Falcke H, Melia F, Agol E
Astrophys J. 2000 Jan 1;528(1):L13-L16. doi: 10.1086/312423.
In recent years, evidence for the existence of an ultracompact concentration of dark mass associated with the radio source Sagittarius A* in the Galactic center has become very strong. However, unambiguous proof that this object is indeed a black hole is still lacking. A defining characteristic of a black hole is the event horizon. To a distant observer, the event horizon casts a relatively large "shadow" with an apparent diameter of approximately 10 gravitational radii that is due to the bending of light by the black hole, and this shadow is nearly independent of the black hole spin or orientation. The predicted size ( approximately 30 µas) of this shadow for Sgr A* approaches the resolution of current radio interferometers. If the black hole is maximally spinning and viewed edge-on, then the shadow will be offset by approximately 8 µas from the center of mass and will be slightly flattened on one side. Taking into account the scatter broadening of the image in the interstellar medium and the finite achievable telescope resolution, we show that the shadow of Sgr A* may be observable with very long baseline interferometry at submillimeter wavelengths, assuming that the accretion flow is optically thin in this region of the spectrum. Hence, there exists a realistic expectation of imaging the event horizon of a black hole within the next few years.
近年来，与银河系中心射电源人马座A相关联的超致密暗物质团存在的证据变得非常确凿。然而，仍缺乏确凿证据证明这个天体确实是一个黑洞。黑洞的一个决定性特征是事件视界。对于遥远的观测者来说，事件视界会投射出一个相对较大的“阴影”，其视直径约为10个引力半径，这是由黑洞对光的弯曲造成的，并且这个阴影几乎与黑洞的自旋或方向无关。人马座A这个阴影的预测大小（约30微角秒）接近当前射电干涉仪的分辨率。如果黑洞以最大速度旋转且从边缘方向观测，那么阴影将从质心偏移约8微角秒，并且在一侧会略微变扁。考虑到星际介质中图像的散射展宽以及望远镜可达到的有限分辨率，我们表明，假设在该光谱区域吸积流是光学薄的，那么人马座A*的阴影可能可以用亚毫米波长的甚长基线干涉测量法观测到。因此，在未来几年内对黑洞的事件视界进行成像存在现实的期望。
