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

We investigate the production of circularly polarized X and gamma-ray signals in cosmic accelerators such as supernova remnants and active galactic nuclei 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 polarization signal as these particles decay radiatively. We find that the fraction of circular polarization thus produced is at the level of $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, 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 polarization 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.