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

Relativistic radiation-mediated shocks are likely formed in prodigious cosmic explosions. The structure and emission of such shocks are 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 non-linear 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 non-adiabatic compression via a Joule-like heating. The heating of the pairs and, conceivably, the formation of non-thermal distributions, arising from the microturbulence, can affect the observed shock-breakout signal in ways unaccounted for by current single-fluid models.

Automated summary

This paper investigates the hierarchy of plasma microinstabilities in relativistic radiation-mediated shocks, specifically focusing on the role of electron-positron pairs in regulating the structure and emission of the shocks. The study reveals that the presence of positrons inside the shock contributes to the growth of plasma instabilities, particularly the current filamentation instability driven by the relative drift between ions and pairs. Particle-in-cell simulations confirm the results obtained from linear stability analysis and provide insights into the non-linear regime of the instabilities. The analysis also suggests that pitch-angle scattering in the microturbulence can couple different species and lead to non-adiabatic compression via Joule-like heating. The heating and formation of non-thermal distributions resulting from microturbulence may impact the observed shock-breakout signal in ways not accounted for by current models.