Surprising Evolution of Faraday Rotation Gradients in the Jet of 1803+784

Abstract

Several multi-frequency polarization studies have shown the presence of systematic Faraday Rotation gradients across the parsec-scale jets of Active Galactic Nuclei (AGN), taken to be due to the systematic variation of the line-of-sight component of a helical magnetic field across the jet. Other studies have confirmed the presence and sense of these gradients in several sources, thus providing evidence that these gradients persist over time and over large distances from the core. However, we find surprising new evidence for a reversal in the direction of the Faraday Rotation gradient across the jet of 1803+784, for which multi-wavelength polarization observations are available at four epochs. At all four epochs, we observe transverse Rotation Measure (RM) gradients across the jet, consistent with the presence of a helical magnetic field wrapped around the jet. However, we also observe a ''flip'' in the direction of the gradient between June 2000 and August 2002. Although the origins of this phenomena are not understood, one way to interpret this change is if the sense of rotation of the central supermassive black hole and accretion disc has remained the same, but the dominant magnetic pole facing the Earth has changed from North to South.