Global Radiation-Magnetohydrodynamic Simulations of Black-Hole Accretion Flow and Outflow: Unified Model of Three States

Abstract

Black-hole accretion systems are known to possess several distinct modes (or spectral states), such as low/hard state, high/soft state, and so on. Since the dynamics of the corresponding flows is distinct, theoretical models were separately discussed for each state. We here propose a unified model based on our new, global, two-dimensional radiation-magnetohydrodynamic simulations. By controlling a density normalization we could for the first time reproduce three distinct modes of accretion flow and outflow with one numerical code. When the density is large (model A), a geometrically thick, very luminous disk forms, in which photon trapping takes place. When the density is moderate (model B), the accreting gas can effectively cool by emitting radiation, thus generating a thin disk, i.e., the soft-state disk. When the density is too low for radiative cooling to be important (model C), a disk becomes hot, thick, and faint; i.e., the hard-state disk. The magnetic energy is amplified within the disk up to about twice, 30%, and 20% of the gas energy in models A, B, and C, respectively. Notably, the disk outflows with helical magnetic fields, which are driven either by radiation pressure force or magnetic pressure force, are ubiquitous in any accretion modes. Finally, our simulations are consistent with the phenomenological α-viscosity prescription, that is, the disk viscosity is proportional to the pressure.

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.…”

“…Taam & Lin 1984) have also been suggested. However, as illustrated in Figure 1, recent radiation MHD simulations of MRI turbulence find that the stress scales best with total thermal pressure, at least on long time scales (Ohsuga et al 2009;). The alpha prescription (2) is usually used to solve for the radial structure of vertically-integrated geometrically thin or slim accretion disks, in which case it enters the equations through the vertically-integrated stress:…”

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

“…Here, the soft component is associated with an optically thick wind ejected by the disc when the accretion rate locally exceeds the Eddington limit (e.g. Poutanen et al 2007;Ohsuga et al 2009;Takeuchi, Ohsuga & Mineshige 2014). The Comptonization component originates from the hot, inner accretion disc (e.g.…”

“…The soft component is thought to originate from the photosphere of a radiatively driven wind that is ejected from the spherization radius of the accretion disc down to its innermost regions, which is expected to occur at super-Eddington accretion rates (e.g. Shakura & Sunyaev 1973;Ohsuga & Mineshige 2007;Poutanen et al 2007;Ohsuga et al 2009). Partially inhomogeneous winds in ULXs may explain the puzzling short-term variability reported in some ULXs (Heil, Vaughan & Roberts 2009), through a combination of inclination angle and accretion rate (e.g.…”

Evidence for a radiatively driven disc-wind in PDS 456?

“…The possible sceCorrespondence to: gabriele.matzeu@brera.inaf.it narios are, radiatively driven winds (e.g., Proga, Stone & Kallman 2000;Proga & Kallman 2004) and/or magneto-centrifugally driven winds (MHD wind hereafter, e.g. Ohsuga et al 2009;Kazanas et al 2012). Most outflow studies so far mainly concentrated on radiatively driven winds (e.g., Sim et al 2010;Reeves et al 2014;Hagino et al 2015;Nomura & Ohsuga 2017).…”