

¹ Institut d'Astrophysique et de Géophysique - Université de Liège, Allée du 6 Août, Bât B5c, B-4000 Liège (Sart Tilman), Belgium
² Department of Astronomy and Space Science, Sejong University, Kunja-dong 98, Kwangjin-gu, Seoul 143-747, Korea

^* Research Fellow FNRS, Belgium
^** Research Associate FNRS, Belgium

⁺ Based on observations collected with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member states and the USA (NASA).

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 180ks 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 x 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 x 10^-15 erg cm^-2 s^-1) with an energy distribution peaked around 1.0-2.0 keV.