Global three-dimensional magnetohydrodynamic simulations of black hole accretion disks: X-ray flares in the plunging region

We present the results of three-dimensional global resistive magnetohydrodynamic (MHD) simulations of black hole accretion flows. General relativistic effects are simulated by using the pseudo-Newtonian potential. The initial state is an equilibrium model of a torus threaded by weak toroidal magnetic fields. As the magnetorotational instability (MRI) grows in the torus, mass accretes to the black hole by losing angular momentum. We found that in the innermost plunging region, nonaxisymmetric accretion flow creates bisymmetric spiral magnetic fields and current sheets. Mass accretion along the spiral channel creates one-armed spiral density distribution. Since the accreting matter carries in magnetic fields that are subsequently stretched and amplified as a result of differential rotation, current density increases inside the channel. Magnetic reconnection taking place in the current sheet produces slow-mode shock waves that propagate away from the reconnection site. Magnetic energy release in the innermost plunging region could be the origin of X-ray shots observed in black hole candidates. Numerical simulations reproduced soft X-ray excess preceding the peak of the shots, X-ray hardening at the peak of the shot, and hard X-ray time lags.