We report the results of one-dimensional particle-in-cell simulations of magnetized ultrarelativistic shock waves in proton-electron-positron plasmas. Relativistic cyclotron instability, as the incoming particles encounter the increasing magnetic field within the shock front provides the basic plasma heating mechanism. The major new results come from simulations with mass ratio mp/m± = 100. When 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 e-e+. Cyclotron absorption by the pairs of the high harmonic ion cyclotron waves, emitted by the protons, provides the nonthermal acceleration mechanism. When the proton fraction is small (pair plasma almost charge-symmetric), the e- and e+ have approximately equal amounts of nonthermal heating. At the lower range of our simulations with mass ratio 100, when the ions contribute 56% of the upstream flow energy flux, the pairs' nonthermal acceleration efficiency by energy is about 1%, increasing to 5% as the ions' energy fraction increases to 72%. When the fraction of upstream flow energy in the ions rises to 84%, the efficiency of nonthermal acceleration of the pairs reaches 30%: the e+ receive most of the nonthermal power and the nonthermal spectra harden. We suggest that the varying power-law spectra observed in synchrotron sources that may be 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.
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