Modeling Emission from the Supermassive Black Hole in the Galactic
  Center with GRMHD Simulations
  Sagittarius A* is a compact radio source at the Galactic center, powered by
accretion of fully ionized plasmas into a supermassive black hole. However, the
radio emission cannot be produced through the thermal synchrotron process by a
gravitationally bounded flow. General relativistic
magneto-hydrodynamical(GRMHD) simulations of black hole accretion show that
there are strong unbounded outflows along the accretion. With the flow
structure around the black hole given by GRMHD simulations, we investigate
whether thermal synchrotron emission from these outflows may account for the
observed radio emission. We find that simulations producing relatively high
values of plasma beta cannot produce the radio flux level without exceeding the
X-ray upper limit set by Chandra observations through the bremsstrahlung
process. The predicted radio spectrum is also harder than the observed spectrum
both for the one temperature thermal model and a simple nonthermal model with a
single power-law electron distribution. The electron temperature needs to be
lower than the gas temperature near the black hole to reproduce the observed
radio spectrum. A more complete modeling of the radiation processes, including
the general relativistic effects and transfer of polarized radiation, will give
more quantitative constraints on physical processes in Sgr A* with the current
multi-wavelength, multi-epoch, and polarimetric observations of this source.
