The circular polarization of black hole accretion flows can encode properties of the underlying magnetic field structure. Using general relativistic magnetohydrodynamic (GRMHD) simulations, we study the imprint of magnetic field geometry on circular polarization images potentially observable by the Event Horizon Telescope (EHT). We decompose images into the different mechanisms that generate circular polarization in these models that are sensitive to both the line-of-sight direction and twist of the magnetic field. In these models, a stable sign of the circular polarization over time, as observed for several sources, can be attributed to a stability of these properties. We illustrate how different aspects of a generic helical magnetic field geometry become imprinted on a circular polarization image. We also identify novel effects of light bending that affect the circular polarization image on event horizon scales. One consequence is the sign flipping of successive photon rings in face-on systems, which if observable and uncorrupted by Faraday rotation, can directly encode the handedness of the approaching magnetic field.

中文翻译：

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黑洞磁场及其在圆偏振图像上的印记

黑洞吸积流的圆极化可以编码潜在磁场结构的特性。使用广义相对论磁流体动力学 (GRMHD) 模拟，我们研究了事件视界望远镜 (EHT) 可能观测到的圆极化图像上的磁场几何印记。我们将图像分解为在这些模型中产生圆极化的不同机制，这些模型对视线方向和磁场的扭曲都敏感。在这些模型中，随着时间的推移，圆极化的稳定信号，正如对几个来源所观察到的，可归因于这些特性的稳定性。我们说明了通用螺旋磁场几何形状的不同方面如何印在圆极化图像上。我们还确定了影响事件视界尺度上圆偏振图像的光弯曲的新效应。一个后果是正面系统中连续光子环的符号翻转，如果可观察到并且不受法拉第旋转的破坏，则可以直接编码接近磁场的旋向性。