出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：Previously, a multi-wavelength (2-cm, 4-cm and 6-cm) polarization study by Gabuzda, Murray
and Cronin 2004, showed systematic Faraday rotation gradients across the parsec-scale jets of
several BL Lac objects, interpreted as evidence for helical magnetic fields — the gradients were
taken to be due to the systematic variation of the line-of-sight magnetic field component across
the jet. We present new results for the parsec-scale Faraday rotation distributions for a number of
BL Lac objects, based on VLBA polarization data obtained from three separate sets of observations
in August 2003, March 2004 and August 2004, at two wavelengths near each of the 2-cm,
4-cm and 6-cm bands. The rotation measure maps for several sources indicate gradients across
their jets, as expected if these jets have helical magnetic fields. In at least one source (1803+784),
we have found evidence for a change in the direction of the gradient across the jet with time, possibly
suggesting the development of a kink in the helical magnetic field, that changed the direction
of the helicity of the field.
