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The colliding-wind region in binary systems made of massive stars allows usto investigate various aspects of shock physics, including particleacceleration. Particle accelerators of this kind are tagged asParticle-Accelerating Colliding-Wind Binaries, and are mainly identified thanksto their synchrotron radio emission. Our objective is first to validate theidea that obtaining snapshot high-resolution radio images of massive binariesconstitutes a relevant approach to unambiguously identify particleaccelerators. Second, we intend to exploit these images to characterize thesynchrotron emission of two specific targets, HD167971 and HD168112, known asparticle accelerators. We traced the radio emission from the two targets at 1.6GHz with the European Very Long Baseline Interferometry Network, with anangular resolution of a few milli-arcseconds. Our measurements allowed us toobtain images for both targets. For HD167971, our observation occurs close toapastron, at an orbital phase where the synchrotron emission is minimum. ForHD168112, we resolved for the very first time the synchrotron emission region.The emission region appears slightly elongated, in agreement with expectationfor a colliding-wind region. In both cases the measured emission issignificantly stronger than the expected thermal emission from the stellarwinds, lending strong support for a non-thermal nature. Our study brings asignificant contribution to the still poorly addressed question of high angularresolution radio imaging of colliding-wind binaries. We show that snapshot VeryLong Baseline Interferometry measurements constitute an efficient approach toinvestigate these objects, with promising results in terms of identification ofadditional particle accelerators, on top of being promising as well to reveallong period binaries.

High resolution radio imaging of the two Particle-Accelerating Colliding-Wind Binaries HD167971 and HD168112