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

Abstract

Previous MHD simulations have shown that wind (i.e., uncollimated outflow) must exist in black hole hot accretion flows. In this paper, we continue our study by investigating the detailed properties of wind, such as mass flux and poloidal speed, and the mechanism of wind production. For this aim, we make use of a three dimensional GRMHD simulation of hot accretion flows around a Schwarzschild black hole. The simulation is designed so that the magnetic flux is not accumulated significantly around the black hole. To distinguish real wind from turbulent outflows, we track the trajectories of the virtual Largrangian particles from simulation data. We find two types of real outflows, i.e., a quasi-relativistic jet close to the axis and a sub-relativistic wind subtending a much larger solid angle. We confirm that the mass flux of wind is very significant and most of the wind originates from the surface layer of the accretion flow. The radial profile of the wind mass flux can be described byṀ wind ≈Ṁ BH (r/20r s ), withṀ BH being the mass accretion rate at the black hole horizon and r s being the Schwarzschild radius. The poloidal wind speed almost remains constant once they are produced, but the fluxweighted wind speed roughly follows v p,wind (r) ≈ 0.25v k (r), with v k (r) being the Keplerian speed at radius r. The mass flux of jet is much lower but the speed is much higher, v p,jet ∼ (0.3 − 0.4)c. Consequently, both the energy and momentum fluxes of the wind are much larger than those of the jet. We find that the wind is produced and accelerated primarily by the combination of centrifugal force and magnetic pressure gradient, while the jet is mainly accelerated by magnetic pressure gradient. Finally, we find that the wind production efficiency ǫ wind ≡Ė wind /Ṁ BH c 2 ∼ 1/1000, in good agreement with the value required from large-scale galaxy simulations with AGN feedback.

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

“…In those works, they also found that inflow is present around the equatorial plane of the accretion flow while wind is present in the polar region. The boundary between inflow and wind obtained in the present paper is even quantitatively similar to that obtained in Yuan et al (2015). In both cases the boundary is located at about θ ∼ 50 • − 60 • .…”

“…3). This structure is same as that obtained in previous numerical simulation works (e.g., Yuan et al 2012;Narayan et al 2012;Sadowski et al 2013;Yuan et al 2015). We have also calculated some properties of winds and found good consistency with those obtained in Yuan et al (2015) and Yuan et al (2012), which are based on the MHD numerical simulation results.…”

“…The strength of the three forces are comparable. This result is again fully consistent with that found in Yuan et al (2015), although here we have an ordered large-scale magnetic field while the field in Yuan et al (2015) is tangled. Jiao & Wu (2011) found that the driving forces are the gas pressure gradient and centrifugal force because they do not include magnetic field in their model.…”

“…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).…”

“…Special attention is paid to whether the winds exist in the solution and what are their driving mechanisms if they exist. The aim is to compare with the numerical results presented in Yuan et al (2015) thus to improve our understanding of numerical studies.…”