Oral Presentation
Global Three-dimensional Radiation MagnetoHydroDynamic Simulations of Hard-to-Soft Transitions in Stellar Mass Black Hole Candidates.
Presenter: Taichi Igarashi (Chiba Univ.)
Black hole candidates show transitions between X-ray hard state and X-ray soft stat. The X-ray hard state in which hard X-ray emission is dominant can be explained by an optically thin, hot, Radiatively Inefficient Accretion Flow (RIAF). On the other hand, X-ray soft state corresponds to an optically thick, cold, standard disk. The hard-to-soft transition takes place when the accretion rate exceeds the threshold for the onset of the cooling instability in RIAF.
Machida et al. (2006) carried out three-dimensional global MHD simulations of the Hard-to-Soft transition by taking into account the radiative cooling. They found that magnetic pressure dominant (i.e. low-β) disk is formed when the disk shrinks in vertical direction by cooling. Machida et al.(2006) could not study further evolution partly because the simulation code became numerically unstable in low-β region, and partly because they assumed optically thin radiative cooling. We updated the MHD code by applying HLLD scheme as Riemann solver and MP5 scheme to achieve 5th accuracy in space. The MHD code CANS+ can handle the region where beta ~0.01. We combined CANS+ with the radiation module based on M1-closure scheme developed by Takahashi and Ohsuga (2013).
We applied the radiation MagnetoHydroDynamic code CANS+R for simulations of black hole accretion flows with moderately high accretion rate, and found that optically thin, hot disk near the black hole co-exist with optically thick cold disk in the outer region. We also reproduced low frequency QPOs observed during the Hard-to-Soft transition.
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