An XMM-Newton observation of the multiple system HD167971 (O5-8V + O5-8V + (O8I)) and the young open cluster NGC6604

M. De Becker^1, G. Rauw^1, R. Blomme^2, J.M. Pittard^3, I.R. Stevens^4, M.C. Runacres^2

1. Institut d'Astrophysique, Université de Liège, Allée du 6
Août, Bât B5c, B-4000 Liège (Sart Tilman), Belgium
2. Royal Observatory of Belgium, Avenue Circulaire 3, 1180 Brussels, Belgium
3. Department of Physics & Astronomy, University of Leeds, Leeds LS2 9JT, UK
4. School of Physics & Astronomy, University of Birmingham, Edgbaston
Birmingham B15 2TT, UK

We discuss the results of two XMM-Newton observations of the open cluster NGC6604 obtained in April and September 2002. We concentrate mainly on the multiple system HD167971 (O5-8V + O5-8V + (O8I)). The soft part of the EPIC spectrum of this system is thermal with typical temperatures of about 2 10^6 to 9 10^6 K. The nature (thermal vs non-thermal) of the hard part of the spectrum is not unambiguously revealed by our data. If the emission is thermal, the high temperature of the plasma (~ 2.3 10^7 to 4.6 10^7 K) would be typical of what should be expected from a wind-wind interaction zone within a long period binary system. This emission could arise from an interaction between the combined winds of the O5-8V + O5-8V close binary system and that of the more distant O8I companion. Assuming instead that the hard part of the spectrum is non-thermal, the photon index would be rather steep (~3). Moreover, a marginal variability between our two XMM-Newton pointings could be attributed to an eclipse of the O5-8V + O5-8V system. The overall X-ray luminosity points to a significant X-ray luminosity excess of about a factor 4 possibly due to colliding winds. Considering HD167971 along with several recent X-ray and radio observations, we propose that the simultaneous observation of non-thermal radiation in the X-ray (below 10.0 keV) and radio domains appears rather unlikely. Our investigation of our XMM-Newton data of NGC6604 reveals a rather sparse distribution of X-ray emitters. Including the two bright non-thermal radio emitters HD168112 and HD167971, we present a list of 31 X-ray sources along with the results of the cross-correlation with optical and infrared catalogues. A more complete spectral analysis is presented for the brightest X-ray sources. Some of the members of NGC6604 present some characteristics suggesting they may be pre-main sequence star candidates.

Reference: Accepted by A&A (complete reference not yet available).
Status: Manuscript has been accepted


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