Paper detail

Photon Ring Symmetries in Simulated Linear Polarization Images of Messier 87*

The Event Horizon Telescope (EHT) recently released the first linearly polarized images of the accretion flow around the supermassive black hole Messier 87*, hereafter \m{}. The spiraling polarization pattern found in EHT images favored magnetically arrested disks (MADs) as the explanation for the EHT image. With next-generation improvements to very long baseline interferometry (VLBI) on the horizon, understanding similar polarized features in the highly lensed structure known as the "photon ring," where photons make multiple half-orbits about the black hole before reaching the observer, will be critical to analysis of future images. Recent work has indicated that this image region may be depolarized relative to more direct emission. We expand this observation by decomposing photon half-orbits in the EHT library of simulated images of the \m{} accretion system and find that images of MAD simulations show a relative depolarization of the photon ring attributable to destructive interference of oppositely spiraling electric field vectors; this antisymmetry, which arises purely from strong gravitational lensing, can produce up to ${\sim}50\%$ depolarization in the photon ring region with respect to the direct image. In systems that are not magnetically arrested and with the exception of systems with high spin and ions and electrons of equal temperature, we find that highly lensed indirect sub-images are almost completely depolarized, causing a modest depolarization of the photon ring region in the complete image. We predict that next-generation EHT observations of \m{} polarization should jointly constrain the black hole spin and the underlying emission and magnetic field geometry.