1. Department of Physics, Xiangtan University, Hunan 411105, China
2. National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
3. CAS Key Laboratory of FAST, Chinese Academy of Sciences, Beijing 100101, China
4. School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 101400, China
5. Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
6. Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi 830011, China
7. Astronomy Department, Beijing Normal University, Beijing 100875, China