Département AGO, Université de Liège, Allée du 6 Août 17, Bât. B5C, 4000 Liège, Belgium
Received: 26 August 2013
Accepted: 19 November 2013
Aims. We wish to study the origin of the X-ray emission of three massive stars in the Cyg OB2 association: Cyg OB2 #5, Cyg OB2 #8A, and Cyg OB2 #12.
Methods. To this aim, dedicated X-ray observations from XMM-Newton and Swift are used, as well as archival ROSAT and Suzaku data.
Results. Our results on Cyg OB2 #8A improve the phase coverage of the orbit and confirm previous studies: the signature of a wind-wind collision is conspicuous. In addition, signatures of a wind-wind collision are also detected in Cyg OB2 #5, but the X-ray emission appears to be associated with the collision between the inner binary and the tertiary component orbiting it with a 6.7 yr period, without a putative collision inside the binary. The X-ray properties strongly constrain the orbital parameters, notably allowing us to discard some proposed orbital solutions. To improve the knowledge of the orbit, we revisit the light curves and radial velocity of the inner binary, looking for reflex motion induced by the third star. Finally, the X-ray emission of Cyg OB2 #12 is also analyzed. It shows a marked decrease in recent years, compatible with either a wind-wind collision in a wide binary or the aftermath of a recent eruption.
Key words: stars: early-type / stars: individual: Cyg OB2 #5 / stars: individual: Cyg OB2 #8A / stars: individual: Cyg OB2 #12 / X-rays: stars / binaries: eclipsing
Based on observations collected at the Observatoire de Haute Provence (OHP) as well as with Swift and XMM-Newton.
Tables 1–3 and 5 are available in electronic form at http://www.aanda.org
© ESO, 2014