NUMERICAL SIMULATION OF HOT ACCRETION FLOWS. II. NATURE, ORIGIN, AND PROPERTIES OF OUTFLOWS AND THEIR POSSIBLE OBSERVATIONAL APPLICATIONS

Abstract

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