Dynamo Effects of Black Hole and Particle Acceleration

黑洞是一种有趣的天体物理学对象，在最近的观测中发现，如果物质发生引力坍缩，它就是最终的状态，并且是高能现象的来源。特别是，人们普遍认为超大质量黑洞存在于活动星系核的中心区域，等离子体在强引力作用下吸积形成了围绕黑洞磁层的磁层，作为活动的发动机。1.首先，利用广义相对论磁流体动力学，研究了静止黑洞周围的旋转磁层，讨论了发电机作用的效率。结果表明，黑洞的自旋运动并不能有效地激发出环场线来对抗磁场的欧姆扩散。这是对以往著作中一个有争议的问题的明确回答。提出了另一种由于流体中电荷分离而产生磁场自激的机制，并发现在磁化流体以接近光速的速度旋转运动的区域中，类似耀斑的活动成为可能。磁场的这种能量释放将有助于产生具有高相对论速度的等离子体射流.其次，引力坍缩在磁层中形成黑洞被认为是电磁波的爆发源。引力场的动力学变化可以直接放大磁场，产生波辐射。发现了引力坍缩的特征爆发模式，这可能使我们能够根据电磁爆发的观测来检查黑洞的形成。对坍缩磁化恒星的更真实模型的详细分析仍有待于未来的研究。

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.