1. Shanghai Astronomical Observatory, Chinese Academy of Sciences; 80 Nandan Road, Shanghai 200030, China.
2. University of Chinese Academy of Sciences; 19A Yuquan Road, Beijing 100049, China.
3. Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shengrong Road 520, Shanghai 201210, China.
4. Center for Astronomy and Astrophysics and Department of Physics, Fudan University, Shanghai 200438, China.
5. Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
6. School of Physics and Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China.
7. Institut für Theoretische Physik, Goethe-Universität Frankfurt, Max-von-Laue-Str. 1, D-60438 Frankfurt am Main, Germany.
8. Key Laboratory of Radio Astronomy and Technology, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing 100101, PR China.
9. Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany.
10. Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China.
11. Department of Astronomy, School of Physics, Peking University, Beijing 100871, China.
12. Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, PL-00-716 Warszawa, Poland.
13. Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany.