Studying black holes using the Event Horizon Telescope

超大质量黑洞是宇宙中发现的最大、最极端的个体物体之一。虽然黑洞本身无法被观察到，但黑洞周围的吸积盘中的气体由于摩擦而被显著加热，导致黑洞发光。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，它被一个明亮的环状结构所包围。爱因斯坦的广义相对论预言了影子和围绕它的环状结构，它们可以用来以令人兴奋的方式检验这一理论。2019年，事件视界望远镜（EHT）发布了超大质量黑洞产生的阴影的第一张图像。利用1.3毫米的全球干涉测量阵列，该合作能够成像附近星系M87中心的超大质量黑洞的阴影。2022年，EHT发布了一张超大质量黑洞位于银河系中心人马座A*的阴影图像。生成这两幅图像的数据是在2017年收集的，由于星际散射和变异性增加两个主要限制，人马座A*的图像比M87*的图像处理时间要长得多。当我们观察银河系的中心时，星际散射对图像有显著的影响，在处理图像时必须考虑到这一点，而在观察M87*时则不是这样。人马座A*的质量大约是M87*的1500倍，因此半径大约是M87*的1500倍。这意味着M87*的动态时间尺度，即最内层稳定圆轨道的周期，要长得多。M87*的动态时间标度估计在5天到1个月之间，这取决于M87*的自旋，而人马座A*的动态时间标度在4到30分钟之间。因此，在对人马座A*的观测中，源结构可能会发生变化，但M87*不会，这也必须考虑在内。这两个因素都增加了充分产生和分析两个黑洞结果所需的时间长度。通过这个项目以及与EHT的合作，我将研究超大质量黑洞人马座A*和M87*周围的阴影。幸运的是，有许多不同的领域可以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，增加了望远镜设施，增加了（u,v）覆盖范围，这将导致比2017年观测结果更好的结果。比较2017年和2018年的数据也是一个值得探索的有趣领域，这可能会让我们更好地理解图像上出现的热点，尤其是在人马座a *上，它有三个亮点。如果这些斑点在相同的位置，或者它们是否在人马座A*环周围移动，可以说明它们是否是环中可以探索的物理亮点。如果它们是静止的，则它们可能是在处理数据时创建的工件。随着卫星网络的不断扩大，以后的观测将进一步改进这一点。另一个可以探索的令人兴奋的领域是黑洞的极化，这已经完成了M87*，但还没有完成人马座A*。从阴影周围的圆盘发出的光的偏振可以揭示黑洞视界附近磁场结构的信息。与构成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理论和磁流体动力学。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捕获的人马座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.