Studying black holes using the Event Horizon Telescope

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.