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

Collisionless shocks are ubiquitous in astrophysical plasmas, and are observed to be the production sites of very high energy particles (which then radiate over a wide range of the EM spectrum). A long-standing, unsolved problem in high energy astrophysics is how magnetic fields are generated in these shocks, and how they 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. Our aim is to examine shock development, magnetic field generation and particle acceleration in the case of mildly relativistic shocks, which are expected when the tidal ejecta of neutron star mergers drive a shock into the external medium.Using LANL's VPIC (vector particle-in-cell), code we have run simulations of such mildly-relativistic, collisionless, weakly magnetized 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 p⁺/e^- mass ratios in VPIC. Our results have implications for observing late-time EM counterparts to gravitational wave detections of neutron star mergers.