出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：We present 18cm Very Long Baseline Array (VLBA) polarisation observations of the 4 BL Lac
objects 0735+178, 0954+658, 1803+784 and BL Lac at two epochs 412
years apart ( Jul 1999 and
Jan 2004). These results are part of an ongoing analysis of 18cm VLBA observations of the 34 1–
Jy BL Lac objects in the complete sample defined by Kühr & Schmidt[4]. The 2004 observations
were made at four distinct wavelengths between 18 and 22cm, allowing for the investigation of the
intrinsic Faraday rotation in each source. The 4 sources each display a spine-sheath polarisation
structure in the VLBI jet, consistent with the presence of helical magnetic (B) fields [2]. The
jet morphology, polarisation structure and degree of linear polarisation are compared for the two
epochs and shown to be very stable on the scales probed by these observations. Some rotation
measure (RM) maps from the 2004 data display transverse gradients in the inner jet, interpreted
as further evidence for the presence of helical B fields in the jet. The fractional polarisation maps
also display gradients increasing away from the core and towards the jet edges, again consistent
with helical jet B fields [5].
