The Power of Imaging: Constraining the Plasma Properties of GRMHD Simulations using EHT Observations of Sgr A*

TL;DR

This study demonstrates that high-resolution imaging from the Event Horizon Telescope can effectively constrain the thermodynamic models of black-hole accretion flows, surpassing spectral data alone.

Contribution

The paper introduces a GPU-accelerated ray-tracing approach to evaluate multiple GRMHD simulations against EHT observations, effectively constraining accretion flow models.

Findings

Spectral data alone cannot distinguish models with different plasma thermodynamics.

EHT imaging rules out models with strong funnel emission due to their extended size.

Images provide critical constraints on accretion flow properties.

Abstract

Recent advances in general relativistic magnetohydrodynamic simulations have expanded and improved our understanding of the dynamics of black-hole accretion disks. However, current simulations do not capture the thermodynamics of electrons in the low density accreting plasma. This poses a significant challenge in predicting accretion flow images and spectra from first principles. Because of this, simplified emission models have often been used, with widely different configurations (e.g., disk- versus jet-dominated emission), and were able to account for the observed spectral properties of accreting black-holes. Exploring the large parameter space introduced by such models, however, requires significant computational power that exceeds conventional computational facilities. In this paper, we use GRay, a fast GPU-based ray-tracing algorithm, on the GPU cluster El Gato, to compute images and spectra for a set of six general relativistic magnetohydrodynamic simulations with different magnetic field configurations and black-hole spins. We also employ two different parametric models for the plasma thermodynamics in each of the simulations. We show that, if only the spectral properties of Sgr A* are used, all twelve models tested here can fit the spectra equally well. However, when combined with the measurement of the image size of the emission using the Event Horizon Telescope, current observations rule out all models with strong funnel emission, because the funnels are typically very extended. Our study shows that images of accretion flows with horizon-scale resolution offer a powerful tool in understanding accretion flows around black-holes and their thermodynamic properties.