标题： 低亮度活动星系核中黑洞阴影的可见性 显示英文标题

标题： Visibility of Black Hole Shadows in Low-luminosity AGN

摘要： 吸积黑洞通常会显示出一个称为黑洞阴影的特征性暗中心区域，该阴影仅依赖于时空/观测者几何结构，并且可以传递关于黑洞质量和自旋的信息。 相反，观测到的中心亮度减弱，即图像阴影，还取决于辐射区域的形态。 本文研究了在基于广义相对论磁流体力学（GRMHD）模型的吸积黑洞中观察有意义的黑洞阴影所需的天体物理学条件。 特别是，我们确定了两种使图像阴影与黑洞阴影不同的过程：吸积流最内层区域的清空，这可以使图像阴影大于黑洞阴影；以及吸积流中光学厚度较大的区域遮挡黑洞阴影，这可以使图像阴影小于黑洞阴影，甚至完全消除它。 我们研究了在哪些模型中我们的模型的图像阴影与其对应的黑洞阴影相匹配，在哪些模型中两者彼此偏离。 我们发现，给定一个紧凑且光学薄的发射区域，我们的模型允许以5%的精度测量黑洞阴影的大小。 我们表明，这些条件通常适用于我们考虑的所有MAD模拟，以及一些SANE模拟。 显示英文摘要

摘要： Accreting black holes tend to display a characteristic dark central region called the black-hole shadow, which depends only on spacetime/observer geometry and which conveys information about the black hole's mass and spin. Conversely, the observed central brightness depression, or image shadow, additionally depends on the morphology of the emission region. In this paper, we investigate the astrophysical requirements for observing a meaningful black-hole shadow in GRMHD-based models of accreting black holes. In particular, we identify two processes by which the image shadow can differ from the black-hole shadow: evacuation of the innermost region of the accretion flow, which can render the image shadow larger than the black-hole shadow, and obscuration of the black-hole shadow by optically thick regions of the accretion flow, which can render the image shadow smaller than the black-hole shadow, or eliminate it altogether. We investigate in which models the image shadows of our models match their corresponding black-hole shadows, and in which models the two deviate from each other. We find that, given a compact and optically thin emission region, our models allow for measurement of the black-hole shadow size to an accuracy of 5%. We show that these conditions are generally met for all MAD simulations we considered, as well as some of the SANE simulations.