Circular polarization in compact extragalactic radio sources

Abstract

Numerical solutions to the transfer equation for an inhomogeneous, optically thick synchrotron source are presented and discussed in the context of compact, extragalactic radio sources. It is shown that the inhomogeneity affects most seriously the circular polarization; for example, assuming a homogeneous plasma when calculating the effects on the circular polarization due to radiative transfer gives results which differ substantially from those for a more realistic, inhomogeneous source structure. Simple analytical expressions are given to estimate the effects of inhomogeneities. Based on the observed properties of compact, extragalactic radio sources, it is argued that in these sources (i) transfer effects are important to the emergent degree of circular polarization, (ii) the magnetic field has a large-scale structure, (iii) the electron distribution is peaked around the energy corresponding to the self-absorption turnover and (iv) the peaked electron distribution is produced directly by the acceleration process. The link between compact extragalactic radio sources and blazars is briefly discussed. It is emphasized that they are likely to be two different aspects of the same phenomenon. The similarities between the inferred properties of the two emission regions indicate a close connection; it is suggested that, in some objects or at some occasions, the two emission regions do in fact coincide.