Ring Position Angles and Spin in M87* and Sgr A*

Abstract

Event Horizon Telescope (EHT) images of black holes appear as rings with a brightness asymmetry. Here, we expand on our previous study of the asymmetry magnitude a 1 to study the position angle of the peak brightness asymmetry P A 1 in general relativistic magnetohydrodynamic (GRMHD) models. For larger spin magnitudes ( a * > 0 and a * ≲ − 0.5 ), the mean P A 1 falls within 1 σ of the approaching limb of the black hole, regardless of viewing inclination, disk magnetization, or source. By comparing the ( a 1 , P A 1 ) distribution in M87* observations with models, we demonstrate that we can mildly disfavor low-magnitude spins and strongly disfavor all spin vectors that point toward Earth. The alignment of P A 1 relative to the large-scale jet axis may suggest that M87 ’s disk does not have a large tilt. By combining P A 1 with the pattern speed measured in optimistic 2026 M87 video conditions, the EHT can constrain whether M87* is prograde or retrograde with ∼ 84 % accuracy. In Sgr A*, we show that a detection of ( a 1 , P A 1 ) could constrain the magnitude and direction of the galactic center spin vector. Finally, if future EHT expansions increase the sample of horizon-scale sources, a simple set of observables (ring diameter, asymmetry magnitude, and asymmetry angle) could enable robust constraints on black hole mass, spin, and inclination.