The Event Horizon Telescope (EHT) has produced images of both total intensity and polarized radiation from plasma around the supermassive black hole in M87 on event horizon scales.
In this talk, I will discuss what these images tell us about the physics of the black hole accretion flow and what may be revealed in future images from more sensitive sub-millimeter Very Long Baseline Interferometry (VLBI) arrays. Polarized synchrotron radiation observed by the EHT probes the plasma properties and the structure of magnetic fields and near the black hole.
In a large library of simulated images from general relativistic magnetohydrodynamic (GRMHD) simulations, the only models consistent with both the EHT observations and the observed jet power from M87 are magnetically arrested accretion disks (MADs), where near-horizon magnetic fields are coherent and dynamically important. MADs naturally launch wide, parabolic jets; simulations predict that future observations of M87 with the will have enough sensitivity to observe the relatively faint jet base in tandem with the bright, near-horizon core. While the jet is most prominent at longer wavelengths, most of the observed emission in MAD models originates from a relatively thin disk in the equatorial plane. In models where equatorial emission extends to the black hole event horizon, the darkest region in the image is restricted to a smaller area than the classic 'black hole shadow' feature.
These models predict that high-sensitivity images of M87 could have two gravitational features accessible to direct observation; a 'photon ring' of strongly lensed images and an 'inner shadow' interior to the photon ring whose edge approaches the direct lensed outline of the equatorial horizon. Measurements of the relative size, shape, and position of these two features can break degeneracies in measurements of the black hole mass and spin using submillimeter VLBI images.