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Paper: Circular Polarization and Helical B Fields in AGN
Volume: 386, Extragalactic Jets: Theory and Observation from Radio to Gamma Ray
Page: 444
Authors: Gabuzda, D.; Vitrishchak, V.M.; Mahmud, M.; O'Sullivan, S.
Abstract: Homan & Lister (2006) have recently published circular polarization (CP) detections for 34 objects in the MOJAVE sample – a set of bright, compact AGN being monitored by the Very Long Baseline Array at 15 GHz. We have also recently detected 15-GHz parsec-scale CP in two more AGN (3C345 and 2231+114). It is generally believed that the most likely mechanism for the generation of this CP is Faraday conversion of linear polarization to CP. A helical jet magnetic-field (B-field) geometry can facilitate this process – linearly polarized emission from the far side of the jet is converted to CP as it passes through the magnetized plasma at the front side of the jet on its way toward the observer. In this case, the sign of the generated CP is essentially determined by the pitch angle and helicity of the helical B field. We have determined the pitch-angle regimes and helicities of the helical jet B fields in 8 AGN for which parsec-scale CP has been detected, and used them to predict the expected CP signs for these AGN if the CP is generated via conversion in these helical fields: in all 8 cases, the observed CP signs agree with the expectations of our model for the case of a South poloidal jet B field, demonstrating that the observed CP in AGN is directly associated with the presence of helical jet B fields. These results suggest that helical B fields may be ubiquitous in AGN jets.
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