2028 20160701171657.0 10.1111/j.1365-2966.2006.10746.x DOI ASTROimport-185 De Becker, M. 2006 XMM-Newton observations of the massive colliding wind binary and non-thermal radio emitter CygOB2#8A [O6If + O5.5III(f)] We report on the results of four XMM-Newton observations separated by about ten days from each other of CygOB2#8A [O6If + O5.5III(f)]. This massive colliding wind binary is a very bright X-ray emitter - one of the first X-ray emitting O-stars discovered by the Einstein satellite - as well as a confirmed non-thermal radio emitter whose binarity was discovered quite recently. The X-ray spectrum between 0.5 and 10.0keV is essentially thermal, and is best fitted with a three-component model with temperatures of about 3, 9 and 20MK. The X-ray luminosity corrected for the interstellar absorption is rather large, i.e. about 1034ergs-1. Compared to the `canonical' LX/Lbol ratio of O-type stars, CygOB2#8A was a factor of 19-28 overluminous in X-rays during our observations. The EPIC spectra did not reveal any evidence for the presence of a non-thermal contribution in X-rays. This is not unexpected considering that the simultaneous detections of non-thermal radiation in the radio and soft X-ray (below 10.0keV) domains is unlikely. Our data reveal a significant decrease in the X-ray flux from apastron to periastron with an amplitude of about 20 per cent. Combining our XMM-Newton results with those from previous ROSAT-PSPC and ASCA-SIS observations, we obtain a light curve suggesting a phase-locked X-ray variability. The maximum emission level occurs around phase 0.75, and the minimum is probably seen shortly after the periastron passage. Using hydrodynamic simulations of the wind-wind collision, we find a high X-ray emission level close to phase 0.75, and a minimum at periastron as well. The high X-ray luminosity, the strong phase-locked variability and the spectral shape of the X-ray emission of CygOB2#8A revealed by our investigation point undoubtedly to X-ray emission dominated by colliding winds. Based on observations with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member states and the USA (NASA). E-mail: debecker@astro.ulg.ac.be ‡ Research Associate FNRS (Belgium). Rauw, G. Sana, H. Pollock, A. M. T. Pittard, J. M. Blomme, R. Stevens, I. R. van Loo, S. Monthly Notices of the Royal Astronomical Society 371 3 2006 1280-1294 http://esoads.eso.org/abs/2006MNRAS.371.1280D published in REFERD