Parsec-Scale Circular Polarization Observations of 40 Blazars

We present circular polarization results from a 5 GHz survey of the parsec-scale polarization properties of 40 active galactic nuclei (AGNs) made with the VLBA. We find 11 circular polarization detections at the 3 σ level or higher. This nearly quadruples the number of sources detected in circular polarization at VLBI resolution. We find no correlation between fractional linear and circular polarization across our sample. A likely explanation is external Faraday depolarization in the cores of AGNs which reduces linear polarization but leaves circular polarization unchanged. In comparing ours and other recent results to observations made ~20 years ago, we find that, in five of six cases, sources have the same sign of circular polarization today as they did 20 years ago. This suggests the presence of a long-term property of the jets, such as the polarity of a net magnetic flux, which is stable on timescales much longer than those of individual outbursts.