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Cyg OB2 #8a is a massive O-type binary displaying strong non-thermal radioemission. Owing to the compactness of this binary, emission of non-thermalX-ray photons via inverse Compton scattering is expected. We first revised theorbital solution for Cyg OB2 #8a using new optical spectra. We then reduced andanalysed X-ray spectra obtained with XMM-Newton, Swift, INTEGRAL, and NuSTAR.The analysis of the XMM-Newton and Swift data allows us to better characterisethe X-ray emission from the stellar winds and colliding winds region atenergies below 10 keV. We confirm the variation of the broad-band light curveof Cyg OB2 #8a along the orbit with, for the first time, the observation of themaximum emission around phase 0.8. The minimum ratio of the X-ray to bolometricflux of Cyg OB2 #8a remains well above the level expected for single O-typestars, indicating that the colliding wind region is not disrupted during theperiastron passage. The analysis of the full set of publicly available INTEGRALobservations allows us to refine the upper limit on the non-thermal X-ray fluxof the Cyg OB2 region between 20 and 200 keV. Two NuSTAR observations (phases0.028 and 0.085) allow us to study the Cyg OB2 #8a spectrum up to 30 keV. Thesedata do not provide evidence of the presence of non-thermal X-rays, but bringmore stringent constraints on the flux of a putative non-thermal component.Finally, we computed, thanks to a new dedicated model, the anisotropic inverseCompton emission generated in the wind shock region. The theoreticalnon-thermal emission appears to be compatible with observational limits and thekinetic luminosity computed from these models is in good agreement with theunabsorbed flux observed below 10 keV.

Search for non-thermal X-ray emission in the colliding wind binary Cyg OB2 #8A