# Circular polarisation in compact radio sources: Constraints on particle acceleration and electron-positron pairs

## TL;DR

This paper investigates how circular polarisation in compact radio sources reveals details about plasma properties, electron energy distribution, and electron-positron pair fractions, especially around the synchrotron turn-over frequency.

## Contribution

It demonstrates that the frequency dependence of circular polarisation constrains plasma characteristics and electron-positron pair presence in synchrotron sources.

## Key findings

- Circular polarisation is mainly influenced by the region near the turn-over frequency.
- Nearly circular characteristic waves are favored by observations.
- Constraints on electron-positron pair fractions are derived from polarisation data.

## Abstract

It is shown that the frequency distribution of the degree of circular polarisation for a homogeneous source is sensitive to the properties of the synchrotron emitting plasma. Most of the circular polarisation comes from the region around the turn-over frequency, where the synchrotron radiation becomes optically thick. However, nearly circular characteristic waves result in circular polarisation dominated by frequencies above the turn-over frequency, while for nearly linear characteristic waves it is dominated by frequencies below. Observations argue in favour of nearly circular characteristic waves. This implies a low energy cut-off in the electron distribution substantially below that corresponding to the turn-over frequency and, simultaneously, provides an upper limit to the fraction of electron-positron pairs.