The 2.35 year itch of Cygnus OB2 #9

Abstract

Context. Cyg OB2 #9 is one of a small set of non-thermal radio emitting massive O-star binaries. The non-thermal radiation is due to synchrotron emission in the colliding-wind region. Cyg OB2 #9 has only recently been discovered to be a binary system, and a multi-wavelength campaign was organized to study its 2011 periastron passage. Aims. We want to better determine the parameters of this system and model the wind-wind collision. This will lead to a better understanding of the Fermi mechanism that accelerates electrons up to relativistic speeds in shocks and its occurrence in collidingwind binaries. We report here on the results of the radio observations obtained in the monitoring campaign and present a simple model to interpret the data. Methods. We used the Expanded Very Large Array (EVLA) radio interferometer to obtain 6 cm and 20 cm continuum fluxes during the Cyg OB2 #9 periastron passage in 2011. We introduce a simple model to solve the radiative transfer in the stellar winds and the colliding-wind region, and thus determine the expected behaviour of the radio light curve. Results. The observed radio light curve shows a steep drop in flux sometime before periastron. The fluxes drop to a level that is comparable to the expected free-free emission from the stellar winds, suggesting that the non-thermal emitting region is completely hidden at that time. After periastron passage, the fluxes slowly increase. We use the asymmetry of the light curve to show that the primary has the stronger wind. This is somewhat unexpected if we use the astrophysical parameters based on theoretical calibrations. But it becomes entirely feasible if we take into account that a given spectral type-luminosity class combination covers a range of astrophysical parameters. The colliding-wind region also contributes to the free-free emission, which can help explain the high values of the spectral index seen after periastron passage. Combining our data with older Very Large Array (VLA) data allows us to derive a period P = 860.0 ± 3.7 days for this system. With this period, we update the orbital parameters that were derived in the first paper of this series. Conclusions. A simple model introduced to explain only the radio data already allows some constraints to be put on the parameters of this binary system. Future, more sophisticated, modelling that will also include optical, X-ray, and interferometric information will provide even better constraints.

The 2.35 year itch of Cygnus OB2 #9

“…k and α are the Castor et al (1975) Notes. We have taken the median value from the ranges noted by Blomme et al (2013). momentum, and energy conservation are:…”

“…The adopted parameters of the system and stars are noted in Tables 1 and 2, respectively. The large variation in observed radio flux across the orbit (van Loo et al 2008;Blomme et al 2013) is characteristic of a highly eccentric binary system. Indeed, the orbital solution derived by Nazé et al (2010) returned e ∼ 0.744.…”

“…Indeed, the orbital solution derived by Nazé et al (2010) returned e ∼ 0.744. This has since been revised to e 0.711, with an orbital period of 860 days, using the recent optical and radio analysis by Nazé et al (2012b) and Blomme et al (2013). The model constructed by Nazé et al (2012b; based on estimates for the stellar parameters) provides valuable physical insight into the system, particularly the anticipated significance of wind acceleration on the importance of radiative cooling.…”

“…Previous models of Cyg OB2#9 have provided tantalising evidence of non-thermal radio emission from a wind-wind collision (van Loo et al 2008;Blomme et al 2013). In the following we proceed using the mass-loss rates determined from Sect.…”

“…Dougherty & Williams 2000;van Loo 2005;De Becker et al 2006;van Loo et al 2006van Loo et al , 2008De Becker 2007;Nazé et al 2008;Montes et al 2009;Blomme et al 2010;A&A 570, A10 (2014) A prime example that fits into this class of X-ray and nonthermal radio emitting massive binary is the O+O-star system Cyg OB2#9 (van Loo et al 2008;Nazé et al 2008Nazé et al , 2010Volpi et al 2011). Our recent campaign to study this object has so far consisted of an analysis of optical and X-ray data by Nazé et al (2012b), and radio observations by Blomme et al (2013), which have refined the orbital solution and provided initial estimates for the stellar parameters − see Tables 1 and 2. The variation of the X-ray flux with orbital phase correlates with the inverse of binary separation (i.e.…”