1. The National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
2. University of Chinese Academy of Sciences, No.19(A) Yuquan Road, Shijingshan District, Beijing 100049, China
3. Cahill Centre for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
4. Dr. Karl Remeis-Observatory and Erlangen Centre for Astroparticle Physics, Sternwartstr 7, D-96049 Bamberg, Germany