Supermassive Black Hole Spin Constraints from Polarimetry in an Equatorial Disk Model
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 semianalyti...
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2024-01-01
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| Online Access: | https://doi.org/10.3847/2041-8213/ad9bb4 |
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| author | Daniel C. M. Palumbo |
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| description | 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 ${\sigma }_{| {a}_{* }| }\sim 0.25$ for radially infalling models but otherwise provides only weak constraints; an error of 1 ^∘ can reach ${\sigma }_{| {a}_{* }| }\sim 0.15$ . 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. |
| format | Article |
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| language | English |
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| spelling | doaj-art-1e7b64e1c298430f9350912d690ef21f2024-12-24T10:43:24ZengIOP PublishingThe Astrophysical Journal Letters2041-82052024-01-019781L410.3847/2041-8213/ad9bb4Supermassive Black Hole Spin Constraints from Polarimetry in an Equatorial Disk ModelDaniel C. M. Palumbo0https://orcid.org/0000-0002-7179-3816Center for Astrophysics ∣Harvard & Smithsonian , 60 Garden Street, Cambridge, MA 02138, USA ; daniel.palumbo@cfa.harvard.edu; Black Hole Initiative at Harvard University , 20 Garden Street, Cambridge, MA 02138, USAThe 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 ${\sigma }_{| {a}_{* }| }\sim 0.25$ for radially infalling models but otherwise provides only weak constraints; an error of 1 ^∘ can reach ${\sigma }_{| {a}_{* }| }\sim 0.15$ . 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.https://doi.org/10.3847/2041-8213/ad9bb4Supermassive black holesPolarimetryVery long baseline interferometry |
| spellingShingle | Daniel C. M. Palumbo Supermassive Black Hole Spin Constraints from Polarimetry in an Equatorial Disk Model The Astrophysical Journal Letters Supermassive black holes Polarimetry Very long baseline interferometry |
| title | Supermassive Black Hole Spin Constraints from Polarimetry in an Equatorial Disk Model |
| title_full | Supermassive Black Hole Spin Constraints from Polarimetry in an Equatorial Disk Model |
| title_fullStr | Supermassive Black Hole Spin Constraints from Polarimetry in an Equatorial Disk Model |
| title_full_unstemmed | Supermassive Black Hole Spin Constraints from Polarimetry in an Equatorial Disk Model |
| title_short | Supermassive Black Hole Spin Constraints from Polarimetry in an Equatorial Disk Model |
| title_sort | supermassive black hole spin constraints from polarimetry in an equatorial disk model |
| topic | Supermassive black holes Polarimetry Very long baseline interferometry |
| url | https://doi.org/10.3847/2041-8213/ad9bb4 |
| work_keys_str_mv | AT danielcmpalumbo supermassiveblackholespinconstraintsfrompolarimetryinanequatorialdiskmodel |