Studying black holes using the Event Horizon Telescope

超大质量黑洞是宇宙中发现的一些最大和最极端的个体物体。虽然黑洞本身无法被观测到，但黑洞周围的吸积盘中的气体因摩擦而显着加热，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，它被明亮的环形结构包围。这个阴影和围绕它的环形结构是由爱因斯坦的广义相对论预测的，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）发布了超大质量黑洞产生的阴影的第一张图像。使用1.3毫米的全球干涉测量阵列，该合作能够对附近星系M87中心的超大质量黑洞的阴影进行成像。2022年，EHT发布了银河系中心超大质量黑洞人马座A* 阴影的图像。产生这两张图像的数据是在2017年收集的，人马座A* 的图像比M87* 的图像处理时间长得多，这是由于两个主要限制，星际散射和增加的可变性。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，但在观察M87* 时情况并非如此。人马座A* 的质量大约是M87 * 的1/1500，因此它的半径大约是M87* 的1/1500。这意味着M87* 的动力学时标，也就是最内层稳定圆形轨道的周期，要长得多。M87* 的动力学时标估计在5天到1个月之间，这取决于M87* 的旋转，人马座A* 的动力学时标在4到30分钟之间。因此，源结构可以在人马座A* 的观测运行中发生变化，而不是M87*，这也必须考虑在内。这两个因素都增加了完全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，这将导致与2017年观测相比改善的结果。2017年和2018年数据之间的比较也是一个有趣的探索领域，这可能会导致更好地了解图像上出现的热点，特别是在人马座A* 上，它有3个亮点。如果这些点在同一个位置，或者它们在人马座环周围移动，A* 可以说它们是环中可以探索的物理亮点。如果它们是固定的，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，观测工作将进一步完善这一点。另一个令人兴奋的领域是黑洞的极化，M87* 已经完成，但人马座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磁场结构的信息。沿着组成EHT的全球射电望远镜阵列，模拟将是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.