1. Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea.
2. Department of Astronomy and Astrophysics, University of Toronto, Toronto, Ontario M5S 3H4, Canada.
3. Space Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125, USA.
4. Visiting Brain Pool Scholar, Korea Astronomy and Space Science Institute, Daejeon 305-348, Korea.
5. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.