Radiative Models of Sgr A* from GRMHD Simulations

TL;DR

This paper develops radiative models of Sgr A* using GRMHD simulations, analyzing how black hole spin, inclination, and temperature ratios affect observable spectra and images, with implications for event horizon visibility.

Contribution

The study introduces a comprehensive GRMHD-based radiative modeling framework for Sgr A* that incorporates synchrotron emission, absorption, and Compton scattering, exploring parameter effects on observables.

Findings

Models with Ti/Te=1 do not match submillimeter spectral slope.

X-ray flux increases with black hole spin and inclination.

Most consistent models have a* around 0.9 and reveal the event horizon at 230 GHz.

Abstract

Using flow models based on axisymmetric general relativistic magnetohydrodynamics (GRMHD) simulations, we construct radiative models for sgr A*. Spectral energy distributions that include the effects of thermal synchrotron emission and absorption, and Compton scattering, are calculated using a Monte Carlo technique. Images are calculated using a ray-tracing scheme. All models are scaled so that the 230 GHz flux density is 3.4 Jy. The key model parameters are the dimensionless black hole spin a*, the inclination i, and the ion-to-electron temperature ratio Ti/Te. We find that: (1) models with Ti/Te=1 are inconsistent with the observed submillimeter spectral slope; (2) the X-ray flux is a strongly increasing function of a*; (3) the X-ray flux is a strongly increasing function of i; (4) 230 GHz image size is a complicated function of i, a*, and Ti/Te, but the Ti/Te = 10 models are generally large and at most marginally consistent with the 230 GHz VLBI data; (5) for models with Ti/Te=10 and i=85 deg the event horizon is cloaked behind a synchrotron photosphere at 230 GHz and will not be seen by VLBI, but these models overproduce NIR and X-ray flux; (6) in all models whose SEDs are consistent with observations the event horizon is uncloaked at 230 GHz; (7) the models that are most consistent with the observations have a* \sim 0.9. We finish with a discussion of the limitations of our model and prospects for future improvements.