1. Physics Department and Tsinghua Center for Astrophysics, Tsinghua University, Beijing 100084, China
2. Yunnan Observatories, Chinese Academy of Sciences, Kunming 650011, China
3. Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, China
4. Department of Physics and Astronomy, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK
5. Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
6. Physics and Astronomy Department, Texas A&M University, College Station, TX 77843, USA
7. Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China