The role of plasma instabilities in relativistic radiation mediated shocks: stability analysis and particle-in-cell simulations

TL;DR

This paper investigates plasma microinstabilities in relativistic radiation mediated shocks, revealing how pair-ion interactions and microturbulence influence shock structure and emission, with implications for cosmic explosion observations.

Contribution

It provides a combined linear stability analysis and particle-in-cell simulations to understand plasma instabilities and their effects in pair-loaded relativistic shocks.

Findings

Identifies dominant current filamentation instability driven by ion-pair drift.

Validates instability growth and nonlinear behavior through PIC simulations.

Derives a reduced transport equation showing microturbulence impacts shock heating and emission.

Abstract

Relativistic radiation mediated shocks (RRMS) likely form in prodigious cosmic explosions. The structure and emission of such shocks is regulated by copious production of electron-positron pairs inside the shock transition layer. It has been pointed out recently that substantial abundance of positrons inside the shock leads to a velocity separation of the different plasma constituents, which is expected to induce a rapid growth of plasma instabilities. In this paper, we study the hierarchy of plasma microinstabilities growing in an electron-ion plasma loaded with pairs and subject to a radiation force. Linear stability analysis indicates that such a system is unstable to the growth of various plasma modes which ultimately become dominated by a current filamentation instability driven by the relative drift between the ions and the pairs. These results are validated by particle-in-cell simulations that further probe the nonlinear regime of the instabilities, and the pair-ion coupling in the microturbulent electromagnetic field. Based on this analysis, we derive a reduced transport equation for the particles via pitch angle scattering in the microturbulence and demonstrate that it can couple the different species and lead to nonadiabatic compression via a Joule-like heating. The heating of the pairs and, conceivably, the formation of nonthermal distributions, arising from the microturbulence, can affect the observed shock breakout signal in ways unaccounted for by current single-fluid models.