COMPARISON OF ALGORITHMS FOR DETERMINATION OF ROTATION MEASURE AND FARADAY STRUCTURE. I. 1100–1400 MHZ
COMPARISON OF ALGORITHMS FOR DETERMINATION OF ROTATION MEASURE AND FARADAY STRUCTURE. I. 1100–1400 MHZ
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DOI：
10.1088/0004-6256/149/2/60
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发表时间：
2014-09
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通讯作者：
Xiaohui Sun;L. Rudnick;T. Akahori;Craig S. Anderson;M. R. Bell;J. Bray;J. Farnes;S. Ideguchi;Kohei Kumazaki;T. O’Brien;S. O'Sullivan;A. Scaife;Rodion Stepanov;J. Stil;Keitaro Takahashi;R. V. Weeren;M. Wolleben
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文献类型：
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作者：
Xiaohui Sun;L. Rudnick;T. Akahori;Craig S. Anderson;M. R. Bell;J. Bray;J. Farnes;S. Ideguchi;Kohei Kumazaki;T. O’Brien;S. O'Sullivan;A. Scaife;Rodion Stepanov;J. Stil;Keitaro Takahashi;R. V. Weeren;M. Wolleben
法拉第旋转测度（RM）和更一般的法拉第结构是研究宇宙磁性的关键参数，也是微弱电离热气体的灵敏探针。需要定义各种科学研究所需的导出量，以及解决确定法拉第结构的挑战。已经提出了各种各样的算法来重建这些结构。为准备将与澳大利亚平方公里阵列探路者和正在进行的银河阿雷西博L波段馈源阵列连续体凌日观测一起进行的宇宙磁性偏振巡天观测（POSSUM）（GALFACTS），我们运行了一个法拉第结构测定数据挑战，以基准目前可用的算法，包括法拉第合成（以前在文献中称为RM合成）、小波、压缩采样和QU-fitting。输入模型包括具有一个法拉第薄组件、两个法拉第薄组件和一个法拉第厚组件的源。频率设置类似于POSSUM/GALFACTS，带宽为300 MHz，从1.1到1.4 GHz。我们定义了三个由基础科学激发的品质因数：（1）由偏振强度加权的平均RM，RM wtd？（2）分离度Δ φ？>的两个法拉第组件，和（3）减少卡方χ r 2？> .基于信噪比约为32的当前测试数据，我们发现以下内容。(1)当只有一个法拉第薄组件时，大多数方法都能达到预期效果，偶尔会出现两个组件被错误发现的故障。（2）对于两个法拉第薄组件，QU-fitting例程执行得最好，误差接近RM wtd？>但Δ φ的误差明显更高？> .所有其他方法，包括标准的法拉第合成法，当Δ φ？>低于或接近法拉第点扩散函数的宽度。(3)由于窄的带宽，目前实施的方法对于法拉第厚的部件没有很好的效果。(4)存在两个法拉第组件的组合，其产生大范围的可接受的拟合，因此在导出的单个RM中存在大的不确定性;在这些情况下，不同的RM导致相同的Q，U？>行为，因此没有方法可以恢复唯一的输入模型。在即将进行的调查能够提供关于法拉第结构的可靠结果之前，需要进一步探索所有这些问题。
Faraday rotation measures (RMs) and more general Faraday structures are key parameters for studying cosmic magnetism and are also sensitive probes of faint ionized thermal gas. A definition of which derived quantities are required for various scientific studies is needed, as well as addressing the challenges in determining Faraday structures. A wide variety of algorithms has been proposed to reconstruct these structures. In preparation for the Polarization Sky Survey of the Universe's Magnetism (POSSUM) to be conducted with the Australian Square Kilometre Array Pathfinder and the ongoing Galactic Arecibo L-band Feeds Array Continuum Transit Survey (GALFACTS), we run a Faraday structure determination data challenge to benchmark the currently available algorithms, including Faraday synthesis (previously called RM synthesis in the literature), wavelet, compressive sampling, and QU-fitting. The input models include sources with one Faraday thin component, two Faraday thin components, and one Faraday thick component. The frequency set is similar to POSSUM/GALFACTS with a 300 MHz bandwidth from 1.1 to 1.4 GHz. We define three figures of merit motivated by the underlying science: (1) an average RM weighted by polarized intensity, RM wtd ?> , (2) the separation Δ φ ?> of two Faraday components, and (3) the reduced chi-squared χ r 2 ?> . Based on the current test data with a signal-to-noise ratio of about 32, we find the following. (1) When only one Faraday thin component is present, most methods perform as expected, with occasional failures where two components are incorrectly found. (2) For two Faraday thin components, QU-fitting routines perform the best, with errors close to the theoretical ones for RM wtd ?> but with significantly higher errors for Δ φ ?> . All other methods, including standard Faraday synthesis, frequently identify only one component when Δ φ ?> is below or near the width of the Faraday point-spread function. (3) No methods as currently implemented work well for Faraday thick components due to the narrow bandwidth. (4) There exist combinations of two Faraday components that produce a large range of acceptable fits and hence large uncertainties in the derived single RMs; in these cases, different RMs lead to the same Q , U ?> behavior, so no method can recover a unique input model. Further exploration of all these issues is required before upcoming surveys will be able to provide reliable results on Faraday structures.