活动星系核喷流中磁塔与法拉第旋转矢量的连接
Connecting Magnetic Towers with Faraday Rotation Gradients in Active Galactic Nuclei Jets
摘要：
自20世纪90年代初以来，活动星系核喷流中系统的法拉第旋转梯度可以揭示螺旋磁场的存在。这些梯度被认为是由于系统的变化的视线-B-字段通过射流。本文根据甚长基线阵列（VLBA）在2cm、4cm和6cm波段（0716+714）附近的两个波长和18-22 cm波段（1749+701）的四个波长处获得的偏振数据，给出BL Lac天体0716+714和1749 + 701的秒差距尺度法拉第旋转分布。这两个源的旋转测量（RM）图表明，系统的梯度在其射流，如预期的那样，如果这些射流有螺旋B场。在所有情况下，这些横向RM梯度的显著性> 3 σ。我们提出的蒙特卡罗模拟的结果直接证明观察这样的横向RM梯度的可能性，即使本征射流结构比观察光束窄得多。我们观察到一个有趣的新功能，在这些来源，在射流的梯度方向的逆转相比，在核心区域的梯度。这提供了新的证据来支持这样的模型：从吸积盘中心区域出现的场线和在吸积盘外部区域闭合的场线都被吸积盘的差速旋转所“缠绕”。观察到的净RM梯度基本上是两个螺旋场区域的总和效应，一个嵌套在另一个内部。净RM梯度的方向将取决于内部或外部螺旋是否主导通过射流积分的RM，并且如果内部和外部螺旋场在射流的不同区域中占主导地位，则将观察到RM梯度反转。这可能提供关于喷流B场的整体配置的新见解。
英文摘要：
The idea that systematic Faraday Rotation gradients across the parsec-scale jets of AGNs can reveal the presence of helical magnetic (B) fields has been around since the early 1990s. These gradients are taken to be due to the systematic variation of the line of sight-B-field across the jet. We present here the parsec-scale Faraday Rotation distributions for the BL Lac objects 0716+714 and 1749+701, based on polarization data obtained with the Very Long Baseline Array (VLBA) at two wavelengths near each of the 2cm, 4cm and 6cm bands (0716+714) and at four wavelengths in the range 18-22 cm (1749+701). The Rotation Measure (RM) maps for both these sources indicate systematic gradients across their jets, as expected if these jets have helical B fields. The significance of these transverse RM gradients is > 3 sigma in all cases. We present the results of Monte Carlo simulations directly demonstrating the possibility of observing such transverse RM gradients even if the intrinsic jet structure is much narrower than the observing beam. We observe an intriguing new feature in these sources, a reversal in the direction of the gradient in the jet as compared to the gradient in the core region. This provides new evidence to support models in which field lines emerging from the central region of the accretion disk and closing in the outer region of the accretion disk are both "wound up" by the differential rotation of the disk. The net observed RM gradient will essentially be the sum effect of two regions of helical field, one nested inside the other. The direction of the net RM gradient will be determined by whether the inner or outer helix dominates the RM integrated through the jet, and RM gradient reversals will be observed if the inner and outer helical fields dominate in different regions of the jet. This potentially provides new insights about the overall configuration of the jet B fields.
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