Preferential Positron Acceleration in Relativistic Magnetized Electron-Positron-Ion Shocks

Relativistic shocks are considered efficient accelerators of charged particles and play crucial roles in high-energy astrophysical phenomena, such as gamma-ray bursts and pulsar winds. This study focuses on positron accelerations in magnetized relativistic shocks in electron-positron-ion plasma. Employing one-dimensional ab initio particle-in-cell simulations, we found a preferential positron acceleration through an interaction with the wakefield associated with a precursor wave in the upstream region. Test particle simulations revealed that the selective acceleration occurs for sufficiently large amplitudes of the wakefield. The mechanism can be understood as the relativistic $\boldsymbol{E}\times\boldsymbol{B}$ acceleration formulated in the upstream frame. A theoretical analysis of the positron acceleration in astrophysical contexts is presented, supporting ultra-relativistic shocks in pulsar winds as a primary source for the high-energy positron excess.