用法拉第旋转矢量探测磁塔模型
Using Faraday Rotation Gradients to probe Magnetic Tower Models
摘要：
基于法拉第旋转（FR）梯度，利用秒差距尺度的多波长VLBA偏振观测可以研究活动星系核（AGN）的磁场结构。已经发现了许多横向FR梯度，并将其解释为对应于缠绕在喷流周围的螺旋磁场;梯度反映了穿过喷流的环形或螺旋磁场的视线分量的系统变化（例如，Gabuzda，Murray & Cronin 2004）。我们对BL Lac天体样本在2、4和6厘米附近的六个波长的观测也揭示了一个以前未被发现的现象：这些横向梯度有时会随着与核心的距离而改变方向。我们已经观察到这种行为，在至少五个源，显示在其VLBI核心区域的梯度相反的射流。我们认为这可能与磁塔模型有关。在磁塔模型中，磁力线随着喷流向外运动，然后返回并在吸积盘中闭合（反之亦然）;吸积盘的差速旋转将内外磁力线缠绕成两个螺旋（内螺旋“嵌套”在外螺旋中）。观察到的总FR梯度是这两个螺旋场的效果的总和。可能的情况是，在离核心相对较远的地方检测到的梯度对应于外螺旋，而在核心区域中检测到的梯度对应于内螺旋的优势。这为螺旋磁场和磁塔模型的统一提供了初步证据，为认识活动星系核喷流提供了重要的新信息。进一步的VLBI研究，分辨率足以可靠地检测这些梯度在厘米波长的核心和内部喷流将是重要的进一步调查这一现象。
英文摘要：
Parsec-scale multi-wavelength VLBA polarization observations can be used to study the magnetic-field structures of Active Galactic Nuclei (AGN) based on Faraday Rotation (FR) gradients. A number of transverse FR gradients have been found, and interpreted as corresponding to helical magnetic fields wrapped around the jets; the gradients reflect the systematic change in the line-of-sight component of a toroidal or helical magnetic field across the jet (e.g Gabuzda, Murray & Cronin 2004). Our observations of a sample of BL Lac objects at six wavelengths near 2, 4 and 6 cm have also revealed a previously undetected phenomena: these transverse gradients sometimes change their direction with distance from the core. We have observed this behaviour in at least five sources, which display gradients in their VLBI core region opposite to those in the jet. We suggest that this may be linked to magnetic tower models. In magnetic tower models, the field lines go outward with the jet and return and close in the accretion disk (or vice versa); differential rotation of the accretion disk winds up the inner and outer field lines into two helices (the inner helix "nested" in the outer helix). The total observed FR gradient is a sum of the effect of these two helical fields. It may be that gradients detected relatively far from the core correspond to the outer helix, while gradients detected in the core region correspond to dominance of the inner helix. This provides tentative evidence for the unification of helical magnetic fields and magnetic tower models, which could provide crucial new information for understanding AGN jets. Further VLBI studies with resolution sufficient to reliably detect these gradients in the cm-wavelength core and inner jet will be important for further investigations of this phenomena.
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