法拉第层析成像的迭代重建算法
An Iterative Reconstruction Algorithm for Faraday Tomography
摘要：
法拉第层析成像技术通过观测类星体、星系或星系团等磁化天体的磁离子介质，提供了重要的信息。所观察到的线偏振光谱进行逆傅立叶变换，以获得法拉第色散函数（FDF），为我们提供了沿视线的磁离子介质的层析分布。然而，由于仪器的有限波长覆盖，这种变换给出了差的FDF重建。目前的法拉第层析成像技术无法可靠地解决上述逆问题，这已经明显困扰着宇宙磁学研究。我们提出了一种新的算法的启发，信号恢复的研究领域，称为约束和恢复迭代算法法拉第层析成像（CRAFT）。这种迭代的模型无关的算法是计算成本低，只需要弱的物理动机的假设，以产生高保真FDF重建。我们展示了一个现实的合成模型FDF的银河系，其中手工艺显示出更大的潜力，比其他流行的模型独立的技术的应用。对于一个更简单的FDF，也证明了观测频率覆盖对各种技术的重建性能的依赖性。CRAFT甚至比依赖于模型的技术（即，QU-fitting）通过捕获源内FDF幅度和偏振角变化的复杂多尺度特征。所提出的方法将是最重要的未来宇宙磁学研究，特别是与宽带偏振数据从平方公里阵列及其前兆。我们公开CRAFT代码。
英文摘要：
Faraday tomography offers crucial information on the magnetized astronomical objects, such as quasars, galaxies, or galaxy clusters, by observing its magnetoionic media. The observed linear polarization spectrum is inverse Fourier transformed to obtain the Faraday dispersion function (FDF), providing us a tomographic distribution of the magnetoionic media along the line of sight. However, this transform gives a poor reconstruction of the FDF because of the instrument's limited wavelength coverage. The current Faraday tomography techniques' inability to reliably solve the above inverse problem has noticeably plagued cosmic magnetism studies. We propose a new algorithm inspired by the well-studied area of signal restoration, called the Constraining and Restoring iterative Algorithm for Faraday Tomography (CRAFT). This iterative model-independent algorithm is computationally inexpensive and only requires weak physically-motivated assumptions to produce high fidelity FDF reconstructions. We demonstrate an application for a realistic synthetic model FDF of the Milky Way, where CRAFT shows greater potential over other popular model-independent techniques. The dependence of observational frequency coverage on the various techniques' reconstruction performance is also demonstrated for a simpler FDF. CRAFT exhibits improvements even over model-dependent techniques (i.e., QU-fitting) by capturing complex multi-scale features of the FDF amplitude and polarization angle variations within a source. The proposed approach will be of utmost importance for future cosmic magnetism studies, especially with broadband polarization data from the Square Kilometre Array and its precursors. We make the CRAFT code publicly available.
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