利用JVLA对天鹅座A的宽带偏振研究。I：观测与数据
A Wideband Polarization Study of Cygnus A with the JVLA. I: The Observations and Data
AI总结：
本文利用JVLA对天鹅座A进行宽带偏振观测，通过高分辨率偏振成像和模型拟合，揭示了低频退偏振源于星系团磁场中约300–700 pc尺度的未分辨涨落，并验证了模型预测与观测的高度一致性。
摘要：
我们展示了利用Jansky甚大阵（JVLA）对邻近明亮射电星系天鹅座A进行的深度、宽带、高空间和高光谱分辨率观测的结果。该源的高表面亮度使得详细的偏振成像成为可能，提供了0.75角秒分辨率、覆盖2–18 GHz的图像，以及0.30角秒分辨率（6–18 GHz）的图像。来自整个射电瓣中2000条独立视线的线偏振比例随频率降低而强烈下降，其中东瓣比西瓣在更高频率处发生退偏振。退偏振表现出相当明显的结构，从单调变化到强烈振荡不等。在给定频率下，线偏振比例通常随分辨率提高而增加，这与预期一致。然而，存在大量具有更复杂行为的视线。我们对0.3角秒图像拟合了一个简单模型，该模型包含星系团磁场中随机的、未分辨的涨落，以确定该源和星系团在高分辨率、高频下的性质。基于这些推导出的性质，我们生成了该源在较低频率下的预测偏振图像，并将其卷积到0.75角秒。这些预测与观测到的辐射高度一致。观测结果与以下解释一致：较低频率的退偏振是由尺度约大于等于300–700 pc的磁场和/或电子密度中未分辨的涨落叠加在部分有序的场分量上造成的。我们的数据中没有迹象表明退偏振屏或大尺度场的位置，二者之一或两者可能位于整个星系团中，或位于射电瓣与星系团之间的边界区域。
英文摘要：
We present results from deep, wideband, high spatial and spectral resolution observations of the nearby luminous radio galaxy Cygnus A with the Jansky Very Large Array. The high surface brightness of this source enables detailed polarimetric imaging, providing images at $0.75''$, spanning 2 - 18 GHz, and at 0.30$''$ (6 - 18 GHz). The fractional polarization from 2000 independent lines of sight across the lobes decreases strongly with decreasing frequency, with the eastern lobe depolarizing at higher frequencies than the western lobe. The depolarization shows considerable structure, varying from monotonic to strongly oscillatory. The fractional polarization in general increases with increasing resolution at a given frequency, as expected. However, there are numerous lines of sight with more complicated behavior. We have fitted the $0.3''$ images with a simple model incorporating random, unresolved fluctuations in the cluster magnetic field to determine the high resolution, high-frequency properties of the source and the cluster. From these derived properties, we generate predicted polarization images of the source at lower frequencies, convolved to 0.75$\arcsec$. These predictions are remarkably consistent with the observed emission. The observations are consistent with the lower-frequency depolarization being due to unresolved fluctuations on scales $\gtrsim$ 300 - 700 pc in the magnetic field and/or electron density superposed on a partially ordered field component. There is no indication in our data of the location of the depolarizing screen or the large-scale field, either, or both of which could be located throughout the cluster, or in a boundary region between the lobes and the cluster.
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