文章基本信息

标题：Variability on Decaparsec scales of a number of BL Lac objects
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作者：R. Hallahan ; D. Gabuzda
期刊名称：PoS - Proceedings of Science
印刷版ISSN：1824-8039
出版年度：2008
卷号：2008
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].