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标题：Using Faraday rotation sign-reversals to study magnetic fields in AGN jets
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作者：S. O'Sullivan ; D. Gabuzda
期刊名称：PoS - Proceedings of Science
印刷版ISSN：1824-8039
出版年度：2006
出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：Most of detected variations in parsec-scale Faraday rotation in AGN have been associated with enhancement of the observed Faraday rotation measure (RM) in the region of the VLBI core, consistent with an increase in the density of free electrons with approach toward the centre of activity. Many of our multi-wavelength VLBA polarisation observations of 1 Jy BL Lac objects show this tendency, and also display higher RMs for the 7-mm/1.3-cm/2-cm cores than for the 2- cm/3.6-cm/6-cm cores, consistent with higher free electron densities being present on the smaller scales probed by the short-wavelength observations. However, in several sources, the signs of the core RMs determined from 7-mm to 2-cm and 2-cm to 6-cm data obtained at different epochs are different. This indicates that the orientation of the line-of-sight magnetic field associated with the Faraday rotation changes with either time or distance from the centre of activity. One possibility is that these RM sign reversals correspond to places where the helical magnetic field surrounding the jet undergoes a change in helicity due to a kink or reconnection. Another possibility is that a bend in the inner jet changes the side of the helical field with dominant polarisation, hence producing RMs with different signs in different lengths of the jet.