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 resistivemagnetohydrodynamic (MHD) simulations of black hole accretion flows. Generalrelativistic effects are simulated by using the pseudo-Newtonian potential.Initial state is an equilibrium model of a torus threaded by weak toroidalmagnetic fields. As the magnetorotational instability (MRI) grows in the torus,mass accretes to the black hole by losing the angular momentum. We found thatin the innermost plunging region, non-axisymmetric accretion flow createsbisymmetric spiral magnetic fields and current sheets. Mass accretion along thespiral channel creates one armed spiral density distribution. Since theaccreting matter carries in magnetic fields which subsequently are stretchedand amplified due to differential rotation, current density increases insidethe channel. Magnetic reconnection taking place in the current sheet producesslow mode shock waves which propagate away from the reconnection site. Magneticenergy release in the innermost plunging region can be the origin of X-rayshots observed in black hole candidates. Numerical simulations reproduced softX-ray excess preceding the peak of the shots, X-ray hardening at the peak ofthe shot, and hard X-ray time lags.