Global 3D MHD simulations of optically thin black hole accretion disks

We present the results of global three-dimensional resistive magnetohydrodynamic simulations of optically thin accretion disks around a black hole. General relativistic effects are simulated by using the pseudo-Newtonian potential. Initial state is a rotating torus threaded by weak toroidal magnetic fields. We assume an anomalous resistivity enhanced in localized current sheets. After several rotation time of the torus, the torus deforms itself into an accretion disk. Numerical results indicate that small-scale magnetic reconnections taking place in the turbulent disk produces l/f-like X-ray fluctuations. When dense blobs infall, since they stretch and twist magnetic field lines, bisymmetric spiral magnetic channels are created in the innermost plunging region of the disk. Magnetic reconnection taking place inside the channel can be the origin of X-ray shots observed in black hole candidates. We also present the results of simulations including the radiative cooling in optically thin disks. When mass accretion rate exceeds a critical accretion rate, the outer regions of the hot accretion disk (r > 10r _g, where r_g is the Schwarzschild radius) make transition to geometrically thin, cool disk. Since the toroidal magnetic flux is conserved while the disk shrinks in vertical direction, magnetic pressure becomes dominant inside the disk. We expect violent magnetic activities in such magnetic pressure dominated disk.