GRMHD simulations of visibility amplitude variability for Event Horizon Telescope images of Sgr A*

TL;DR

This study uses high-cadence GRMHD simulations to analyze the variability of visibility amplitudes in EHT images of Sgr A*, revealing how accretion flow dynamics affect image interpretation.

Contribution

It demonstrates the limitations of stationary image assumptions in EHT data analysis by characterizing variability patterns in simulated visibility amplitudes.

Findings

Visibility amplitudes decrease smoothly perpendicular to the spin axis.

Significant structure and minima are observed parallel to the spin axis.

Variability impacts the accuracy of inferring black hole parameters.

Abstract

Synthesis imaging of the black hole in the center of the Milky Way, Sgr A*, with the Event Horizon Telescope (EHT) rests on the assumption of a stationary image. We explore the limitations of this assumption using high-cadence GRMHD simulations of Sgr A*. We employ analytic models that capture the basic characteristics of the images to understand the origin of the variability in the simulated visibility amplitudes. We find that, in all simulations, the visibility amplitudes for baselines oriented perpendicular to the spin axis of the black hole typically decrease smoothly over baseline lengths that are comparable to those of the EHT. On the other hand, the visibility amplitudes for baselines oriented parallel to the spin axis show significant structure with one or more minima. This suggests that fitting EHT observations with geometric models will lead to reasonably accurate determination of the orientation of the black-hole on the plane of the sky. However, in the disk-dominated models, the locations and depths of the minima in the visibility amplitudes depend primarily on the width and asymmetry of the crescent-like images and are highly variable. In the jet-dominated models, the locations of the minima are determined by the separation of the two image components but their depths depend primarily on the relative brightness of the two components and are also variable. This suggests that using time-independent models to infer additional black-hole parameters, such as the shadow size or the spin magnitude, will be severely affected by the variability of the accretion flow.