1. Department of Astronomy and Applied Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
2. Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Dipartimento di Scienze Fisiche, Università di Napoli, I-80126 Napoli, Italy
3. CCAST (World Lab.), P. O. Box 8730, Beijing 100080, China
4. Department of Astronomy and Applied Physics, University of Science and Technology of China, Hefei, Anhui 230026, China