Non-thermal radio emission from O-type stars - IV. Cygnus OB2 No. 8A

摘要

Context. Several early-type colliding-wind binaries areknown to emit synchrotron radiation due to relativistic electrons,which are most probably accelerated by the Fermi mechanism. By studyingsuch systems we can learn more about this mechanism, which is alsorelevant in other astrophysical contexts. Colliding-wind binaries arefurthermore important for binary frequency determination in clustersand for understanding clumping and porosity in stellar winds. Aims. We study the non-thermal radio emission of the binaryCygOB2 No.8A, to see if it is variable and if thatvariability is locked to the orbital phase. We investigate if thesynchrotron emission generated in the colliding-wind region of thisbinary can explain the observations and we verify that our proposedmodel is compatible with the X-ray data. Methods. We use both new and archive radio data from the VeryLarge Array (VLA) to construct a light curve as a function of orbitalphase. We also present new X-ray data that allow us to improve theX-ray light curve. We develop a numerical model for the colliding-windregion and the synchrotron emission it generates. The model alsoincludes free-free absorption and emission due to the stellar winds ofboth stars. In this way we construct artificial radio light curves andcompare them with the observed one. Results. The observed radio fluxes show phase-lockedvariability. Our model can explain this variability because thesynchrotron emitting region is not completely hidden by the free-freeabsorption. In order to obtain a better agreement for the phases ofminimum and maximum flux we need to use stellar wind parameters for thebinary components which are somewhat different from typical values forsingle stars. We verify that the change in stellar parameters does notinfluence the interpretation of the X-ray light curve. Our model hastrouble explaining the observed radio spectral index. This couldindicate the presence of clumping or porosity in the stellar wind,which - through its influence on both the Razin effect and thefree-free absorption - can considerably influence the spectral index.Non-thermal radio emitters could therefore open a valuable pathway toinvestigate the difficult issue of clumping in stellar winds. Key words: stars: individual: Cyg OB2 No. 8A - stars: early-type - stars: mass-loss - radiation mechanisms: non-thermal - acceleration of particles - radio continuum: stars