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Particle-Accelerating Colliding-Wind Binaries (PACWB)
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Binary, or higher multiplicity systems made of massive stars constitute highly adequate environments to investigate shock physics. The stellar winds collide and generate a wind interaction region, whose main components are two strong shocks. The subset of colliding-wind binaries where observational evidence for the existence of relativistic particles has been provided is referred to Particle-Accelerating Colliding-Wind Binaries. The particle acceleration mechanism is most likely Diffusive Shock Acceleration (DSA). The presence of high energy particles is revealed mainly thanks to the identification of synchrotron radio emission, even though a few cases display also non-thermal high energy emission (inverse Compton scattering, neutral pion decay). The current census of PACWB is compiled in a regularly updated online Catalogue (based on the original version published by De Becker & Raucq 2013).
Left : schematic view of a colliding-wind binary including shocked gas between two hydrodynamic shocks. Right : radio map of the colliding wind region in the Apep system (Marcote et al. 2021).
The identification of the synchrotron radio emission relies on detailed radio observations (multi-band, high resolution,...). Considering the thermal nature of the radio emission from the stellar winds, the investigation of low frequencies is more adequate to optimize the measurement of the non-thermal component, although recent result allow to measure more clearly the strong impact of free-free absorption that prevents the detection of the synchrotron radiation. The sychrotron radio emission and the inverse Compton high energy radiation reveal the leptonic part of the non-thermal processes. The hadronic part can only be revealed by gamma-ray measurements.