How narrow is the M87* ring? II. A new geometric model

Abstract

The 2017 Event Horizon Telescope (EHT) observations of M87* detected a ring-shaped feature $\sim 40\mu$as in diameter, consistent with the event horizon scale of a black hole of the expected mass. The thickness of this ring, however, proved difficult to measure, despite being an important parameter for constraining the observational appearance. In the first paper of this series we asked whether the width of the ring was sensitive to the choice of likelihood function used to compare observed closure phases and closure amplitudes to model predictions. In this paper we investigate whether the ring width is robust to changes in the model itself. We construct a more realistic geometric model with two new features: an adjustable radial falloff in brightness, and a secondary "photon ring" component in addition to the primary annulus. This thin, secondary ring is predicted by gravitational lensing for any black hole with an optically thin accretion flow. Analyzing the data using the new model, we find that the primary annulus remains narrow (fractional width $\le 0.25$) even with the added model freedom. This provides further evidence in favor of a narrow ring for the true sky appearance of M87*, a surprising feature that, if confirmed, would demand theoretical explanation. Comparing the Bayesian evidence for models with and without a secondary ring, we find no evidence for the presence of a lensed photon ring in the 2017 observations. However, the techniques we introduce may prove useful for future observations with a larger and more sensitive array.

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• Demonstrating Photon Ring Existence with Single-Baseline Polarimetry
• A Lensing-Band Approach to Spacetime Constraints
• Near-Horizon Polarization as a Diagnostic of Black Hole Spacetime
• Is a photon ring invariably a closed structure?
• Bayesian Black Hole Photogrammetry
• Polarimetric Geometric Modeling for mm-VLBI Observations of Black Holes
• Event-horizon-scale Imaging of M87* under Different Assumptions via Deep Generative Image Priors
• Extreme lensing induces spectro-temporal correlations in black-hole signals
• Photon ring polarimetry with next-generation black hole imaging I. M87*
• Assessing the impact of instrument noise and astrophysical fluctuations on measurements of the first black hole photon ring
• Ridges in rotating neutron-star properties due to first order phase transitions
• Gravitational Lensing in the Kerr Spacetime: An Analytic Approach for Light and High-Frequency Gravitational Waves
• Asymptotic Schwarzschild solutions in $f(R)$ gravity and their observable effects on the photon sphere of black holes
• Extreme-Lensing Signatures Revealed by Correlations of Simulated Black-Hole Movies
• Optical appearance of regularized compact objects without an exterior photon sphere
• Observational distinguishability of the Kerr and Kerr-Hayward metrics to EHT