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.
