  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.

High Energy Astrophysical Phenomena