We study the non-thermal radio emission of the binary Cyg OB2 No. 8A, to see if it is variable and if that variability is locked to the orbital phase. We investigate if the synchrotron emission generated in the colliding-wind region of this binary can explain the observations and we verify that our proposed model is compatible with the X-ray data. We use both new and archive radio data from the Very Large Array (VLA) to construct a light curve as a function of orbital phase. We also present new X-ray data that allow us to improve the X-ray light curve. We develop a numerical model for the colliding-wind region and the synchrotron emission it generates. The model also includes free-free absorption and emission due to the stellar winds of both stars. In this way we construct artificial radio light curves and compare them with the observed one. The observed radio fluxes show phase-locked variability. Our model can explain this variability because the synchrotron emitting region is not completely hidden by the free-free absorption. In order to obtain a better agreement for the phases of minimum and maximum flux we need to use stellar wind parameters for the binary components which are somewhat different from typical values for single stars. We verify that the change in stellar parameters does not influence the interpretation of the X-ray light curve. Our model has trouble explaining the observed radio spectral index. This could indicate the presence of clumping or porosity in the stellar wind, which - through its influence on both the Razin effect and the free-free absorption - can considerably influence the spectral index. Non-thermal radio emitters could therefore open a valuable pathway to investigate the difficult issue of clumping in stellar winds.
Modelling the radio emission from Cyg OB2 #5: a quadruple system?
利用甚大天线阵对天鹅座OB2#5超巨星双星进行了20年的50次射电观测，以确定先前探测到的射电辐射的位置和特征。该系统的射电辐射由一个与双星相关的主成分和一个非热源（NE）组成，该非热源被认为是双星的恒星风和NE的B型星（D星）的恒星风之间的风碰撞区（WCR）。NE显示没有变化的证据，在23个历元，它是从主要的无线电组件单独解决，表明可变的排放产生的主要。由于NE是不变的，现在可以很好地确定来自主要的无线电通量第一次，特别是在观测，不解决的主要和NE分量。主星的射电辐射变化周期为6.7+/-0.3年，这可以用一个简单的模型来描述，即在双星的恒星风包层内运行的非热源。这样的模型意味着存在第三颗未被发现的恒星伴星（恒星C），以6.7年的周期绕着6.6天的双星运行。可变的非热辐射来自恒星C和双星系统之间的WCR，或者可能直接来自恒星C。该模型给出天鹅座OB 2 #5的质量损失率为3.4 x 10^{-5}太阳质量/年，对于超巨星来说异常高，与WR星相当，并且与异常强的He I 1.083微米发射线一致，也散发着WR星的气味。对径向速度观测的检验表明，双星的反射运动是由于恒星C。NE和星D的性质也被检查。（节略）
