A Polarimetric View of the Edge of M87's Black Hole

For astronomers, black holes are not just solutions to Einstein's equations. They are objects that can be observed, studied and used to test Einstein's Theory of General Relativity and models of cosmic plasma flows. The Event Horizon Telescope (EHT) is the Earth-size virtual radio telescope array built with a goal to obtain high-resolution images and movies of supermassive black holes nearby. In 2019, the EHT collaboration team successfully produced the first-ever image of a black hole back. Further, in early 2021, the EHT collaboration revealed a new view of the massive object at the centre of the M87 galaxy: how it looks in polarized light. This is the first time astronomers have been able to measure polarization, a signature of magnetic fields, this close to the edge of a black hole. In this talk, these new observational results and their theoretical interpretations will be presented. A detailed theoretical understanding of black hole accretion is crucial to interpret the new black hole images. The numerical models of accretion are based on general relativistic magnetohydrodynamics simulations, which can capture the complex dynamics of magnetic fields and their interactions with plasma very close to the event horizon. It is important to understand the emission and propagation of polarized light in these models to interpret black holes' polarimetric properties correctly. A particular focus will be given on the modelling of the circular polarization in the near-horizon emission as well as its spatial and temporal handedness stability. These are keys to understanding how the M87 black hole launches its energetic plasma jets and measuring the black hole's spin.