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.
Versions
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BibTeX
@article{conroy2026ring,
title = {{Ring Position Angles and Spin in M87* and Sgr A*}},
author = {Nicholas S. Conroy and Michi Bauböck and Vadim Bernshteyn and Paul Tiede and Abhishek V. Joshi and Cora Prather and Charles F. Gammie},
journal = {The Open Journal of Astrophysics},
year = {2026},
month = oct,
doi = {10.33232/001c.172040},
url = {https://astro.theoj.org/article/172040-ring-position-angles-and-spin-in-m87-and-sgr-a},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Ring Position Angles and Spin in M87* and Sgr A*
AU - Nicholas S. Conroy
AU - Michi Bauböck
AU - Vadim Bernshteyn
AU - Paul Tiede
AU - Abhishek V. Joshi
AU - Cora Prather
AU - Charles F. Gammie
PY - 2026
DA - 2026/10/02
JO - The Open Journal of Astrophysics
DO - 10.33232/001c.172040
UR - https://astro.theoj.org/article/172040-ring-position-angles-and-spin-in-m87-and-sgr-a
AB - 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.
SN - 2565-6120
ER -