Kilonova Emissions -- Particle-In-Cell Simulations of Mildly Relativistic Outflows.

Collisionless shocks are ubiquitous in astrophysical plasmas, and are observed to be the sites of very high energy particles (which then radiate photons over a wide range of energies). ~A long-standing, unsolved problems in high energy astrophysics how magnetic fields are generated in these shocks, and how these fields relate to the process of particle acceleration. ~Particle-in-cell codes are ideally suited to address this question and previous work has looked at cases of magnetic field generation and particle acceleration in both highly relativistic and non-relativistic shocks.~~The aim of this project is to examine shock development, magnetic field generation and particle acceleration in the case of~\textit{mildly~}relativistic shocks, which are expected when the tidal ejecta of neutron star mergers shocks with the external medium. Using LANL's VPIC~(vector particle-in-cell), we have run simulations of such mildly-relativistic, collisionless, (initially unmagnetized) plasmas and compute the resultant magnetic fields and particle energy spectra.~ We show~the effects of varying plasma conditions, as well as explore the~validity of using different and often unrealistic proton~to electron mass ratios in VPIC.~ Our results have implications for observing late-time electromagnetic counterparts to gravitational wave detections of neutron star mergers.