The X-ray bright massive stars in Cyg OB2

Yaël Nazé, Constantin Cazorla, Gregor Rauw
2012 unpublished
Cyg OB2 #5 Cyg OB2 #5 was recently found to be a quadruple system (Kennedy et al. 2010) consisting of an eclipsing binary with 6.6d period, a tertiary star in a 6.7 yrs orbit around it, and a fourth object at 0.8". The X-ray emission can be fitted with two thermal components, with constant temperatures and ratio between the normalization factors. However, both flux and absorption vary. The flux has a complex behaviour, incompatible with changes over a 6.6d period; the X-ray emission rather
more » ... mission rather appears to be modulated by the 6.7yrs period: it is thus associated with the wind-wind collision between the binary and the third star, as is the radio emission. This is supported by the variation of the absorption since the latter appears stronger when the binary is in front. Since the collision should be adiabatic, its theoretical behaviour is known and our study therefore allowed us to test (and discard) some orbital solutions proposed for this triple system. Reflex motion induced by the third star was detected for eclipse times and radial velocity curves of the binary, but additional data must be gathered before a secure solution is achieved.