Kinetic Simulations of Particle Acceleration in Relativistic Perpendicular Electron–Positron Shocks with Proton Admixture

Abstract Particle acceleration in relativistic shocks of electron–positron plasmas with proton admixture is investigated through two-dimensional particle-in-cell simulations. The upstream plasma, with a bulk Lorentz factor of 10 and a magnetization parameter of 0.02, includes a small fraction of protons (∼5% by number). A relativistic perpendicular shock is formed by reflecting the flow off a conducting wall. The shock structure, electromagnetic fields, and particle energy spectra are analyzed. The particle density and the magnetic field have fluctuations. In the far-downstream region of the shock, positrons are accelerated to energies comparable to protons and develop a hard nonthermal component with a spectral index of ∼2 in their energy spectrum, while electrons remain confined to lower energies. This asymmetry is attributed to the polarization properties of proton-driven electromagnetic waves, which favor positron acceleration. The results highlight the importance of plasma composition in shaping particle acceleration and nonthermal emission in relativistic shocks. These findings provide new insights into the microphysics of particle acceleration in astrophysical sources containing relativistic shocks.