Circular polarisation and the magnetic dipole model

Abstract

THE magnetic dipole model for a compact extragalactic radio source proposed by Sanders^1,2 and discussed by Milgrom and Bahcall³ and by Sanders and DaCosta⁴, accounts for one of the features observed in a few of these sources—the apparently superluminal separation of two components (see ref. 5). However, the magnetic dipole model requires a strong, well-ordered magnetic field and predicts that the radio emission will have a substantial degree of circular polarisation unless the double source appears highly symmetric to the observer, or the radiation is by positrons and electrons in nearly equal numbers. For the past five years we have been monitoring extragalactic variable radio sources for circular polarisation at 8 GHz, and we have recently begun observations at 4.8 GHz. We present evidence here that the circular polarisation observations made by us and by other observers conflict with the predictions of the magnetic dipole model.