Interferometric Measurement of the Magnetization and Spin of Supermassive Black Holes

Abstract

Supermassive black hole (SMBH) event horizons are now observationally accessible. The Event Horizon Telescope (EHT) has recently imaged the two known black holes of largest angular size with very-long-baseline interferometry (VLBI): that in Messier 87 (M 87*), and in our own Galactic Center (Sgr A*). In this thesis, I develop observational and theoretical tools for studies of synchrotron emission from SMBH accretion flows. Together, these results yield promising pathways to measurements of black hole spin and the accretion magnetization state, crucial properties that encode the accretion and merger history of the black hole. First, I specify a Fourier decomposition of rotational polarization structure in images of modestly inclined accretion flows. I find that spiral structure in synchrotron polarization directly probes both accretion magnetization and spin. Next, I demonstrate that lensing symmetries of the Penrose-Walker constant in axisymmetric equatorial flows simplify to a polarization B-mode antisymmetry that is apparent in the strongly lensed “photon rings” of favored simulations for M 87* and Sgr A*. I then propose a VLBI experiment that could use these symmetries to detect the photon ring. Finally, I develop a semi-analytic equatorial disk model for optically thin accretion flows. I embed this model in an inference framework to measure black hole mass, spin, inclination, and magnetic field geometry directly from interferometric data. I conclude with an overview of recent and forthcoming observational results using these techniques, including the EHT’s measurement of the magnetization state of M 87*.