High-resolution radio imaging of the two particle-accelerating colliding-wind binaries HD167971 and HD168112

The colliding-wind region in binary systems made of massive stars allows us
to investigate various aspects of shock physics, including particle
acceleration. Particle accelerators of this kind are tagged as
Particle-Accelerating Colliding-Wind Binaries, and are mainly identified thanks
to their synchrotron radio emission. Our objective is first to validate the
idea that obtaining snapshot high-resolution radio images of massive binaries
constitutes a relevant approach to unambiguously identify particle
accelerators. Second, we intend to exploit these images to characterize the
synchrotron emission of two specific targets, HD167971 and HD168112, known as
particle accelerators. We traced the radio emission from the two targets at 1.6
GHz with the European Very Long Baseline Interferometry Network, with an
angular resolution of a few milli-arcseconds. Our measurements allowed us to
obtain images for both targets. For HD167971, our observation occurs close to
apastron, at an orbital phase where the synchrotron emission is minimum. For
HD168112, we resolved for the very first time the synchrotron emission region.
The emission region appears slightly elongated, in agreement with expectation
for a colliding-wind region. In both cases the measured emission is
significantly stronger than the expected thermal emission from the stellar
winds, lending strong support for a non-thermal nature. Our study brings a
significant contribution to the still poorly addressed question of high angular
resolution radio imaging of colliding-wind binaries. We show that snapshot Very
Long Baseline Interferometry measurements constitute an efficient approach to
investigate these objects, with promising results in terms of identification of
additional particle accelerators, on top of being promising as well to reveal
long period binaries.