Correlating Particle Acceleration Rates with Plasma Conditions in Colliding Wind Binaries

TL;DR

This study uses advanced simulations to explore how plasma conditions in colliding-wind binaries influence particle acceleration, highlighting turbulence and magnetic fields as key factors in producing high-energy cosmic rays.

Contribution

It demonstrates that turbulence and magnetic complexity, rather than classical shock acceleration, primarily drive particle acceleration in CWBs, with potential for PeV energies.

Findings

CWBs can accelerate particles up to PeV energies at moderate magnetization.

Turbulence and magnetic field complexity dominate acceleration processes.

Classical diffusive shock acceleration plays a limited role.

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 (CWBs) 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 CWBs. 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 ($\gt$1 G), CWBs can accelerate hadronic particles up to hundreds of TeV or even PeV energies, with more than 1\% of particles reaching the very-high-energy range. By correlating the local acceleration rate with plasma quantities - \textit{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. LACT and CTAO).