Global Structure of Accretion Flows in Sgr A*

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 $β$ 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.

💡 Research Summary

The paper investigates the large‑scale structure of the accretion flow onto the supermassive black hole at the Galactic centre, Sgr A*, by combining recent multi‑frequency radio measurements of the nearby magnetar PSR J1745‑2900 with Event Horizon Telescope (EHT) Faraday‑rotation data and X‑ray constraints. Both the pulsar (RM ≈ ‑6.7 × 10⁴ rad m⁻² at 0.12 pc) and the EHT measurement of Sgr A* (RM ≈ ‑4.6 × 10⁵ rad m⁻² at 230 GHz) imply a magnetic field strength of at least 8 mG on scales from tens to tens of thousands of Schwarzschild radii (r_S). The corresponding plasma β is far below unity, indicating that the gas is magnetically dominated at large distances.

Using these observational inputs, the authors construct a simple one‑dimensional model of accretion along magnetic field lines, essentially a Bondi‑type spherical inflow modified to include magnetic pressure. The basic equations are the hydrostatic balance in the z‑direction (including gravity), an adiabatic equation of state (γ = 5/3), and a prescribed radial dependence of the mass flux along a field line: ρ v = ρ₀ c_s (r/r₀)^α. The outer boundary is set at R = 0.4 pc with n ≈ 26 cm⁻³ and kT ≈ 3.5 keV. By comparing magnetic pressure (B²/8π) with gas pressure, they find a critical radius R₀ ≈ 3 × 10⁴ r_S where the two become equal; inside this radius gas pressure dominates.

Choosing representative parameters (r₀ = 30 r_S, α = ‑0.5, c_s ≈ 5.5 × 10⁷ cm s⁻¹, ρ₀ ≈ 4.3 × 10⁻²³ g cm⁻³) the solution exhibits two distinct regimes. For r < r₀ the flow is supersonic (Mach > 1), interpreted as a wind launched from the corona of the inner accretion flow. For r > r₀ the flow is subsonic, representing the quasi‑spherical inflow that feeds the black hole. The density and pressure profiles inside r₀ are nearly proportional, implying an almost isothermal wind, while beyond r₀ they follow the classic Bondi scalings (ρ ∝ z⁻³ᐟ², p ∝ z⁵ᐟ²).

Integrating the mass flux from r₀ to the outer limit R₀ yields the total accretion rate. For a fiducial \dot M ≈ 4 × 10¹⁷ g s⁻¹ (consistent with spectral modeling of Sgr A*), the authors derive empirical relations between α and r₀, showing that the accretion rate peaks near r₀ and declines both inward (due to the wind) and outward (because magnetic pressure prevents further inflow). This behavior naturally produces a convection‑dominated accretion flow (CDAF) in the region from a few × 10³ r_S to a few × 10⁴ r_S, while a thin, possibly magnetically arrested disk (MAD) exists at radii of a few × 10 r_S, as suggested by EHT imaging.

The paper situates its findings within the broader theoretical landscape. It reconciles aspects of MAD (strong magnetic flux accumulation near the horizon), ADIOS (mass loss via winds), and CDAF (flattened density profile due to convective turbulence) into a unified picture where large‑scale magnetic fields suppress accretion at large radii, convection dominates the intermediate zone, and a supersonic wind emerges from the inner disk. The authors stress that the model is highly idealized—ignoring radiative cooling, three‑dimensional turbulence, and detailed magnetic reconnection—but it captures the essential scaling relations implied by the observations.

In the discussion, they call for dedicated 3‑D MHD simulations that incorporate realistic stellar wind feeding, magnetic flux transport, and radiative processes to test the proposed structure. They also highlight future observational prospects: higher‑resolution RM maps from next‑generation radio interferometers, time‑resolved EHT polarimetry to track wind variability, and deep X‑ray spectroscopy to constrain gas pressure profiles. Such data would allow a stringent test of whether a supersonic wind coexists with a convection‑dominated inflow and a compact MAD disk in Sgr A*, thereby refining our understanding of low‑luminosity accretion onto supermassive black holes.