Demonstrating Photon Ring Existence with Single-Baseline Polarimetry

Abstract

Images of supermassive black hole accretion flows contain features of both curved spacetime and plasma structure. Inferring properties of the spacetime from images requires modeling the plasma properties, and vice versa. The Event Horizon Telescope Collaboration has imaged near-horizon millimeter emission from both Messier 87* (M87*) and Sagittarius A* (Sgr A*) with very-long-baseline interferometry (VLBI) and has found a preference for magnetically arrested disk (MAD) accretion in each case. MAD accretion enables spacetime measurements through future observations of the photon ring, the image feature composed of near-orbiting photons. The ordered fields and relatively weak Faraday rotation of MADs yield rotationally symmetric polarization when viewed at modest inclination. In this letter, we utilize this symmetry along with parallel transport symmetries to construct a gain-robust interferometric quantity that detects the transition between the weakly lensed accretion flow image and the strongly lensed photon ring. We predict a shift in polarimetric phases on long baselines and demonstrate that the photon rings in M87* and Sgr A* can be unambiguously detected {with sensitive, long-baseline measurements. For M87* we find that photon ring detection in snapshot observations requires $\sim 1$ mJy sensitivity on $>15$ G$\lambda$ baselines at 230 GHz and above, which could be achieved with space-VLBI or higher-frequency ground-based VLBI. For Sgr A*, we find that interstellar scattering inhibits photon ring detectability at 230 GHz, but $\sim 10$ mJy sensitivity on $>12$ G$\lambda$ baselines at 345 GHz is sufficient, which is accessible from the ground. For both sources, these sensitivity requirements may be relaxed by repeated observations and averaging.

arXiv comment14 pages, 7 figures, Accepted to ApJL

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Cited by in corpus (12)

• Black Hole Polarimetry I: A Signature of Electromagnetic Energy Extraction
• The Black Hole Explorer: Photon Ring Science, Detection and Shape Measurement
• First Very Long Baseline Interferometry Detections at 870μm
• A Lensing-Band Approach to Spacetime Constraints
• Measuring Black Hole Light Echoes with Very Long Baseline Interferometry
• Gravity versus astrophysics in black hole images and photon rings: Equatorial emissions and spherically symmetric space-times
• Spacetime Measurements with the Photon Ring
• Photon ring polarimetry with next-generation black hole imaging I. M87*
• The Japanese Vision for the Black Hole Explorer Mission
• Observable strong field effects of extra spacetime dimension in the braneworld black hole
• Effect of Earth's Oblateness on Black Hole Imaging Through Earth-Space and Space-Space VLBI
• Orbit design for mitigating interstellar scattering effects in Earth-space VLBI observations of Sgr A*