Polarimetry of Black Hole Spin and Axisymmetric Emission Geometry
- 1University of Arizona
- 2Center for Astrophysics ∣ Harvard & Smithsonian
- 3Black Hole Initiative at Harvard University
Abstract
The Event Horizon Telescope array has measured the average polarimetric spiral morphology of black hole accretion disks using the so-called β ₂ coefficient. This measurement is dominated by the weakly lensed, direct ( n = 0, counting photon half-orbits) image of the disk, providing insight into magnetic field structure around the M87* black hole. The future space-based black hole Explorer mission is expected to extend these capabilities, allowing for β ₂ measurement of the strongly lensed n = 1 image. In this paper, we explore the constraints β ₂ measurements would impose on spacetime and plasma geometries using a semianalytic conical emission model. We show that magnetic field orientation with respect to the emission cone qualitatively changes the dependence of β ₂ modes on model parameters. For cone-aligned magnetic fields, we find that the leading-order theoretical prediction from equatorial emitters of β 2 , n = 0 ≈ β 2 , n = 1 * is the dominant trend even in off-equatorial models. Specifically, we show arg ( β 2 , n = 0 β 2 , n = 1 * ) ≈ 4 χ , where χ is the angle determining in-cone magnetic field orientation. We also find that arg ( β 2 , n = 0 β 2 , n = 1 ) minimizes the effect of magnetic field geometry while preserving a dependence on spin that may be used to measure spin direction. We find that observations of the photon ring’s β ₂ complement the constraint from direct image polarization and can greatly constrain magnetic field structure even with an unknown external rotation measure. We also find that black hole spin and emission cone opening angle would not be tightly constrained by β ₂ alone, unless strong priors are placed on the magnetic field geometry.
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BibTeX
@article{bernshteyn2026polarimetry,
title = {{Polarimetry of Black Hole Spin and Axisymmetric Emission Geometry}},
author = {Vadim Bernshteyn and Daniel C. M. Palumbo and Dominic Chang and Tintin Nguyen},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/aea49f},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea49f},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Polarimetry of Black Hole Spin and Axisymmetric Emission Geometry
AU - Vadim Bernshteyn
AU - Daniel C. M. Palumbo
AU - Dominic Chang
AU - Tintin Nguyen
PY - 2026
DA - 2026/10/05
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/aea49f
UR - https://iopscience.iop.org/article/10.3847/1538-4357/aea49f
AB - The Event Horizon Telescope array has measured the average polarimetric spiral morphology of black hole accretion disks using the so-called β ₂ coefficient. This measurement is dominated by the weakly lensed, direct ( n = 0, counting photon half-orbits) image of the disk, providing insight into magnetic field structure around the M87* black hole. The future space-based black hole Explorer mission is expected to extend these capabilities, allowing for β ₂ measurement of the strongly lensed n = 1 image. In this paper, we explore the constraints β ₂ measurements would impose on spacetime and plasma geometries using a semianalytic conical emission model. We show that magnetic field orientation with respect to the emission cone qualitatively changes the dependence of β ₂ modes on model parameters. For cone-aligned magnetic fields, we find that the leading-order theoretical prediction from equatorial emitters of β 2 , n = 0 ≈ β 2 , n = 1 * is the dominant trend even in off-equatorial models. Specifically, we show arg ( β 2 , n = 0 β 2 , n = 1 * ) ≈ 4 χ , where χ is the angle determining in-cone magnetic field orientation. We also find that arg ( β 2 , n = 0 β 2 , n = 1 ) minimizes the effect of magnetic field geometry while preserving a dependence on spin that may be used to measure spin direction. We find that observations of the photon ring’s β ₂ complement the constraint from direct image polarization and can greatly constrain magnetic field structure even with an unknown external rotation measure. We also find that black hole spin and emission cone opening angle would not be tightly constrained by β ₂ alone, unless strong priors are placed on the magnetic field geometry.
SN - 0004-637X
SN - 1538-4357
ER -