1999AJ....118.1942H

We present five epochs of 15 GHz VLBA observations of 13 AGNs. These observations were specially calibrated to detect parsec-scale circular polarization and our calibration techniques are discussed and analyzed in detail. We obtained reliable detections of parsec-scale circular polarization in the radio jets of four AGNs: 3C 84, PKS 0528+134, 3C 273, and 3C 279. For each of these objects our detections are at the level of approximately 0.3%-1% local fractional circular polarization. Each individual detection has a significance in the range of 3 to 10 σ. Our observations are consistent across multiple epochs (and different calibration techniques) in the sign and magnitude of the circular polarization observed. 3C 273 and 3C 279 both undergo core outbursts during our observations and changes in the circular polarization of both sources are correlated with these outbursts. In general, we observe the circular polarization to be nearly coincident with the strong VLBI cores of these objects; however, in 3C 84 the circular polarization is located a full milliarcsecond south of the source peak, and in the 1996.73 epoch of 3C 273 the circular polarization is predominately associated with the newly emerging jet component. Our observations support the theoretical conclusion that emission of circular polarization is a sensitive function of opacity, being strongest when the optical depth is near unity. Circular polarization may be produced as an intrinsic component of synchrotron radiation or by the Faraday conversion of linear to circular polarization. Our single-frequency observations do not easily distinguish between these possible mechanisms, but independent of mechanism, the remarkable consistency across epoch of the sign of the observed circular polarization suggests the existence of a long-term, stable, unidirectional magnetic field. Single-dish observations of 3C 273 and 3C 279 at 8 GHz by Hodge and Aller suggest that this stability may persist for decades in our frame of observation.