Studies of State Transitions in Black Hole Candidates by Collaboration between Magnetohydrodynamic Simulations and X-ray Observations

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State transitions of black hole candidates are studied by collaboration between theoretical/simulation studies and X-ray observations. By applying a higher accuracy magnetohydrodynamic code based on the HLLD scheme to hard-to-soft transitions, we found that luminous hard state is maintained by balancing the magnetic energy release and radiative cooling through magnetic reconnection between dynamo fields near the black hole and strong azimuthal fields in the outer disk. We also explained the state transitions observed during a tidal disruption event Swift J1644+57 by radiation hydrodynamic simulations of supercritical accretion onto a supermassive black hole.