Studying black holes using the Event Horizon Telescope

超大质量黑洞是宇宙中发现的最大、最极端的单个天体之一。虽然黑洞本身不能被观测到，但黑洞周围吸积盘中的气体受到摩擦的显著加热，使其发光。这在黑暗的中心区域之间形成了一个明显的边界，称为黑洞阴影，被明亮的环形结构包围。爱因斯坦的广义相对论预言了黑影及其周围的环结构，它们可以用来以令人兴奋的方式测试该理论。2019年，事件视界望远镜(EHT)发布了第一张由超大质量黑洞产生的阴影图像。使用1.3毫米的全球干涉测量阵列，合作伙伴能够拍摄到附近M87星系中心的超大质量黑洞的阴影。2022年，EHT发布了一张银河系中心超大质量黑洞人马座A*的阴影图像。这两张图像的数据都是在2017年收集的，由于星际散射和变化性的增加，射手A*的图像处理时间比M87*的图像长得多。当观察我们银河系的中心时，星际散射对图像有很大的影响，在处理图像时必须考虑到这一点，但在观测M87*时情况并非如此。射手座A*的质量大约小1500倍，因此半径大约比M87*小1500倍。这意味着对于M87*来说，动态时间尺度，也就是最内部稳定圆形轨道的周期，要长得多。根据M87*的自转，M87*的动态时间刻度估计在5天到1个月之间，而射手座A*的动态时间刻度在4到30分钟之间。因此，射手座A*的源结构可以在一次观测运行中改变，但不能改变M87*，这也必须考虑到。这两个因素都增加了完全产生和分析两个黑洞的结果所需的时间。通过这个项目和与EHT的合作，我将研究超大质量黑洞射手座A*和M87*周围的阴影。幸运的是，有许多不同的区域可以用来进一步了解黑洞阴影。我希望探索的一个有希望的领域是2018年观测运行的新数据，加上额外的望远镜设施，增加了(u，v)覆盖，这将导致与2017年的观测相比，结果有所改善。2017年和2018年的数据对比也是一个值得探索的有趣领域，这可能会让我们更好地了解图像中出现的热点，特别是在射手座A*上，它有3个亮点。如果这些斑点位于相同的位置，或者它们围绕射手座的光环移动，A*可以说它们是否是射手座光环中可以探索的物理亮点。如果它们是固定的，那么它们可能是在处理数据时创建的人工制品。随后，随着卫星网络的不断扩大，观测将进一步完善。另一个可以探索的令人兴奋的领域是黑洞的极化，这已经在M87*上完成了，但对于人马座A*还没有完成。从阴影周围的圆盘发出的光的偏振可以揭示黑洞事件视界附近的磁场结构的信息。与组成EHT的全球射电望远镜阵列一起，模拟将成为整个项目的关键工具。广义相对论磁流体动力学(GRMHD)模拟描述了爱因斯坦的广义相对论和磁流体动力学。因此，它们可以用来模拟黑洞附近的吸积和喷流形成，这对于理解EHT拍摄的人马座A*和M87*的图像至关重要。

Supermassive black holes are some of the largest and most extreme individual objects found in the Universe. While the black hole itself cannot be observed, the gas in an accretion disk surrounding the black hole is significantly heated by friction, causing it to glow. This forms a distinct boundary between the dark central region, called a black hole shadow, which is surrounded by a bright ring structure. The shadow and the ring structure surrounding it are predicted by Einstein's theory of general relativity and they can be used to test the theory in exciting ways.In 2019 the Event Horizon Telescope (EHT) released the first image of the shadow created by a supermassive black hole. Using a global interferometry array at 1.3mm, the collaboration was able to image the shadow of the supermassive black hole at the centre of the nearby galaxy M87. In 2022 the EHT released an image of the shadow of the supermassive black hole at the centre of the Milky Way, Sagittarius A*.The data that produced both images was collected in 2017, the image of Sagittarius A* took much longer to process than the image of M87* due to two main constraints, the interstellar scattering, and the increased variability. When looking into the centre of our galaxy, the Milky Way, interstellar scattering has a significant effect on the images, this had to be accounted for when processing the image, this is not the case when observing M87*. Sagittarius A* is approximately 1500 times less massive and therefore has a radius approximately 1500 times smaller than M87*. This means the dynamical timescale, which is the period of the innermost stable circular orbit, is much longer for M87*. The dynamical timescale for M87* is estimated to be between 5 days and 1 month, depending on the spin of M87*, for Sagittarius A* the dynamical timescale is between 4 and 30 minutes. So, the source structure can change over an observational run for Sagittarius A*, but not M87*, this also had to be accounted for. Both factors increased the length of time required to fully produce and analyse the results for both black holes.Through this project and the collaboration with the EHT, I will investigate the shadows around the supermassive black holes Sagittarius A* and M87*. Fortunately, there are many different areas available to further understand black hole shadows. One promising area that I hope to explore is the new data from the 2018 observing run, with additional telescope facilities, which increased the (u,v) coverage, which will lead to improved results compared to the 2017 observations. Comparisions between the 2017 and 2018 data is also an interesting area to explore, this could lead to a better understanding of the hotspots that appear on the images, especially on Sagittarius A*, which has 3 bright spots. If these spots are in the same position or if they have moved around the ring of Sagittarius A* could say if they are physical bright spots in the ring which could be explored. If instead they are stationary, they might be an artifact created while processing the data. Later observing runs with an ever-growing network of satellites will improve on this further.Another exciting area that can be explored is the polarization of the black holes, this has been completed for M87*, but not for Sagittarius A* yet. The polarization of the light being emitted from the disk around the shadow can reveal information about the magnetic field structure near the event horizon of the black hole. Along with the global array of radio telescopes that make up the EHT, simulations will be a key tool throughout this project. General Relativistic Magnetohydrodynamical Dynamics (GRMHD) simulations, describe both Einstein's theory of general relativity and magnetohydrodynamics. Therefore, they can be used to model accretion and jet formation in the vicinity of black holes which is critical for understanding the images of Sagittarius A* and M87* captured by the EHT.