Discussion and conclusions

Partial conversion of linear into circular polarization is possible in synchrotron sources due to the relativistic electrons contributing an intrinsically linearly polarized component to the natural wave modes of the ambient medium. This effect may be characterized by the parameter RRM defined by (5).

The conditions required for the observed circular polarization of some synchrotron sources to be due to this effect are restrictive but not impossibly so. We discuss two situations in which the mechanism might operate. One situation is when cold plasma is absent, and then one requires either in a small portion of the source or over a large portion of the source. Estimates based on (6) or (8) suggest that this condition may be satisfied for plerions and for compact extragalactic sources; the magnetic field strengths required are of the same order of magnitude as those required to account for the circular polarization in terms of the intrinsic circular polarization of synchrotron radiation. The other situation, which has been discussed previously in this context by Pacholczyk (1973), is when there is a mixture of cold plasma and relativistic particles in the source. Provided that the relative phase difference across the source (which is dominated by the cold electrons in this case) is large, the emerging polarization should have a circular polarization similar to that of the natural modes, given roughly by (7).

It is highly desirable to model in detail all the different ways in which circular polarization might be produced in synchrotron sources in order to compare and contrast them. Although it is a small effect, the circular polarization of synchrotron sources is potentially significant because, whatever its cause, it depends on parameters different to those that determine the other observable properties of synchrotron radiation (e.g., Weiler & de Pater 1980).

© Copyright Astronomical Society of Australia 1997