The spectacular images of the M87 black hole taken by the Event Horizon Telescope (EHT) have opened a new era of black hole research. One of the next issues is to take polarization images around the central black hole (BH). Since radio emission is produced by synchrotron process, polarization properties should vividly reflect the magnetic field structure at the jet base and thus provide good information regarding the magnetic mechanism of jet formation. With this kept in mind we perform general relativistic (GR) radiative transfer calculations of polarized light based on the GR magnetohydrodynamic (MHD) simulation data of accretion flow and outflow in M87, to obtain their linear and circular polarization images in the horizon-scale. We found that the linear polarization components from the jet base and inner accretion flow should experience Faraday rotation when passing through magnetized plasmas around the BH, thus sensitively depending on the BH spin. Through the comparison with intensity image at 1.3mm by EHT and the polarized degree and the rotation measure (RM) at 1.3mm with the Submillimeter Array, the model with the spin parameter of a=0.9M_BH (with M_BH being the BH mass) is favored over other models with a=0.5M_BH or 0.99M_BH, though we need further systematic studies for confirmation. We also find in low-temperature models clear ring-like image in the circular polarization map, which arises because of Faraday conversion of the linearly polarized synchrotron emission and is thus indicative of magnetic field. This occurs only when the emission region is threaded with well-ordered magnetic fields and hence no clear images are expected in high-temperature disk models, in which disk emission is appreciable. We will be able to elucidate the field configuration through the comparison between the simulated polarization images and future polarimetry with EHT and other VLBI observations.

中文翻译：

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基于 GRMHD 模拟的广义相对论辐射传递计算对 M 87 喷流的偏振成像

事件视界望远镜 (EHT) 拍摄的 M87 黑洞壮观图像开启了黑洞研究的新纪元。接下来的问题之一是在中央黑洞 (BH) 周围拍摄偏振图像。由于射电发射是由同步加速器过程产生的，极化特性应能生动地反映喷流底部的磁场结构，从而为有关喷流形成的磁机制提供良好的信息。牢记这一点，我们根据 M87 中吸积流和流出的 GR 磁流体动力学 (MHD) 模拟数据执行偏振光的广义相对论 (GR) 辐射传输计算，以获得水平尺度的线偏振和圆偏振图像。我们发现，当穿过 BH 周围的磁化等离子体时，来自喷流底部和内部吸积流的线性极化分量应该经历法拉第旋转，因此敏感地取决于 BH 自旋。通过与EHT在1.3mm处的强度图像和亚毫米阵列在1.3mm处的偏振度和旋转测量（RM）的比较，自旋参数为a=0.9M_BH（M_BH为BH质量）的模型为优于其他 a=0.5M_BH 或 0.99M_BH 的模型，尽管我们需要进一步的系统研究来确认。我们还在低温模型中发现圆极化图中清晰的环状图像，这是由于线极化同步加速器发射的法拉第转换而产生的，因此表示磁场。仅当发射区域与有序磁场串连时才会发生这种情况，因此在高温盘模型中预计不会出现清晰的图像，其中盘发射是可观的。我们将能够通过模拟偏振图像与未来偏振测量与 EHT 和其他 VLBI 观测的比较来阐明场配置。