000002138 001__ 2138
000002138 005__ 20160706150153.0
000002138 0247_ $$2DOI$$a10.1051/0004-6361/201423833
000002138 037__ $$aASTROimport-295
000002138 100__ $$aParkin, E. R.
000002138 245__ $$aThe 2.35 year itch of Cygnus OB2 #9. III. X-ray and radio emission analysis based on 3D hydrodynamical modelling
000002138 260__ $$c2014
000002138 520__ $$aContext. 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 - 52G.
000002138 700__ $$aPittard, J. M.
000002138 700__ $$aNazé, Y.
000002138 700__ $$aBlomme, R.
000002138 773__ $$cA10$$pAstronomy and Astrophysics$$v570$$y2014
000002138 85642 $$ahttp://esoads.eso.org/abs/2014A%26A...570A..10P
000002138 905__ $$apublished in
000002138 980__ $$aREFERD