An XMM-Newton view of the young open cluster NGC 6231 -- I. The catalogue

H.Sana$^1$, E. Gosset$^1$, G. Rauw$^1$, H. Sung$^2$, J.-M. Vreux$^1$

1. Institut d'Astrophysique et de Geophysique, University of Liege, Belgium \\
2. Department of Astronomy and Space Science, Sejong University, Seoul, Korea

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 6E-15 erg/s/cm$^2$ 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 arcsec 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 arcmin$^2$ 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 ($\sim$ 5E-15 erg/s/cm$^2$) with an energy distribution peaked around 1.0-2.0 keV.

Reference: Astronomy & Astrophysics 454, 1047 (astro-ph/0603783)
Status: Manuscript has been accepted


Comments: Tables 3 and 6 are available at the CDS only