Testing the theory of colliding winds: the periastron passage of 9 Sagittarii

Abstract

Context. The long-period, highly eccentric O-star binary 9 Sgr, known for its non-thermal radio emission and its relatively bright X-ray emission, went through its periastron in 2013. Aims. Such an event can be used to observationally test the predictions of the theory of colliding stellar winds over a broad range of wavelengths. Methods. We conducted a multi-wavelength monitoring campaign of 9 Sgr around the 2013 periastron. In this paper, we focus on X-ray observations and optical spectroscopy. Results. The optical spectra allow us to revisit the orbital solution of 9 Sgr and to refine its orbital period to 9.1 years. The X-ray flux is maximum at periastron over all energy bands, but with clear differences as a function of energy. The largest variations are observed at energies above 2 keV, whilst the spectrum in the soft band (0.5−1.0 keV) remains mostly unchanged, indicating that it arises far from the collision region, in the inner winds of the individual components. The level of the hard emission at periastron clearly deviates from the 1/r relation expected for an adiabatic wind-interaction zone, whilst this relation seems to hold at the other phases that are covered by our observations. The spectra taken at phase 0.946 reveal a clear Fe xxv line at 6.7 keV, but no such line is detected at periastron (φ = 0.000), although a simple model predicts a strong line that should be easily visible in the data. Conclusions. The peculiarities of the X-ray spectrum of 9 Sgr could reflect the effect of radiative inhibition as well as a phasedependent efficiency of particle acceleration on the shock properties.

The long-period massive binary HD 54662 revisited

“…The Xray emission of such an adiabatic interaction region should vary as the inverse of the distance d between the stars (Stevens et al 1992). Whilst this picture is consistent with the observations of Cyg OB2 #9 (Nazé et al 2012), it fails in the case of 9 Sgr (Rauw et al 2016a). However, the latter O + O binaries are also nonthermal radio emitters, which implies the presence of a population of relativistic electrons that could modify the properties of the wind interaction (Pittard & Dougherty 2006), thereby causing deviations from the theoretical 1/d relation.…”

“…FRS-FNRS Postdoctoral Researcher vational bias: none of the most commonly used techniques are very sensitive in this regime and only long-term spectroscopic monitoring has been able so far to partly fill the gap (see, e.g., the cases of Cyg OB2 #9 and 9 Sgr, Nazé et al 2012;Rauw et al 2016a).…”

Resolved astrometric orbits of ten O-type binaries

“…Our combined fit confirms that the resolved pair corresponds to the long-period SB2 presented in Rauw et al (2012Rauw et al ( , 2016. We had to introduce a constant RV shift of 10.0 km s −1 between the two components in order to achieve a correct fit of the SB2 radial velocities.…”

“…Thus, in the long controversy about the distance to NGC 6530, and assuming the total mass from spectroscopy is correct, our observations confirm the distance from Prisinzano et al (2005) and Kharchenko et al (2005). This is significantly lower than the 1.78 ± 0.08 kpc assumed by Rauw et al (2016) and based on Sung et al (2000).…”

“…The deduced individual masses and the distance are even smaller. We note that, fitting the same RV data, Rauw et al (2016) obtain similar radial velocity amplitudes. They conclude that the masses were compatible with expectation, but assuming an inclination of 45 ± 1 deg.…”