Global Structure of Accretion Flows in Sgr A*

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 $β$ 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.

Figures (6)

• Figure 1: The cylindrical coordinates adopted in this study. The magnetic field $\vec{B}$ is in the $z$ direction. $\vec{r}$ is the distance of magnetic field line to the origin, where the black hole locates. $\vec{R}$ indicates the location of a gas element, that moves along a magnetic field line with a velocity $\vec{v}$ in the magnetic pressure dominated region at large radii.
• Figure 2: Locations where the gas pressure is equal to the magnetic pressure for $B=8$ mG. Left: For the case of pressure equilibrium. Right: For an accretion model with $r_0=30\ r_S$, $\alpha=-0.5$.
• Figure 3: Dependence of $r_0$ on $\alpha$ for given values of $\dot{M}$. The lines show binomial exponential fits to the data. Left: For $\dot{M}=4\times10^{17}$ g s$^{-1}$. The fitting function is $\log_{10}(r_0/r_S)=-15.74e^{2.009\alpha}+3.179e^{-0.04702\alpha}$. Right: For $\dot{M}=4\times10^{18}$ g s$^{-1}$. The fitting function is $\log_{10}(r_0/r_S)=-9.284e^{11.92\alpha}+2.782e^{-0.4012\alpha}$.
• Figure 4: Mach number (upper panels), gas density (lower left) and gas pressure (lower right) profiles with $\rho_{0}=4.3\times 10^{-23}$ g cm$^{-3}$ and $kT = 3.5$ keV at $R=0.4$ pc, $r_0=30\ r_S$, $\alpha=-0.5$, $\gamma={5/3}$. Top left: Mach number for $r=1,5,10,15,20,25\ r_S$. Top right: Mach number for $r=30,10^2,10^3,10^4,10^5\ r_S$. Bottom left: gas density for $r=1,5,10,15,20,25,30,10^2,10^3,10^4,10^5\ r_S$. Bottom right: gas pressure for $r=1,5,10,15,20,25,30,10^2,10^3,10^4,10^5\ r_S$. Solid lines are the gas pressure, and the dashed line is the magnetic pressure for $B=8$ mG.
• Figure 5: $\dot{M}$ as a function of $r$ .