| Research Abstract |
Black holes are interesting astrophysical objects which are final states if gravitional collapse if matter and work as sources of highly energetic phenomena found in recent observations. In particular, it is widely believed that supermassive black holes exist in the central region of active galactic nuclei, and plasma accreted by strong gravity forms a magnetosphere around the black-hole magnetosphere as an engine of the activity.
1. First, using general relativistic magnetohydrodynamics,a rotating magnetosphere around a stationary black hole was investigated with the purpose of discussing the efficiently of dynamo action. It was shown that spinning motion of black hole is not so efficient to excite loop field lines against ohmic diffusion of magnetic fields. This is a clear answer for a controversial point among previous works. The alternative mechanism for self-excitation of magnetic fields due to charge separation in fluids was proposed, and it was found that flare-like activity becomes possible in the region where magnetized fluids have rotational motion with speeds nearly equal to the light velocity. This energy release of magnetic fields will be useful to produce plasma jet with highly relativistic speeds.
2. Next, gravitational collapse to form a black hole in the magnetosphere was considered as a burst source of electromagnetic waves. Dynamical variation of gravitational fields can give a direct amplification of magnetic fields to generate wave emission. The characteristic burst pattern of gravitational collapse was found, which may allow us to check black hole formation in terms of observations of electromagnetic bursts. A detailed analysis for more realistic models of collapsing magnetized stars remains in future investigations.
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