Global Structure of Three Distinct Accretion Flows and Outflows Around Black Holes From Two-Dimensional Radiation-Magnetohydrodynamic Simulations

Abstract

We present the detailed global structure of black hole accretion flows and outflows through newly performed two-dimensional radiation-magnetohydrodynamic simulations. By starting from a torus threaded with weak toroidal magnetic fields and by controlling the central density of the initial torus, ρ 0 , we can reproduce three distinct modes of accretion flow. In model A with the highest central density, an optically and geometrically thick supercritical accretion disk is created. The radiation force greatly exceeds the gravity above the disk surface, thereby driving a strong outflow (or jet). Because of the mild beaming, the apparent (isotropic) photon luminosity is ∼ 22L E (where L E is the Eddington luminosity) in the face-on view. Even higher apparent luminosity is feasible if we increase the flow density. In model B with a moderate density, radiative cooling of the accretion flow is so efficient that a standard-type, cold, and geometrically thin disk is formed at radii greater than ∼ 7R S (where R S is the Schwarzschild radius), while the flow is radiatively inefficient otherwise. The magnetic-pressure-driven disk wind appears in this model. In model C the density is too low for the flow to be radiatively efficient. The flow thus becomes radiatively inefficient accretion flow, which is geometrically thick and optically thin. The magnetic-pressure force, in cooperation with the gas-pressure force, drives outflows from the disk surface, and the flow releases its energy via jets rather than via radiation. Observational implications are briefly discussed.

General Overview of Black Hole Accretion Theory

“…Unfortunately, this still leaves major unanswered questions as to the global structure of the flow. Two dimensional (axisymmetric) global simulations have also been done which have confirmed the existence of discrete flow states (Ohsuga et al 2009;Ohsuga & Mineshige 2011, see also Chapters 2.4 and 5.3), but these cannot be run over long time scales as MRI turbulence cannot be sustained in axisymmetry. With the ongoing increase of computer power, combined with the development of new radiation transport algorithms, it should be possible to do global 3D simulations of accretion flows in optically thick regimes too.…”

The ultraluminous X-ray source NGC 5643 ULX1: a large stellar mass black hole accreting at super-Eddington rates?

“…Stobbart et al 2006;Roberts 2007;Gladstone et al 2009;Heil et al 2009;Middleton et al 2011;Sutton et al 2013b;Pintore et al 2014;Middleton et al 2015). If the power-law-dominated spectra of NGC 5643 ULX1 are indeed simply due to the poor data quality and the intrinsic spectra are actually similar to the highest quality observation, then the marginally different levels of short-term variability could be explained by the effects of an outflow ejected by the accretion disc at radii where it is locally super-Eddington (Poutanen et al 2007;Ohsuga & Mineshige 2011;Takeuchi et al 2014). If the wind is turbulent, it could increase the short-term variability when our line of sight encounters turbulences in the wind (Takeuchi et al 2014;Middleton et al 2015).…”

An Optically Thick Disk Wind in Gro J1655–40?

“…Recent radiation (magneto-)hydrodynamic simulations predict radiation-pressure-dominated, optically thick disk winds to be driven in supercritical accretion disks (e.g., Ohsuga et al 2005;Dotan & Shaviv 2011;Ohsuga & Mineshige 2011). Although the ionized Fe-K absorption lines like those observed in Galactic BH binaries have never been observed in ultraluminous X-ray sources (ULXs; Walton et al 2012Walton et al , 2013, Middleton et al (2014) argued that uneven spectral structures often seen at »1 keV could be explained by broad blueshifted absorption lines.…”