On the Microphysics of Relativistic Collisionless Electron-ion-positron Shocks *

We present a set of first-principles simulations to elucidate the microscale physics of relativistic, pair-plasma-loaded weakly magnetized shocks. We show that even moderate changes in the plasma composition significantly impact the shock dynamics. When the bulk of the upstream momentum is carried by ions we find that (i) the strength of the mean magnetic field required for the transition from a Weibel to a Larmor mediated shock drops as a function of the pair-loading factor Z, (ii) the energy fraction transferred from ions to pairs is only weakly dependent on Z , and (iii) pair-loaded shocks are efficient particle accelerators only in the limit of vanishing external magnetization. The acceleration is enhanced through the formation of intense magnetic cavities that populate the precursor during late stages of shock evolution. Alongside simulations, we develop a set of theoretical estimates which yield predictions consistent with numerical results. Our findings have important implications for the modeling of the early afterglow emission of both short and long GRBs and place additional constraints on the maximum value of external magnetization that allows for particle acceleration.