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 a1 to study the position angle of the peak brightness asymmetry PA1 in general relativistic magnetohydrodynamic (GRMHD) models. For larger spin magnitudes (a*>0 and a*≲−0.5), the mean PA1 falls within 1σ of the approaching limb of the black hole, regardless of viewing inclination, disk magnetization, or source. By comparing the (a1, PA1) 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 PA1 relative to the large-scale jet axis may suggest that M87*'s disk does not have a large tilt. By combining PA1 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 (a1, PA1) 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
@misc{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},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2606.11322},
eprint = {2606.11322},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.11322},
note = {CC BY 4.0},
}
RIS
TY - UNPB
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 - 2026/09/30
PB - arXiv
DO - 10.48550/arxiv.2606.11322
UR - https://arxiv.org/abs/2606.11322
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 PA_1 in general relativistic magnetohydrodynamic (GRMHD) models. For larger spin magnitudes (a_*>0 and a_*≲−0.5), the mean PA_1 falls within 1σ of the approaching limb of the black hole, regardless of viewing inclination, disk magnetization, or source. By comparing the (a_1, PA_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 PA_1 relative to the large-scale jet axis may suggest that M87*'s disk does not have a large tilt. By combining PA_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, PA_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.
ER -