A&A 454, 1047-1063 (2006)
DOI: 10.1051/0004-6361:20053224

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

I. The catalogue

H. Sana¹, E. Gosset¹, G. Rauw¹, H. Sung² and J.-M. Vreux<sup>1

1</sup>  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.

Key words: open clusters and associations: individual: NGC 6231 -- X-rays: individuals: NGC 6231 -- X-rays: stars -- stars: early-type -- catalogues

© ESO 2006

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