[论文解读] Full-Stokes polarimetry with circularly polarized feeds - Sources with stable linear and circular polarization in the GHz regime

I. Myserlis, E. Angelakis+9 more|arXiv (Cornell University)|Jun 13, 2017

Astrophysics and Cosmic Phenomena参考文献 26被引用 3

一句话总结

该论文提出了一种针对圆极化馈电的新型全斯托克斯偏振测量流程，该流程在计算斯托克斯参数之前校正仪器效应（如线性与圆极化、与光束相关的系统旋转），实现了偏振度0.1–0.2%的精度和偏振角约1°的精度。该方法在5.5年内对150个AGN源在4.85 GHz和8.35 GHz频段的应用中，识别出五个具有稳定线性偏振的源，十一个具有稳定圆偏振度的源（其中四个具有非零$m_{\mathrm{c}}$），以及八个具有稳定偏振角的源，在低信噪比环境下优于穆勒矩阵方法。

ABSTRACT

We present a pipeline that allows recovering reliable information for all four Stokes parameters with high accuracy. Its novelty relies on the treatment of the instrumental effects already prior to the computation of the Stokes parameters contrary to conventional methods, such as the Müller matrix one. The instrumental linear polarization is corrected across the whole telescope beam and significant Stokes $Q$ and $U$ can be recovered even when the recorded signals are severely corrupted. The accuracy we reach in terms of polarization degree is of the order of 0.1-0.2 %. The polarization angles are determined with an accuracy of almost 1$^{\circ}$. The presented methodology was applied to recover the linear and circular polarization of around 150 Active Galactic Nuclei. The sources were monitored from July 2010 to April 2016 with the Effelsberg 100-m telescope at 4.85 GHz and 8.35 GHz with a cadence of around 1.2 months. The polarized emission of the Moon was used to calibrate the polarization angle. Our analysis showed a small system-induced rotation of about 1$^{\circ}$ at both observing frequencies. Finally, we identify five sources with significant and stable linear polarization; three sources remain constantly linearly unpolarized over the period we examined; a total of 11 sources have stable circular polarization degree $m_\mathrm{c}$ and four of them with non-zero $m_\mathrm{c}$. We also identify eight sources that maintain a stable polarization angle over the examined period. All this is provided to the community for polarization observations reference. We finally show that our analysis method is conceptually different from the traditionally used ones and performs better than the Müller matrix method. Although it was developed for a system equipped with circularly polarized feeds it can easily be modified for systems with linearly polarized feeds as well.

研究动机与目标

• 开发一种高精度偏振测量流程，在计算斯托克斯参数之前校正仪器效应，克服传统方法（如穆勒矩阵方法）的局限性。
• 实现在河外射电源中可靠检测低水平线性和圆偏振（低至0.3%）的能力，特别是在GHz频段。
• 识别并表征在长时间基线（5.5年）内具有稳定线性和圆偏振特性的源，为未来观测提供参考。
• 利用月球已知的径向偏振图案校准偏振角，最大限度减少系统引起的旋转误差。
• 将该方法推广至适用于圆极化和线极化馈电，提升其在各类射电望远镜中的适用性。

提出的方法

• 利用无偏振源响应在整个望远镜光束范围内建模仪器线性偏振，从而实现对点源和扩展源的校正。
• 该方法采用逐子扫描的仪器贡献清理，确保在低信噪比环境下的高精度。
• 采用两种独立方法——$m_{\mathrm{c}}$的零点校准与奇异值分解（SVD）——对仪器圆偏振进行校正，结果高度一致。
• 应用了光学厚度和仰角增益校正，且证明这些校正与入射辐射的偏振状态无关。
• 通过月球观测量化仪器旋转，发现4.85 GHz处存在1.26°的小但可测量的偏移，8.35 GHz处为-0.5°，且可能存在仰角依赖性。
• 该方法避免了对大参数空间覆盖的依赖，与穆勒矩阵方法相比，在短观测周期中表现更优。

实验结果

研究问题

• RQ1 能否在计算斯托克斯参数之前有效校正圆极化馈电中的仪器效应，从而提高偏振测量精度？
• RQ2 该方法在低通量源中可实现的最小可检测偏振度是多少，特别是对低信噪比源？
• RQ3 哪些射电源在5.5年基线内表现出线性和圆偏振参数的长期稳定性？
• RQ4 该方法在斯托克斯Q和U的会话内稳定性方面与穆勒矩阵方法相比，其定量差异如何？
• RQ5 仪器旋转角在不同源仰角下是否稳定，是否需要进行绝对校准？

主要发现

• 该方法实现了0.1–0.2%的偏振度精度和约1°的偏振角精度，显著优于传统方法。
• 五个源在整个5.5年观测期间表现出显著且稳定的线性偏振，其中三个始终无偏振。
• 共11个源表现出稳定的圆偏振度（$m_{\mathrm{c}}$），其中四个具有非零$m_{\mathrm{c}}$，表明存在长期磁场或辐射稳定性。
• 八个源在观测期间保持稳定的偏振角，表明辐射几何结构或磁场取向一致。
• PKS 1127-14表现出稳定的圆偏振度和手征性，与1984年的测量结果一致，表明其稳定性持续约40年。
• 对于低偏振源（15–100 mJy），该方法在斯托克斯Q和U的会话内稳定性上比穆勒矩阵方法高出28%，尤其在低信噪比环境下表现更优。