Circular and linear polarization variations of compact radio sources

Abstract

For 22 compact extragalactic sources, 5-GHz measurements of all four Stokes parameters were made at Parkes at quarterly intervals from 1976 December to 1982 March. Circular polarization significant at the 4σ level or greater was measured in 16 of these sources. The fractional variability of the circularly polarized flux density was found to be greater than that of the linearly polarized flux density, which in turn was greater than that of the total flux density. Attributing the observed large changes in circularly polarized flux density accompanied by only small changes in total flux density to the appearance of new components would require such a high degree of polarization for the new components as to argue against the circular polarization being the intrinsic polarization of synchrotron radiation. In an alternative model the circular polarization is the intrinsic polarization of the synchrotron emission, not of a new component but of an existing, non-uniform, partly optically-thick source in which some of the parameters vary. In another alternative model, the medium through which the radiation propagates converts a small fraction of the linear polarization to circular polarization, and it is assumed that the degree of conversion varies without the magnitude of the total emission being affected. There was little if any correlation between the fluctuations of the total intensity and the fluctuations of either the circularly or the linearly polarized components. For at least some of the sources the variations of linear polarization are consistent with a model comprising two components, approximately orthogonally polarized, for which the relative intensities of the two components vary with time.