1. Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan
2. MAXI team, Institute of Physical and Chemical Research (RIKEN), Wako, 2-1 Hirosawa, Saitama 351-0198, Japan
3. Department of Electrical and Information Systems, Shibuya Institute of Technology, 307 Fukasaku, Minuma, Saitama City, Saitama 337-8570, Japan