New Methods for Geometric Image Modeling of the Black Hole in M87

Abstract

Recent observations of the galaxy M87 by the Event Horizon Telescope (EHT) indicate that the appearance of the galactic core M87* is dominated by a ring-shaped structure. The size and shape of this ring match expectations for matter orbiting near the event horizon of a black hole weighing several billion solar masses, providing the most direct evidence to date that M87* is a supermassive black hole (SMBH). However, many details of the observational appearance remain unknown. Re-examining the public datasets with a code base built from scratch by the author, two aspects of the analysis pipeline are scrutinized: (1) approximations in the statistical likelihood functions used to fit image models to the data, and (2) the design of the model itself. An alternative (but equally valid) likelihood function is tried, and a new parametric model is introduced -- the first such model to include the ultra-thin "photon ring" component expected from gravitational lensing. Particular attention is paid to the thickness of the main annular feature, which has implications for the nature of the accretion flow around the black hole. It is shown that the width of the annulus is difficult to constrain, and is indeed sensitive to choices made in the analysis. Nevertheless a discrepancy persists between the ring width inferred from geometric modeling and that inferred from non-parametric imaging algorithms. Evidence for the presence of the photon ring is found to be inconclusive, and the photon ring degrees of freedom have little effect on the main conclusions. As the EHT array is expanded and more observations are made in the coming years, the techniques developed in this work have the potential to aid in further constraining the observational appearance of M87*.