The Supermassive Black Hole at the Center of M87 as Seen by the EHT

Black holes are predicted by general relativity (GR). The most intriguing feature of black holes is their event horizon, from which even light cannot escape. The black hole’s event horizon and strongly curved spacetime around it cause photons approaching it with an impact parameter smaller than 2.6 Rs (Schwarzschild radius) to be captured and sucked into the black hole. Consequently, a lensed photon ring with a diameter of ~5.2 Rs is expected to form around a black hole. The first ever image of the black hole in M87, showing an asymmetric ring-like structure, was observed with the Event Horizon Telescope (EHT) in April 2017. The observed diameter of the ring is in good agreement with the size of the photon ring predicted by GR. The asymmetry in brightness in the ring can be explained in terms of relativistic beaming of the emission from a plasma rotating close to the speed of light around a black hole. The black hole image in polarized light has also been recently published. The resolved fractional linear polarization has a maximum located in the southwest part of the ring, where it rises to the level of ∼15%. The polarization position angles are arranged in a nearly azimuthal pattern. We compared a number of images constructed from general relativistic hydrodynamic (GRMHD) simulations with the observation and was able to put a tight constraint on the physical properties of the plasma in the vicinity of the black hole. Also, the inferred magnetic field structure is consistent with a picture where there is a strong, ordered, poloidal magnetic field in the vicinity of the black hole. This magnetic field can regulate the mass accretion process, and can play a critical role in launching powerful jets. We will briefly overview the observation, data calibration and analysis, and physical interpretation of the results.