Supermassive Black Hole Spin Constraints from Polarimetry in an Equatorial Disk Model
Year: 2024
Type: article
Abstract: The Event Horizon Telescope has released polarized images of the supermassive black holes Messier 87* (M87*) and Sagittarius A* accretion disks. As more images are produced, our understanding of the average polarized emission from near the event horizon improves. In this Letter, we use a semianalytic model for optically thin, equatorial emission near a Kerr black hole to study how spin constraints follow from measurements of the average polarization spiral pitch angle. We focus on the case of M87* and explore how the direct, weakly lensed image spiral is coupled to the strongly lensed indirect image spiral, and how a precise measurement of both provides a powerful spin tracer. We find a generic result that the spin twists the direct and indirect image polarization in opposite directions. Using a grid search over model parameters, we find a strong dependence of the resulting spin constraint on plasma properties near the horizon. Grid constraints suggest that, under reasonable assumptions for the accretion disk, a measurement of the direct and indirect image spiral pitch angles to ±5° yields a dimensionless spin amplitude measurement with uncertainty σ ∣ a * ∣ ∼ 0.25 for radially infalling models but otherwise provides only weak constraints; an error of 1 ∘ can reach σ ∣ a * ∣ ∼ 0.15 . We also find that a well-constrained rotation measure greatly improves spin measurements. Assuming that equatorial velocity and magnetic field are oppositely oriented, we find that the observed M87* polarization pattern favors models with strong radial velocity components, which are close to optimal for future spin measurements.
Author: Daniel C. M. Palumbo
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $2,400
OpenAlex ID: W4405731168