2019ApJ...873...55B

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