Black hole shadows, photon rings, and lensing rings
Black hole shadows, photon rings, and lensing rings
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DOI：
10.1103/physrevd.100.024018
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发表时间：
2019-07-12
影响因子：
5
通讯作者：
Wald, Robert M.
中科院分区：
文献类型：
--
作者：
Gralla, Samuel E.;Holz, Daniel E.;Wald, Robert M.
围绕着黑暗的“黑洞阴影”的明亮的“光子环”的存在被认为是黑洞附近发射的观测外观的一个重要特征。我们澄清的意义和相关性，这些分析计算和数字玩具模型。术语“阴影”的标准用法描述了从各个方向照亮的黑洞的外观，包括从观察者后面。一个背光的黑洞投下了一个更大的阴影。这两种“阴影”启发式方法都与理解黑洞附近的发射现象无关，在黑洞附近，发射轮廓和引力红移在确定中心暗区的观测大小方面起着主导作用。光子环是由在逃逸到无穷远之前在近场区域围绕黑洞运行的光线产生的，在那里它们到达像平面上的环形“临界曲线”附近。虽然亮度可以成为任意大的情况下，在黑洞附近的光学薄发射物质的临界曲线附近，我们表明，增强只是对数，因此是没有相关性的目前的观测。对于从几何上薄或厚的圆盘发出的光学薄辐射，仅占轨道一小部分的光子通常会产生更宽的“透镜环”，这是圆盘背面的缩小图像，叠加在直接辐射的顶部。对于近正面的观察，透镜环的中心半径比光子环大5%，并且根据发射的细节，其宽度类似于0.5- 1 M（其中M是黑洞的质量）。与图像的周围部分相比，它可以相对亮2-3倍，因此可以在高分辨率图像中提供重要特征。然而，观测图像的特征主要取决于黑洞附近发射物质的位置和性质。本文评述了最近M87* 事件视界望远镜的观测和质量测量。
The presence of a bright "photon ring" surrounding a dark "black hole shadow" has been discussed as an important feature of the observational appearance of emission originating near a black hole. We clarify the meaning and relevance of these heuristics with analytic calculations and numerical toy models. The standard usage of the term "shadow" describes the appearance of a black hole illuminated from all directions, including from behind the observer. A backlit black hole casts a somewhat larger shadow. Neither "shadow" heuristic is particularly relevant to understanding the appearance of emission originating near the black hole, where the emission profile and gravitational redshift play the dominant roles in determining the observed size of the central dark area. A photon ring results from light rays that orbit around the black hole in the near-field region before escaping to infinity, where they arrive near a ring-shaped "critical curve" on the image plane. Although the brightness can become arbitrarily large near this critical curve in the case of optically thin emitting matter near the black hole, we show that the enhancement is only logarithmic, and hence is of no relevance to present observations. For optically thin emission from a geometrically thin or thick disk, photons that make only a fraction of an orbit will generically give rise to a much wider "lensing ring," which is a demagnified image of the back of the disk, superimposed on top of the direct emission. For nearly face-on viewing, the lensing ring is centered at a radius similar to 5% larger than the photon ring and, depending on the details of the emission, its width is similar to 0.5-1M (where M is the mass of the black hole). It can be relatively brighter by a factor of 2-3, as compared to the surrounding parts of the image, and thus could provide a significant feature in high-resolution images. Nevertheless, the characteristic features of the observed image are dominated by the location and properties of the emitting matter near the black hole. We comment on the recent M87* Event Horizon Telescope observations and mass measurement.