  Relativistic collisionless shocks in electron-ion plasma are thought to occur
in the afterglow phase of Gamma-Ray Bursts (GRBs), and in other environments
where relativistic flows interact with the interstellar medium. A particular
regime of shocks in an unmagnetized plasma has generated much interest for GRB
applications. In this paper we present ab-initio particle-in-cell simulations
of unmagnetized relativistic electron-ion shocks. Using long-term
2.5-dimensional simulations with ion-electron mass ratios from 16 to 1000 we
resolve the shock formation and reach a steady-state shock structure beyond the
initial transient. We find that even at high ion-electron mass ratios initially
unmagnetized shocks can be effectively mediated by the ion Weibel instability
with a typical shock thickness of ~50 ion skin-depths. Upstream of the shock
the interaction with merging ion current filaments heats the electron
component, so that the postshock flow achieves near equipartition between the
ions and electrons, with electron temperature reaching 50% of the ion
temperature. This energy exchange helps to explain the large electron energy
fraction inferred from GRB afterglow observations.
