Optimum frequency band for radio polarization observations
Optimum frequency band for radio polarization observations
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DOI：
10.1111/j.1365-2966.2011.19623.x
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发表时间：
2011-01
影响因子：
4.8
通讯作者：
T. Arshakian;R. Beck
中科院分区：
文献类型：
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作者：
T. Arshakian;R. Beck
来自星际、星系团内和星系际磁场的偏振射电同步辐射受频率相关的法拉第退偏振的影响。最大偏振强度取决于退偏振介质的物理性质。新一代射电望远镜，如低频阵列（LOFAR），平方公里阵列（SKA）及其前身需要广泛的频率来覆盖所有的物体。假设磁离子介质的法拉第谱由一个分量或介质是湍流的，计算了磁离子介质在规则磁场（微分法拉第旋转）或湍流磁场（内部或外部法拉第色散）作用下去极化的最大极化强度（PI）的最佳频率。明亮的星系盘、旋臂和星系团核心的偏振发射在几厘米以下的波长（频率超过约10千兆赫）、星系晕和星系团在分米波长左右（频率低于约2千兆赫）时观测得最好。星系际细丝需要在米波长（频率低于300兆赫）上进行观测。具有极大内禀旋转测度的源|RM|或RM色散可以用毫米波望远镜来搜索。PI光谱的测量允许我们导出平均法拉第|RM|或源内的法拉第色散，如螺旋星系NGC 6946的情况所示。PI在低频的周期性波动是微分法拉第旋转的特征。内部和外部的法拉第色散可以区分由不同的斜率的PI频谱在低频。最佳频率附近的宽频带对于区分各种去极化效应是重要的。
Polarized radio synchrotron emission from interstellar, intracluster and intergalactic magnetic fields is affected by frequency-dependent Faraday depolarization. The maximum polarized intensity depends on the physical properties of the depolarizing medium. New-generation radio telescopes such as Low Frequency Array (LOFAR), the Square Kilometre Array (SKA) and its precursors need a wide range of frequencies to cover the full range of objects. The optimum frequency of maximum polarized intensity (PI) is computed for the cases of depolarization in magneto-ionic media by regular magnetic fields (differential Faraday rotation) or by turbulent magnetic fields (internal or external Faraday dispersion), assuming that the Faraday spectrum of the medium is dominated by one component or that the medium is turbulent. Polarized emission from bright galaxy discs, spiral arms and cores of galaxy clusters are best observed at wavelengths below a few centimetres (at frequencies beyond about 10 GHz), haloes of galaxies and clusters around decimetre wavelengths (at frequencies below about 2 GHz). Intergalactic filaments need observations at metre wavelengths (frequencies below 300 MHz). Sources with extremely large intrinsic rotation measure | RM | or RM dispersion can be searched with mm-wave telescopes. Measurement of the PI spectrum allows us to derive the average Faraday | RM | or the Faraday dispersion within the source, as demonstrated for the case of the spiral galaxy NGC 6946. Periodic fluctuations in PI at low frequencies are a signature of differential Faraday rotation. Internal and external Faraday dispersion can be distinguished by the different slopes of the PI spectrum at low frequencies. A wide band around the optimum frequency is important to distinguish between varieties of depolarization effects.