1. Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China
2. Guangxi Key Laboratory for Relativistic Astrophysics, Nanning 530004, China
3. School of Astronomy and Space Sciences, University of Science and Technology of China, Hefei 230026, China
4. Joint Center for Particle, Nuclear Physics and Cosmology, Nanjing University-Purple Mountain Observatory, Nanjing 210008, China