Modeling Emission from the Supermassive Black Hole in the Galactic Center with GRMHD Simulations

TL;DR

This study uses GRMHD simulations to explore if outflows from the supermassive black hole Sagittarius A* can produce the observed radio emission, revealing challenges in matching observed spectra and flux levels.

Contribution

It demonstrates that current GRMHD models struggle to reproduce the radio emission of Sgr A* without conflicting with X-ray constraints and highlights the need for more detailed radiation modeling.

Findings

High plasma beta simulations cannot match radio flux without exceeding X-ray limits.

Predicted radio spectrum is harder than observed for simple thermal and nonthermal models.

Electron temperature must be lower than gas temperature to fit the observed radio spectrum.

Abstract

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.