\begin{center}{\Large\bf 
X-ray emission from massive stars in Cyg OB2}\end{center} 

\centerline{\bf G. Rauw (1), Y. Naze (1), N.J. Wright (2), J.J. Drake (2), M.G. Guarcello (2), R.K. Prinja (3), L.W. Peck (3), J.F. Albacete Colombo (4), A. Herrero (5), H.A. Kobulnicky (6), S. Sciortino (7), J.S. Vink (8)
}{\footnotesize (1) - Institut d’Astrophysique & Geophysique, University of Liege, Belgium
(2) - Smithsonian Astrophysical Observatory, Cambridge, USA
(3) - Department of Physics & Astronomy, University College London, United Kingdom
(4) - Centro regional Zona Atlantica, Universidad Nacional del Comahue, Viedma, Argentina
(5) - Instituto de Astrofısica de Canarias, Universidad de La Laguna, La Laguna, Spain
(6) - Department of Physics & Astronomy, University of Wyoming, Laramie, USA
(7) - INAF - Osservatorio Astronomico di Palermo, Italy
(8) - Armagh Observatory, College Hill, Armagh, Northern Ireland
}
 
We report on the analysis of the Chandra-ACIS data of O, B and WR stars in the young association Cyg OB2. X-ray spectra of 49 O-stars, 54 B-stars and 3 WR-stars are analyzed and for the brighter sources, the epoch dependence of the X-ray fluxes is investigated. The O-stars in Cyg,OB2 follow a well-defined scaling relation between their X-ray and bolometric luminosities: log(Lx/Lbol) = -7.2 +/- 0.2. This relation is in excellent agreement with the one previously derived for the Carina OB1 association. Except for the brightest O-star binaries, there is no general X-ray overluminosity due to colliding winds in O-star binaries. Roughly half of the known B-stars in the surveyed field are detected, but they fail to display a clear relationship between Lx and Lbol. Out of the three WR stars in Cyg OB2, probably only WR144 is itself responsible for the observed level of X-ray emission, at a very low log(Lx/Lbol) = -8.8 +/- 0.2. The X-ray emission of the other two WR-stars (WR145 and 146) is most probably due to their O-type companion along with a moderate contribution from a wind-wind interaction zone.

{\bf Reference: Accepted for an ApJS Special Issue devoted to the Chandra Cygnus OB2 Legacy Survey} \\{\it Status:} Manuscript has been accepted
 \\ {\it Comments:} 

{\it On the web at:} {\tt http://arxiv.org/abs/1401.8098} 

{\it Preprints from:} {\tt  rauw@astro.ulg.ac.be} 
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