The 2.35 year itch of Cygnus OB2 #9

Abstract

Context. The wind-wind collision in a massive star binary system leads to the generation of high temperature shocks that emit at X-ray wavelengths and, if particle acceleration is effective, may exhibit non-thermal radio emission. Cyg OB2#9 is one of a small number of massive star binary systems in this class. Aims. X-ray and radio data recently acquired as part of a project to study Cyg OB2#9 are used to constrain physical models of the binary system, providing in-depth knowledge about the wind-wind collision and the thermal, and non-thermal, emission arising from the shocks. Methods. We use a 3D, adaptive mesh refinement simulation (including wind acceleration, radiative cooling, and the orbital motion of the stars) to model the gas dynamics of the wind-wind collision. The simulation output is used as the basis for radiative transfer calculations considering the thermal X-ray emission and the thermal/non-thermal radio emission. Results. The flow dynamics in the simulation show that wind acceleration (between the stars) is inhibited at all orbital phases by the opposing star's radiation field, reducing pre-shock velocities below terminal velocities. To obtain good agreement with the X-ray observations, our initial mass-loss rate estimates require a down-shift by a factor of ∼7.7 to 6.5 × 10 −7 M yr −1 and 7.5 × 10 −7 M yr −1 for the primary and secondary star, respectively. Furthermore, the low gas densities and high shock velocities in Cyg OB2 #9 are suggestive of unequal electron and ion temperatures, and the X-ray analysis indicates that an immediately post-shock electron-ion temperature ratio of 0.1 is also required. The radio emission is dominated by non-thermal synchrotron emission. A parameter space exploration provides evidence against models assuming equipartition between magnetic and relativistic energy densities. However, fits of comparable quality can be attained with models having stark contrasts in the ratio of magnetic-to-relativistic energy densities. Both X-ray and radio lightcurves are largely insensitive to viewing angle. The variations in X-ray emission with orbital phase can be traced back to an inverse relation with binary separation and pre-shock velocity. The radio emission also scales with pre-shock velocity and binary separation, but to positive powers (i.e. not inversely). The radio models also reveal a subtle effect whereby inverse Compton cooling leads to an increase in emissivity as a result of the synchrotron characteristic frequency being significantly reduced. Finally, using the results of the radio analysis, we estimate the surface magnetic field strengths to be ≈0.3−52 G.

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

“…5 of Stevens & Pollock 1994). In their 3D hydrodynamical simulations of Cyg OB2 #9, Parkin et al (2014) reported a drop of the pre-shock wind velocity by 18% at periastron. 9 Sgr and Cyg OB #9 have very similar eccentricities, but the orbital separation of 9 Sgr is twice as wide, and its components are slightly less luminous than those of Cyg OB2 #9.…”

“…Both effects should reduce the importance of radiative inhibition in 9 Sgr compared to Cyg OB2 #9. A detailed assessment of the importance of radiative inhibition requires 3D hydrodynamic simulations as performed by Parkin et al (2014), which is beyond the scope of the present work.…”

“…For 9 Sgr, Neiner et al (2015) reported upper limits on the longitudinal magnetic field of |B l | < 64 G, resulting in an upper limit on the dipolar field strength of B pol,max = 605 G. Either the magnetic field involved in the production of the synchrotron emission is stellar but sufficiently weak to remain below the detection threshold (e.g. Parkin et al 2014), or it is generated inside the wind-interaction zone. In either case, it is very challenging to estimate U B .…”

Search for magnetic fields in particle-accelerating colliding-wind binaries

“…In their work, the magnetic field strength scales with ζ 1/2 B and V rot . The assumptions on the field configuration and slow rotation used by Parkin et al (2014) are probably generally not adapted to PACWB. In particular, if the field is strong, the impact of the magnetic field on the wind, for example, magnetic wind confinement, should be taken into account (ud-Doula & Owocki 2002;ud-Doula et al 2008), and massive stars are often rapid rotators (e.g.…”

“…Extrapolating this to the surface of the stars with typical distances between the stagnation point and the photosphere, we obtain values of the stellar magnetic field strength between one G and a few thousand G, depending on the system and on the assumptions (e.g. Parkin et al 2014). Magnetic fields detected in single massive stars have a polar field strength between one hundred and several thousand G (see e.g.…”