标题： 第一M87事件视界望远镜结果。 IX。 近视界圆偏振的探测 显示英文标题

标题： First M87 Event Horizon Telescope Results. IX. Detection of Near-horizon Circular Polarization

摘要： 事件视界望远镜（EHT）的观测揭示了在M87星系中心超大质量黑洞周围存在一个明亮的辐射环。 EHT线性偏振图像进一步识别出了围绕黑洞的相干螺旋图案，这是由穿过发射等离子体的有序磁场产生的。 在这里，我们展示了首次使用2017年获取的EHT数据进行的圆偏振分析，这可能会为黑洞附近的磁场和等离子体组成提供额外的见解。 干涉仪闭合量提供了在事件视界尺度上存在圆偏振辐射的有力证据。 我们使用传统方法和新开发的方法生成了圆偏振的图像。 所有方法都在图像中发现了一个中等水平的解析圆偏振（$\langle|v|\rangle < 3.7\%$），与ALMA阵列测量的低图像积分圆偏振分数（$|v_{\rm int}| < 1\%$）一致。 尽管有这种广泛的共识，这些方法在圆偏振辐射的形态上表现出显著的差异，表明我们的结论强烈依赖于成像假设，这是由于基线覆盖有限、望远镜增益校准不确定以及弱偏振信号所致。 我们将这个上限包含在对广义相对论磁流体动力学（GRMHD）模拟模型的更新比较中。 这项分析加强了之前报告的对磁约束吸积流模型的偏好。 我们发现大多数模拟自然产生一个与我们的上限一致的低水平圆偏振，并且法拉第转换可能是M87*在230 GHz处圆偏振的主要产生机制。 显示英文摘要

摘要： Event Horizon Telescope (EHT) observations have revealed a bright ring of emission around the supermassive black hole at the center of the M87 galaxy. EHT images in linear polarization have further identified a coherent spiral pattern around the black hole, produced from ordered magnetic fields threading the emitting plasma. Here, we present the first analysis of circular polarization using EHT data, acquired in 2017, which can potentially provide additional insights into the magnetic fields and plasma composition near the black hole. Interferometric closure quantities provide convincing evidence for the presence of circularly polarized emission on event-horizon scales. We produce images of the circular polarization using both traditional and newly developed methods. All methods find a moderate level of resolved circular polarization across the image ($\langle|v|\rangle < 3.7\%$), consistent with the low image-integrated circular polarization fraction measured by the ALMA array ($|v_{\rm int}| < 1\%$). Despite this broad agreement, the methods show substantial variation in the morphology of the circularly polarized emission, indicating that our conclusions are strongly dependent upon the imaging assumptions because of the limited baseline coverage, uncertain telescope gain calibration, and weakly polarized signal. We include this upper limit in an updated comparison to general relativistic magnetohydrodynamic (GRMHD) simulation models. This analysis reinforces the previously reported preference for magnetically arrested accretion flow models. We find that most simulations naturally produce a low level of circular polarization consistent with our upper limit, and that Faraday conversion is likely the dominant production mechanism for circular polarization at 230 GHz in M87*.