Numerical Simulation of Hot Accretion Flows (II): Nature, Origin, and Properties of Outflow and Their Possible Observational Applications

Abstract

Previous hydrodynamical (HD) and magnetohydrodynamical (MHD) numerical simulations of hot accretion flows have shown that the mass accretion rate decreases with decreasing radius. Two models have been proposed to explain this result. In the ADIOS model, the inward decrease of accretion rate is because of the loss of gas in the outflow. In the CDAF model, the gas is assumed to be locked in convective eddies, which results in the inward decrease of the accretion rate. We investigate the nature of inward decrease of accretion rate using HD and MHD simulations. We calculate various properties of inflow and outflow, including the mass flux, radial and rotational velocities, temperature, and the Bernoulli parameter ($Be$). Systematic and significant differences between inflow and outflow are found. These results suggest that the inflow and outflow are not dominated by convective turbulence, but are systematic inward and outward motion. We have also analyzed the convective stability of MHD accretion flow and found that they are convectively stable. These results indicate that the ADIOS scenario is favored. The different properties of inflow and outflow also suggest that the mechanisms of producing outflow in HD and MHD flows are buoyancy and centrifugal force associated with the magnetic field, respectively. The latter mechanism is similar to the Blandford & Payne mechanism. We also study the effect of initial conditions in the simulations. We find that the value of $Be$ is mainly determined by the value of $Be$ of the initial condition. We discuss some possible observational applications of our outflow model. These observations include the Fermi bubble observed in the Galaxy center, and winds widely observed in AGNs and black hole X-ray binaries.