Circular Polarization Variability in Extragalactic Sources on Time Scales of Months to Decades

Abstract

We present previously unpublished circular polarization (cp) measurementsat 4.8 and 8.0 GHz made with the University of Michigan 26-meter primefocus telescope during 1978–1984 and results of a newobservational program during the past two years. Based on the preliminary analysis of our recent data, eight sources were detected at 4.8 GHz with average degrees of cp ranging from ≤ 0.1% to ≥ 1%.The results are compared with observations at other frequencies and with linearpolarization and total flux density variability in the sources.The behavior of the cp variability observed to date is consistentwith stochastic variations produced by mode conversion in transient,opaque emitting regions in the sources. The observed sign changes observedbetween different epochs and different frequencies are not consistentwith the hypothesis that sources maintain a fixed handedness of cp.