Global Structures of Optically Thin Black Hole Accretion Flows Obtained from Direct Magnetohydrodynamic Simulations

Abstract

We studied the global structures of radiatively inefficient black hole accretion flows by three-dimensional magnetohydrodynamic simulations. The initial state is a constant angular-momentum torus threaded by weak toroidal magnetic fields. The torus is assumed to be embedded in a low-density, spherical, isothermal halo. We found that in the innermost region ($\varpi < 15 \,r_g$: $r_g$ is the Schwarzschild radius) of optically thin hot disks formed by accretion from the torus, the radial structure can be described by a one-dimensional steady transonic ADAF (Advection Dominated Accretion Flow) solution that takes into account the radial dependence of $\alpha$. Numerical results indicate that the magnetic flux emerging from the disk creates a magnetically active layer in which magnetic reconnection generates outflows along magnetic field lines. We also studied the dependence of the disk structure on the external pressure of the initial torus. The saturation level of magnetic field amplification is higher for a model with a larger external pressure. Thus, external pressure enhances the magnetic activity of the disk. When the external pressure is 5–20% of the maximum pressure of the torus, the kinetic and Poynting energy flux of the outflow is about 1% of the energy flux advected to the black hole through accretion.