Ab Initio Horizon-Scale Simulations of Magnetically Arrested Accretion in Sagittarius A* Fed by Stellar Winds

TL;DR

This paper presents horizon-scale GRMHD simulations of Sagittarius A*'s accretion flow, driven by stellar wind properties, revealing a magnetically arrested disk with variable 230 GHz images and coherent polarization.

Contribution

It introduces ab initio simulations of accretion onto Sagittarius A* based on stellar wind data, reducing free parameters and linking large-scale winds to horizon-scale phenomena.

Findings

Formation of a magnetically arrested flow near the horizon

Simulated 230 GHz images match observational constraints

Polarization properties are consistent with weak Faraday rotation

Abstract

We present 3D general relativistic magnetohydrodynamic (GRMHD) simulations of the accretion flow surrounding Sagittarius A* that are initialized using larger-scale MHD simulations of the $\sim$ 30 Wolf--Rayet (WR) stellar winds in the Galactic center. The properties of the resulting accretion flow on horizon scales are set not by ad hoc initial conditions but by the observationally constrained properties of the WR winds with limited free parameters. For this initial study we assume a non-spinning black hole. Our simulations naturally produce a $\sim 10^{-8} M_\odot$ yr$^{-1}$ accretion rate, consistent with previous phenomenological estimates. We find that a magnetically arrested flow is formed by the continuous accretion of coherent magnetic field being fed from large radii. Near the event horizon, the magnetic field is so strong that it tilts the gas with respect to the initial angular momentum and concentrates the originally quasi-spherical flow to a narrow disk-like structure. We also present 230 GHz images calculated from our simulations where the inclination angle and physical accretion rate are not free parameters but are determined by the properties of the WR stellar winds. The image morphology is highly time variable. Linear polarization on horizon scales is coherent with weak internal Faraday rotation.