Polarimetry of Black Hole Spin and Axisymmetric Emission Geometry
Abstract
The Event Horizon Telescope array has measured the average polarimetric spiral morphology of black hole accretion disks using the so-called ``β2'' 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 β2 measurement of the strongly lensed n=1 image. In this paper, we explore the constraints β2 measurements would impose on spacetime and plasma geometries using a semi-analytic conical emission model. We show that magnetic field orientation with respect to the emission cone qualitatively changes the dependence of β2 modes on model parameters. For cone-aligned magnetic fields, we find that the leading-order theoretical prediction from equatorial emitters of β2, n=0 ≈β2, n=1* is the dominant trend even in off-equatorial models. Specifically, we show arg(β2, n=0 β2, n=1*) ≈ 4χ, where χ is the angle determining in-cone magnetic field orientation. We also find that arg(β2, n=0β2, 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 β2 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 β2 alone, unless strong priors are placed on the magnetic field geometry.
Related
- A Pipeline for the AI-Assisted Analysis of Imaging X-ray Polarimetry Explorer Observations of Black Hole X-ray Binaries and Limits on Their Polarization Variability
- Quantum Recovery from Black-Hole Radiation Beyond the Emission Yield
- Electromagnetic Emission and Orbital Evolution of Eccentric Supermassive Black Hole Binaries in Retrograde Disks
- Thermal and non-thermal emission from supermassive black hole circumbinary disks: Disks, Coronae, Streams, and Cavities
- JWST/MIRI Spectroscopy of the Quiescent Black Hole X-ray Binary XTE J1118+480: Constraints on Jet and Circumbinary Dust Contributions to the Mid-infrared Emission
BibTeX
@misc{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},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.03943},
eprint = {2610.03943},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.03943},
note = {CC BY 4.0},
}
RIS
TY - UNPB
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 - 2026/10/02
PB - arXiv
DO - 10.48550/arxiv.2610.03943
UR - https://arxiv.org/abs/2610.03943
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 semi-analytic 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^*) ≈ 4χ, where χ is the angle determining in-cone magnetic field orientation. We also find that arg(β_2, n=0β_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.
ER -