Because these stars live together and orbit each other while following a precise trajectory, the stellar winds that accompany them collide, resulting in a huge impact. “Imagine two jets colliding at ten million kilometers per hour. The collision creates very hot plasma and this energy becomes transformed into heat and gas which can reach temperatures of several millions degrees resulting in the emission of X rays,” explains Yaël Nazé

Or at least that is the theory, because, against all expectations, Cyg OB2#9 did not seem to emit any peculiar X ray emission. This mystery attracted the attention of the researchers. “We had known for at least fifty years that there was an anomaly due to the radio radiation, but nobody had studied it in detail. We decided to monitor this system to find out the reason for this peculiarity. We realized that all the X observations were taken when the two stars were quite far away from each other. Therefore we had to find a moment when they were closer to each other,” explains the Liege-based researcher.

 The first “encounter” was observable in 2009. Unfortunately the system was hidden by the sun. Because this stellar system has an orbital period of 2.4 years, it was necessary to wait until June 2011 for new observations and measurements. This was the amount of time necessary for Yaël Nazé to set up a large operation on behalf of the University of Liege, but also bringing together researchers from the University of Wyoming (United States), the University of Leeds (England), Australia and the Royal Observatory of Brussels.

Under NASA’s eyes

For several months, the spatial telescopes XMM-Newton (ESA) and Swift (NASA) regularly monitored variations in the system. “We noticed that the X ray emissions began slowly to increase, reaching a climax when the two stars were closest to each other, causing a spectacular increase in the luminosity of the system and then  the luminosity diminished as the two stars moved away from each other, ” which corresponded perfectly to the theory. She goes on to add, “Perhaps even a bit too perfectly. When I saw these results for the first time I thought to myself that it was too good to be true. However, after verification this was confirmed! It was the fact that we studied the two stars at their closest point to each other which made these interesting observations possible.” The results of this discovery have just been published in the European journal Astronomy and Astrophysics. (1)

In theory, there are two kinds of possible collisions: radiative (and therefore turbulent and difficult to model) or adiabatic “but the collisions observed tended not to remain adiabatic.” There is a third possibility: one of the stellar winds crashes into the neighboring star when the two objects are close resulting in a reduction of X-ray emissions.

(1) The 2.35 year itch of Cygnus OB2 #9. I. Optical and X-ray monitoring, Y. Nazé, L. Mahy, Y. Damerdji, O. Absil et al. , 2012, EDP Sciences, Astronomy and Astrophysics, vol. 546; id.A37, 14pp.