Global Structure of Accretion Flows in Sgr A*

TL;DR

This paper models the structure of accretion flows in Sgr A*, revealing a transition from magnetic pressure dominance to convection-dominated flow, with implications for the observed accretion disk and wind near the black hole.

Contribution

It introduces a model of accretion flow structure in Sgr A* that accounts for magnetic fields, wind formation, and convection from large to small scales.

Findings

Gas begins to be captured within tens of thousands of Schwarzschild radii.

A supersonic wind is expected above the small accretion disk.

The flow may be convection-dominated over a wide radial range.

Abstract

Sagittarius A* (Sgr A*) is a compact radio source at the Galactic center. Observations have confirmed that its mass is approximately (4.1)*10$^{6}$ M$_{\odot}$, and Sgr A* is generally believed to be powered by gas accretion onto a supermassive black hole. Multifrequency radio observations of the pulsar J1745-2900, about 0.12 pc away from Sgr A*, reveal an unusually large Faraday rotation. Combined with X-ray observations, this indicates that there is a strong magnetic field (greater than 8 mG) leading to a low $\beta$ plasma at large scales.We show that the gas starts to be captured by the black hole below tens of thousands of the Schwarzschild radii $r_S$, where the gas pressure starts to dominate. Assuming that the accretion rate along magnetic fields at large scales decreases with the distance to the black hole following a power law, it is shown that, with an accretion disk below tens of $r_S$, as revealed with the EHT observations, there should be a supersonic wind above such a small accretion disk, and the accretion flow may be convection-dominated from tens of $r_S$ to tens of thousands of $r_S$. Detailed modeling is warranted.