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Astronomy & Astrophysics

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

M. De Becker1, G. Rauw1,*, R. Blomme2, J.M. Pittard3, I.R. Stevens4 and M.C. Runacres2

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

* Research Associate FNRS, Belgium

+ Based on observations with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member states and the USA (NASA). Partly based on observations collected at the European Southern Observatory (La Silla, Chile).


     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 2 106 to 9 106 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 (~ 2.3 107 to 4.6 107 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.

Key Words
Radiation mechanisms: non-thermal -- Stars: early-type -- Stars: individual: HD167971 -- Stars: winds, outflow -- X-rays: stars

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