Heating and Non-thermal Particle Acceleration in Relativistic, Transverse Magnetosonic Shock Waves in Proton-Electron-Positron Plasmas

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 mp/m± = 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 e − e. 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 e − e power-law spectra harden. When the proton fraction is small (pair plasma almost charge symmetric), the e and e have approximately equal amounts of non-thermal 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’ non-thermal acceleration efficiency by energy is about 1%, increasing to 5% as the ions’ fraction of upstream flow energy increases to 72%. When the fraction of upstream flow energy in the ions rises to 84%, the efficiency of non-thermal acceleration of the pairs reaches 30%, with the e receiving most of the non-thermal power. 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