Massive non-thermal radio emitters: new data and their modelling

arXiv:1110.2622v1 [astro-ph.HE] 12 Oct 2011 **Volume Title** ASP Conference Series, Vol. **Volume Number** **Author** c⃝**Copyright Year** Astronomical Society of the Pacific Massive non-thermal radio emitters: new data and their modelling Delia Volpi,1 Ronny Blomme,1 Michael De Becker2,3, and Ya¨el Naz´e2 1Royal Observatory of Belgium, Ringlaan 3, B-1180 Brussel, Belgium Delia.Volpi@oma.be 2Institut d’Astrophysique, Universit´e de Li`ege, All´ee du 6 Aoˆut, 17, Bˆat B5c, B-4000 Li`ege (Sart-Tilman), Belgium 3Observatoire de Haute-Provence, F-04870 Saint-Michel L’Observatoire, France Abstract. During recent years some non-thermal radio emitting OB stars have been discovered to be binary, or multiple systems. The non-thermal emission is due to syn- chrotron radiation that is emitted by electrons accelerated up to high energies. The electron acceleration occurs at the strong shocks created by the collision of radiatively- driven winds. Here we summarize the available radio data and more recent observations for the binary Cyg OB2 No. 9. We also show a new emission model which is being developed to compare the theoretical total radio flux and the spectral index with the ob- served radio light curves. This comparison will be useful in order to solve fundamental questions, such as the determination of the stellar mass loss rates, which are perturbed by clumping. 1. Introduction Several massive stars, of both Wolf-Rayet and OB-type, show evidence of non-thermal radio emission (Bieging et al. 1989). This non-thermal radiation is produced by rel- ativistic electrons accelerated at collisionless shocks through the Fermi mechanism. Following the theoretical model of Eichler & Usov (1993) the shocks are created by the strong radiatively-driven winds that collide in the binary or multiple systems. This model was confirmed by the observations of WR140 (Dougherty et al. 2005). The ac- celerated electrons advect away cooling down due to adiabatic and inverse Compton losses. In the presence of the magnetic field, they emit synchrotron radiation in the radio band. The synchrotron radiation is partially absorbed by the free-free absorption mechanism, leading to flux variability locked to the orbital phase. This variability con- sists of two contributions: the intrinsic synchrotron variations due to the changes in collision energy linked to the orbital phase and the changing free-free absorption along the line of sight. Recently new observations for the O5 + O6-7 binary Cyg OB2 No. 9 were ob- tained. Here these new radio data together with previously published ones are shown (Fig. 1). A new emission model, which is applicable both to Wolf-Rayet and OB stars, is presented. This model is being developed in order to compare the observations with the theoretical results. The comparison is essential to investigate important issues re- lated to massive stars astrophysics, such as the clumping effect on the mass loss rate 1 2 Volpi, Blomme, De Becker, and Naz´e Figure 1. Cyg OB2 No. 9: 6 cm flux versus phase during the complete orbital period of 852.9 days. The black points are the available VLA radio data from Van Loo et al. (2008), the red ones are our preliminary reductions of the recent ob- servations from EVLA. A sharp drop during periastron passage is clearly observable. determination of single stars, the particle acceleration mechanism at the shocks and the binary frequency of early-type stars. 2. Observations: Cyg OB2 No. 9 Van Loo et al. (2008) discovered that the observed VLA radio fluxes of Cyg OB2 No. 9 at 3.6, 6, and 20 cm show recurrent variations with a period of ≈2.355 years. They accordingly suggested that the star might be a binary system. Binarity was established soon afterwards by the discovery of line splitting in the optical (Naz´e et al. 2008) and a first orbital solution of Cyg OB2 No. 9 was obtained by Naz´e et al. (2010), giving a period of 852.9 days. The 6 cm radio fluxes from Van Loo et al. (2008) are shown in Fig. 1 as black points. The phase-locked flux variability, which is a fingerprint of non- thermal emission in a binary, is clearly observable. The measurements cover a period of about 20 years but with only a few data points around periastron passage. In the framework of a large multi-wavelength campaign initiated by one of us (YN) with the goal to monitor the 2011 periastron passage of Cyg OB2 No. 9, the first observable since the discovery of binarity, we obtained new EVLA data. They are superimposed on the old ones as red points (the data reduction is only preliminary). The new data agree very well with the old ones and they show a very sharp drop just before periastron. 3. Modelling Cyg OB2 No. 9 A previous version of the emission code was developed and applied to the binary sys- tems Cyg OB2 No. 8A (Blomme et al. 2010) and No. 9 (Volpi 2011; Volpi et al. 2011). Modelling of non-thermal radio emitters 3 The code presents several approximations: the input hydrodynamical variables were determined analyticall

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