P86	December 2003

¹ Institut d'Astrophysique et de Géophysique - Université de Liège, Allée du 6 Août, 17 - Bât B5c, B-4000 Liège (Sart Tilman), Belgium
² Department of Physics & Astronomy, University of Leeds, Leeds LS2 9JT, UK
³ Department of Physics & Astronomy, University of Glasgow, Kelvin Building, Glasgow G12 8QQ, Scotland, UK
⁴ On leave from: Sternberg Astronomical Institute, Moscow University, Universitetskij Prospect, 13, Moscow 119899, Russia
⁵ School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
⁶ Bartol Research Institute, University of Delaware, Newark, DE 19716, USA

^* 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).

Abstract

     We report the analysis of an XMM-Newton observation of the close binary HD159176 (O7V + O7V). The observed L_X/L_bol ratio reveals an X-ray luminosity exceeding by a factor ~ 7 the expected value for X-ray emission from single O-stars, therefore suggesting a wind-wind interaction scenario. EPIC and RGS spectra are fitted consistently with a two temperature mekal optically thin thermal plasma model, with temperatures ranging from ~ 2 to 6 10⁶ K. At first sight, these rather low temperatures are consistent with the expectations for a close binary system where the winds collide well before reaching their terminal velocities. We also investigate the variability of the X-ray light curve of HD159176 on various short time scales. No significant variability is found and we conclude that if hydrodynamical instabilities exist in the wind interaction region of HD159176, they are not sufficient to produce an observable signature in the X-ray emission. Hydrodynamic simulations using wind parameters from the literature reveal some puzzling discrepancies. The most striking one concerns the predicted X-ray luminosity which is one or more orders of magnitude larger than the observed one. A significant reduction of the mass loss rate of the components compared to the values quoted in the literature alleviates the discrepancy but is not sufficient to fully account for the observed luminosity. Because hydrodynamical models are best for the adiabatic case whereas the colliding winds in HD159176 are most likely highly radiative, a totally new approach has been envisaged, using a geometrical steady-state colliding wind model suitable for the case of radiative winds. This model successfully reproduces the spectral shape of the EPIC spectrum, but further developments are still needed to alleviate the disagreement between theoretical and observed X-ray luminosities.