1. National Astronomical Observatories, Chinese Academy of Sciences , Beijing 100012 , China
2. Shanghai Astronomical Observatory, Chinese Academy of Sciences , 80 Nandan Road, Shanghai 200030 , China
3. School of Astronomy and Space Science, University of Chinese Academy of Sciences , Beijing 100049 , China
4. Department of Astronomy, Tsinghua University , Beijing 100084 , China
5. Institute of Physics, Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny , CH-1290 Versoix , Switzerland
6. Key Laboratory of Radio Astronomy and Technology, Chinese Academy of Sciences , A20 Datun Road, Chaoyang District, Beijing 100101 , China