2013A&A...550A..90B

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. 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 colliding-wind 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. We used the Expanded Very Large Array (EVLA) radio interferometer to obtain 6cm and 20cm 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. 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. 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.