Numerical Simulation of Hot Accretion Flows. III. Revisiting Wind Properties Using the Trajectory Approach

Abstract

Previous MHD simulations have shown that wind must exist in black hole hot accretion flows. In this paper, we continue our study by investigating the detailed properties of wind and the mechanism of wind production. For this aim, we make use of a 3D general relativistic MHD simulation of hot accretion flows around a Schwarzschild black hole. To distinguish real wind from turbulent outflows, we track the trajectories of the virtual Lagrangian particles from simulation data. We find two types of real outflows, i.e., a jet and a wind. The mass flux of wind is very significant, and its radial profile can be described by M Mr r » ( ) ˙ ˙ wind BH s 20 , with M˙BH being the mass accretion rate at the black hole horizon and rs being the Schwarzschild radius. The poloidal wind speed almost remains constant once they are produced, but the flux-weighted wind speed roughly follows v r vr p,wind ( ) 0.25 ( ) » k , with vk(r) being the Keplerian speed at radius r. The mass flux of the jet is much lower, but the speed is much higher, vp,jet ∼ (0.3–0.4)c. Consequently, both the energy and momentum fluxes of the wind are much larger than those of the jet. The wind is produced and accelerated primarily by the combination of centrifugal force and magnetic pressure gradient, while the jet is mainly accelerated by the magnetic pressure gradient. Finally, we find that the wind production efficiency  wind wind BH º ~ E Mc ˙ ˙ 1 1000 2 is in good agreement with the value required from large-scale galaxy simulations with active galactic nucleus feedback.