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Cyg OB2 is one of the most massive associations of O-type stars in ourGalaxy. Despite the large interstellar reddening towards Cyg OB2, many studies,spanning a wide range of wavelengths, have been conducted to more clearlyunderstand this association. X-ray observations provide a powerful tool toovercome the effect of interstellar absorption and study the most energeticprocesses associated with the stars in Cyg OB2. We analyse XMM-Newton data toinvestigate the X-ray and UV properties of massive O-type stars as well aslow-mass pre-main sequence stars in Cyg OB2. We obtained six XMM-Newtonobservations of the core of Cyg OB2. In our analysis, we pay particularattention to the variability of the X-ray bright OB stars, especially theluminous blue variable candidate Cyg OB2 #12. We find that X-ray variability isquite common among the stars in Cyg OB2. Whilst short-term variations arerestricted mostly to low-mass pre-main sequence stars, one third of the OBstars display long-term variations. The X-ray flux of Cyg OB2 #12 varies by37%, over timescales from days to years, whilst its mean log(L_X/L_bol) amountsto -6.10. These properties suggest that Cyg OB2 #12 is either aninteracting-wind system or displays a magnetically confined wind. Two otherX-ray bright O-type stars (MT91 516 and CPR2002 A11) display variations thatsuggest they are interacting wind binary systems.

A multi-epoch XMM-Newton campaign on the core of the massive Cyg OB2 association