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 (B) 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 B1803+784, for which multi-frequency polarization observations are available at four epochs. At our three epochs and the epoch of Zavala & Taylor (2003), 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 entirely clear, possibly explanations include (i) 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; (ii) a change in the direction of the azimuthal B field component as a result of torsional oscillations of the jet; and (iii) a change in the relative contributions to the observed rotation measures of the "inner" and "outer" helical fields in a magnetic-tower model. Although we cannot entirely rule out the possibility that the observed changes in the RM distribution are associated instead with changes in the thermal-electron distribution in the vicinity of the jet, we argue that this explanation is unlikely.
Discovery of A Variable Broad Absorption Line in the BL Lac object PKS 0138-097
我们报告在先前已知的BL Lac天体PKS 0138-097中发现了宽为\sim 10^4 km s-1的宽吸收线（BAL），我们暂时将其确定为Mg II BAL。这是第一次检测到BAL，这是有时看到的强大的类星体具有高吸积率，在BL Lac对象。在斯隆数字巡天（SDSS）拍摄的高光度状态下的两个时期的光谱中，BAL被清楚地检测到，而在低光度状态下拍摄的三个SDSS光谱中，它消失了。BAL及其变率模式在文献中也被发现在其历史多时期谱中，但以前被忽略了。在其高分辨率射电图中，PKS 0138-097显示了一个核心和一个单侧的秒差距尺度喷流。BAL的变化可以解释如下：光学发射在高状态下由核心主导，在低状态下由射流主导，并且BAL材料位于核心和射流之间，使得BAL仅在核心发光时出现。我们的发现表明，外流也可能产生在活动星系核在一个低吸积状态。
