# Towards understanding the physics of collisionless relativistic shocks

## TL;DR

This paper explores the physical mechanisms behind collisionless relativistic shocks, emphasizing the role of filamentation instabilities and presenting a Monte Carlo model for particle acceleration that extends beyond PIC simulation capabilities.

## Contribution

It provides a theoretical analysis of shock physics and introduces a non-linear Monte Carlo model for particle acceleration in relativistic shocks.

## Key findings

- Filamentation instabilities seed particle scattering and preheat the plasma.
- The Monte Carlo model captures particle back-reaction effects beyond PIC simulation limits.
- Insights into shock dissipation and particle acceleration mechanisms.

## Abstract

Relativistic astrophysical collisionless shocks represent outstanding dissipation agents of the huge power of relativistic outflows produced by accreting black holes, core collapsed supernovae and other objects into multi-messenger radiation (cosmic rays, neutrinos, electromagnetic radiation). This article provides a theoretical discussion of the fundamental physical ingredients of these extreme phenomena. In the context of weakly magnetized shocks, in particular, it is shown how the filamentation type instabilities, which develop in the precursor of pair dominated or electron-ion shocks, provide the seeds for the scattering of high energy particles as well as the agent which preheats and slows down the incoming precursor plasma. This analytical discussion is completed with a mesoscopic, non-linear model of particle acceleration in relativistic shocks based on Monte Carlo techniques. This Monte Carlo model uses a semi-phenomenological description of particle scattering which allows it to calculate the back-reaction of accelerated particles on the shock structure on length and momentum scales which are currently beyond the range of microscopic particle-in-cell (PIC) simulations.