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

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.