1. School of Astronomy and Space Science, Nanjing University, Nanjing 210093, China
2. Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, China
3. Max-Planck-Institut für Kernphysik, Heidelberg D-69117, Germany
4. Deutsches Elektronen Synchrotron (DESY), Platanenallee 6, Zeuthen D-15738, Germany