Astronomy and Astrophysics – Astrophysics – High Energy Astrophysical Phenomena
Scientific paper
2009-03-31
Astronomy and Astrophysics
Astrophysics
High Energy Astrophysical Phenomena
5 pages, 2 figures, accepted for publication in PASJ Letters
Scientific paper
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 alpha-viscosity prescription, that is, the disk viscosity is proportional to the pressure.
Kato Yoshiaki
Mineshige Shin
Mori Masao
Ohsuga Ken
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