Abstract
We present results from two long-duration GRMHD simulations of advection-dominated accretion around a non-spinning black hole. The first simulation was designed to avoid significant accumulation of magnetic flux around the black hole. This simulation was run for a time of 200,000GM/c^3 and achieved inflow equilibrium out to a radius \sim90GM/c^2. Even at this relatively large radius, the mass outflow rate \dot{M}_{out} is found to be only 60% of the net mass inflow rate \dot{M}_{BH} into the black hole. The second simulation was designed to achieve substantial magnetic flux accumulation around the black hole in a magnetically arrested disc. This simulation was run for a shorter time of 100,000GM/c^3. Nevertheless, because the mean radial velocity was several times larger than in the first simulation, it reached inflow equilibrium out to a radius \sim170GM/c^2. Here, \dot{M}_{out} becomes equal to \dot{M}_{BH} at r\sim 160GM/c^2. Since the mass outflow rates in the two simulations do not show robust convergence with time, it is likely that the true outflow rates are lower than our estimates. The effect of black hole spin on mass outflow remains to be explored. Neither simulation shows strong evidence for convection, though a complete analysis including the effect of magnetic fields is left for the future.
Highlights
Thin discs are present around stellar-mass and supermassive Black hole (BH) that accrete at a substantial fraction ∼ a few to 100 per cent of the Eddington rate, while advectiondominated accretion flow (ADAF) are typically found at lower accretion rates M
The strong averaging of the simulation data eliminates most of the turbulent fluctuations that were evident in Fig. 4, and enables us to focus on mean properties of the flow
With respect to the latter, we have considered two very different limits: (1) an ADAF/SANE simulation (SANE = ‘standard and normal evolution’), which is a good proxy for an ADAF model in which the magnetic field is merely an agent that causes angular momentum transport (‘viscosity’) but plays no important dynamical role, and (2) an ADAF/MAD simulation (MAD = ‘magnetically arrested disc’), where the magnetic field is strong enough to alter substantially the dynamics of the gas and to drive the system to a magnetically arrested state (Igumenshchev et al 2003; Narayan et al 2003; Tchekhovskoy et al 2011; McKinney et al 2012)
Summary
Black hole (BH) accretion occurs via at least two distinct modes: (1) a standard thin accretion disc (Shakura & Sunyaev 1973; Novikov & Thorne 1973; Frank, King & Raine 2002), and (2) an advectiondominated accretion flow (ADAF; Narayan & Yi 1994, 1995b; Abramowicz et al 1995; Ichimaru 1977; see Narayan, Mahadevan & Quataert 1998; Frank et al 2002; Kato, Fukue & Mineshige 2008; Narayan & McClintock 2008 for reviews). Analytical models of convection-dominated accretion flows (CDAFs; Narayan, Igumenshchev & Abramowicz 2000; Quataert & Gruzinov 2000b) have been developed, but their relevance to real ADAFs is unclear (see Narayan et al 2002; Balbus & Hawley 2002 for conflicting views) Both mass-loss and convection involve multi-dimensional flows, which are best studied via numerical simulations. A simulation by Igumenshchev, Narayan & Abramowicz (2003), which was initialized with purely toroidal magnetic field, showed significant convection, and appeared to be similar to a CDAF. The poloidal case led to a configuration in which the magnetic field strongly resisted the accreting gas, leading to what the authors later called a ‘magnetically arrested disc’ (MAD; Narayan, Igumenshchev & Abramowicz 2003).
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