Abstract
We present three-dimensional MHD simulations of rotating radiatively inefficient accretion flows onto black holes. We continuously inject magnetized matter into the computational domain near the outer boundary and run the calculations long enough for the resulting accretion flow to reach a quasi-steady state. We have studied two limiting cases for the geometry of the injected magnetic field: pure toroidal field and pure poloidal field. In the case of toroidal field injection, the accreting matter forms a nearly axisymmetric, geometrically thick, turbulent accretion disk. The disk resembles in many respects the convection-dominated accretion flows found in previous numerical and analytical investigations of viscous hydrodynamic flows. Models with poloidal field injection evolve through two distinct phases. In an initial transient phase, the flow forms a relatively flattened, quasi-Keplerian disk with a hot corona and a bipolar outflow. However, when the flow later achieves steady state, it changes in character completely. The magnetized accreting gas becomes two-phase, with most of the volume being dominated by a strong dipolar magnetic field from which a thermal low-density wind flows out. Accretion occurs mainly via narrow slowly rotating radial streams that "diffuse" through the magnetic field with the help of magnetic reconnection events.
Highlights
This study is a continuation of our efforts during the last several years to understand the nature of underluminous accreting black holes
By ADAFs one understands optically thin accretion flows with very low mass accretion rates, M ≪ M Edd, but ADAFs are possible as optically thick flows with high accretion rates, M > M Edd, where M Edd is the Eddington accretion rate
We argue that the toroidal field case has many points of similarity with the convection-dominated accretion flow’ (CDAF) model (§4.1), while the poloidal field case behaves very differently (§4.2)
Summary
This study is a continuation of our efforts during the last several years to understand the nature of underluminous accreting black holes. A class of radiatively inefficient solutions (Ichimaru 1977; Rees et al 1982; Narayan & Yi 1994, 1995a,b; Abramowicz et al 1995; Narayan, Mahadevan & Quataert 1998) has been influential in this field. A key feature of these solutions is that radiative energy losses are small so that most of the energy is advected with the gas. Narayan & Yi (1994) came up with the name ‘advection-dominated accretion’ to describe such flows, and Lasota (1996) suggested the acronym ADAF (advectiondominated accretion flow). One uses the name ‘thick accretion disk’ Jaroszynski, Abramowicz & Paczynski, 1980) or ‘slim accretion disk’ (Abramowicz et al, 1988) rather than ADAF One uses the name ‘thick accretion disk’ (see e.g. Jaroszynski, Abramowicz & Paczynski, 1980) or ‘slim accretion disk’ (Abramowicz et al, 1988) rather than ADAF
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