Demonstrating Photon Ring Existence with Single-Baseline Polarimetry

TL;DR

This paper proposes a new polarimetric interferometric method to detect the photon ring around supermassive black holes, demonstrating its feasibility with specific sensitivity and baseline requirements for M87* and Sgr A*.

Contribution

It introduces a gain-robust interferometric quantity leveraging polarization symmetry to detect photon rings unambiguously in black hole images.

Findings

Photon rings can be detected with long-baseline polarimetry.

Detection sensitivity requirements are specified for M87* and Sgr A*.

Interstellar scattering affects photon ring detectability at certain frequencies.

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 $\sim1$ 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 $\sim10$ 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.