# Using Circular Polarisation to Test the Composition and Dynamics of Astrophysical Particle Accelerators

## TL;DR

This paper explores how circular polarisation in X-ray and gamma-ray signals from cosmic accelerators can reveal the presence of high-energy protons, offering a new observational signature for astrophysical particle acceleration.

## Contribution

It demonstrates that charge asymmetries in meson and muon production lead to a small but detectable circular polarisation signal in high-energy emissions from cosmic sources.

## Key findings

- Circular polarisation fraction is about 5 x 10^{-4} for proton-dominated spectra.
- Detection of circular polarisation alongside neutrinos indicates high-energy protons.
- Circular polarisation signals can confirm proton acceleration in supernova shocks.

## Abstract

We investigate the production of circularly polarised X and gamma-ray signals in cosmic accelerators such as supernova remnants and AGN jets. Proton-proton and proton-photon collisions within these sites produce a charge asymmetry in the distribution of mesons and muons that eventually leads to a net circular polarisation signal as these particles decay radiatively. We find that the fraction of circular polarisation thus produced is at the level of $ 5 \times 10^{-4}$, regardless of the exact beam spectrum, as long as it is made predominantly of protons. While this fraction is very small, the detection of circular polarisation signals in conjunction with high-energy neutrinos would provide an unambiguous signature of the presence of high-energy protons in cosmic accelerators. In Supernovae shocks in particular, this would indicate the presence of relativistic protons hitting stationary protons and/or low-energy photons in the intergalactic or interstellar medium.