  We report the results of 1D particle-in-cell simulations of ultrarelativistic
shock waves in proton-electron-positron plasmas. We consider magnetized shock
waves, in which the upstream medium carries a large scale magnetic field,
directed transverse to the flow. Relativistic cyclotron instability of each
species as the incoming particles encounter the increasing magnetic field
within the shock front provides the basic plasma heating mechanism. The most
significant new results come from simulations with mass ratio $m_p/m_\pm =
100$. We show that if the protons provide a sufficiently large fraction of the
upstream flow energy density (including particle kinetic energy and Poynting
flux), a substantial fraction of the shock heating goes into the formation of
suprathermal power-law spectra of pairs. Cyclotron absorption by the pairs of
the high harmonic ion cyclotron waves, emitted by the protons, provides the
non-thermal acceleration mechanism. As the proton fraction increases, the
non-thermal efficiency increases and the pairs' power-law spectra harden.
  We suggest that the varying power law spectra observed in synchrotron sources
powered by magnetized winds and jets might reflect the correlation of the
proton to pair content enforced by the underlying electrodynamics of these
sources' outflows, and that the observed correlation between the X-ray spectra
of rotation powered pulsars with the X-ray spectra of their nebulae might
reflect the same correlation.
