Correlating particle acceleration rates with plasma conditions in colliding wind binaries

ABSTRACT Recent observations have revealed star-forming regions as possible origin sites of very high-energy (TeV) cosmic rays, not associated with supernova remnants. Colliding-wind binaries are strong X-ray and radio synchrotron emitters and have been proposed as potential accelerators of such particles. We perform high-resolution three-dimensional magnetohydrodynamic simulations coupled with test-particle integration to investigate how local plasma conditions affect particle acceleration in wind–wind collisions. We find that the maximum particle energies and the hardness of the energy distributions depend on the shock magnetization and cooling efficiency. For moderate magnetization (>1 G), CWBs can accelerate hadronic particles up to hundreds of TeV or even PeV energies, with more than 1 per cent of particles reaching the >TeV energy range. By correlating the local acceleration rate with plasma quantities – magnetic field strength, current density, vorticity, and velocity divergence – we show that turbulence and magnetic field complexity dominate the acceleration, while classical diffusive shock acceleration plays a limited role. These results suggest that turbulent, magnetically driven processes are key to producing relativistic particles in CWBs, with implications for future high sensitivity $\gamma$-ray observations (e.g. Large Array of Imaging Atmospheric Cherenkov Telescopes (LACT) and Cherenkov Telescope Array Observatory).