The Submillimeter Bump in Sgr A* from Relativistic MHD Simulations

TL;DR

This study uses relativistic MHD simulations to interpret millimeter observations of Sgr A*, constraining the black hole's inclination, accretion rate, and electron temperature, and predicting observable signatures of the black hole shadow.

Contribution

It provides the first detailed comparison of 3D relativistic MHD models with high-resolution millimeter observations of Sgr A*, constraining key physical parameters and predicting observational signatures.

Findings

Inclination angle estimated at 50+35-15 degrees.

Black hole shadow likely detectable at 1.3mm and 0.87mm baselines.

Millimeter flaring caused by magnetic turbulence in the accretion flow.

Abstract

Recent high resolution observations of the Galactic center black hole allow for direct comparison with accretion disk simulations. We compare two-temperature synchrotron emission models from three dimensional, general relativistic magnetohydrodynamic simulations to millimeter observations of Sgr A*. Fits to very long baseline interferometry and spectral index measurements disfavor the monochromatic face-on black hole shadow models from our previous work. Inclination angles \le 20 degrees are ruled out to 3 \sigma. We estimate the inclination and position angles of the black hole, as well as the electron temperature of the accretion flow and the accretion rate, to be i=50+35-15 degrees, \xi=-23+97-22 degrees, T_e=(5.4 +/- 3.0)x10^10 K and Mdot=(5+15-2)x10^-9 M_sun / yr respectively, with 90% confidence. The black hole shadow is unobscured in all best fit models, and may be detected by observations on baselines between Chile and California, Arizona or Mexico at 1.3mm or .87mm either through direct sampling of the visibility amplitude or using closure phase information. Millimeter flaring behavior consistent with the observations is present in all viable models, and is caused by magnetic turbulence in the inner radii of the accretion flow. The variability at optically thin frequencies is strongly correlated with that in the accretion rate. The simulations provide a universal picture of the 1.3mm emission region as a small region near the midplane in the inner radii of the accretion flow, which is roughly isothermal and has \nu/\nu_c ~ 1-20, where \nu_c is the critical frequency for thermal synchrotron emission.