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 simulations of hot accretion flows have indicated that the inflow (gas with inward radial velocity) accretion rate decreases with decreasing radius. Two models have been proposed to explain this result. In the adiabatic inflowoutflow solution (ADIOS), the inward decrease of accretion rate is because of the loss of gas in the outflow. In the alternative convection-dominated accretion flow (CDAF) model, the accretion flow is thought to be convectively unstable; the gas is then assumed to be locked in convective eddies, which results in the inward decrease of the accretion rate. In the present paper 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. For example, for HD flows, the temperature of outflow is significantly higher than inflow; while for MHD flows, the specific angular momentum of outflow is nearly Keplerian, which is significantly higher than inflow. These results suggest that the inflow and outflow are not dominated by convective turbulence, but they 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 inward decrease of inflow rate is because of the mass loss in outflow. The different properties of inflow and outflow also suggest that the mechanisms of producing outflow in HD and MHD flows are buoyancy associated with the convection and centrifugal force associated with the angular momentum transport mediated by the magnetic field, respectively. The latter mechanism is similar to the Blandford & Payne mechanism but no large-scale open magnetic field is required; it is kind of "micro-Blandford & Payne" mechanism. We also study the effect of initial conditions in the simulations. We find that the value of Be, whose sign determines whether the outflow can escape to infinity or not, is mainly determined by the value of Be of the initial condition. We discuss some possible observational applications, including the Fermi bubble observed in the Galaxy center and winds widely observed in active galactic nuclei and black hole X-ray binaries.

Two-dimensional inflow-wind solution of black hole accretion with an evenly symmetric magnetic field

“…This result means that wind can transfer the angular momentum of the accretion flow. This result is again consistent with the numerical simulation result presented in Yuan et al (2012a).…”

“…We integrate these equations from the equatorial plane (θ = 90 • ) towards the rotational axis (θ = 0 • ). We adopt the values of β θ0 and β φ0 with reference to the numerical simulation data of Yuan et al (2012a), but our results are not sensitive to the exact values. As we have explained before, the toroidal component of the magnetic field, which is produced by shear, should be negative.…”

“…As a comparison, Yuan et al (2012a) find vp,term(r) ≈ 0.5vK (r) while Yuan et al (2015) find vp,term(r) ∼ (0.2 − 0.4)vK (r).…”

“…As a comparison, the numerical simulation result obtained in Yuan et al (2012a;eq. 19) is that Be(r)/v 2 K (r) ≈ (0.1−0.2).…”

“…So the present result is again in good agreement with that obtained by numerical simulations 5 . Now, as in Yuan et al (2012a), we try to estimate the terminal poloidal velocity of wind based on Be(r). To do this, we assume that, once launched, the viscous stress in the wind can be ignored and Be(r) is conserved.…”