Surprising evolution of the parsec-scale Faraday Rotation gradients in the jet of the BL Lac object B1803+784

Abstract

Several multi-frequency polarization studies have shown the presence of\nsystematic Faraday Rotation gradients across the parsec-scale jets of Active\nGalactic Nuclei (AGN), taken to be due to the systematic variation of the\nline-of-sight component of a helical magnetic (B) field across the jet. Other\nstudies have confirmed the presence and sense of these gradients in several\nsources, thus providing evidence that these gradients persist over time and\nover large distances from the core. However, we find surprising new evidence\nfor a reversal in the direction of the Faraday Rotation gradient across the jet\nof B1803+784, for which multi-frequency polarization observations are available\nat four epochs. At our three epochs and the epoch of Zavala & Taylor (2003), we\nobserve transverse Rotation Measure (RM) gradients across the jet, consistent\nwith the presence of a helical magnetic field wrapped around the jet. However,\nwe also observe a "flip" in the direction of the gradient between June 2000 and\nAugust 2002. Although the origins of this phenomena are not entirely clear,\npossibly explanations include (i) the sense of rotation of the central\nsupermassive black hole and accretion disc has remained the same, but the\ndominant magnetic pole facing the Earth has changed from North to South; (ii) a\nchange in the direction of the azimuthal B field component as a result of\ntorsional oscillations of the jet; and (iii) a change in the relative\ncontributions to the observed rotation measures of the "inner" and "outer"\nhelical fields in a magnetic-tower model. Although we cannot entirely rule out\nthe possibility that the observed changes in the RM distribution are associated\ninstead with changes in the thermal-electron distribution in the vicinity of\nthe jet, we argue that this explanation is unlikely.\n