1 Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China; Key laboratory of Radio Astronomy, Chinese Academy of Sciences, Nanjing 210008, China. Electronic address: antao@shao.ac.cn.
2 Osserv Astron Brera, Ist Nazl Astrofis, Via E Bianchi 46, Merate I23807, Italy.
3 Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China; University of Chinese Academy of Sciences, Beijing 100049, China.
4 Ist Nazl Astrofis, Ist Radioastron, Bologna I40129, Italy; Department of Physics and Astronomy, University of Bologna, Bologna I40129, Italy.
5 Ist Nazl Astrofis, Ist Radioastron, Bologna I40129, Italy.
6 National Astronomical Observatory of Japan, Tokyo 181-8588, Japan.
7 Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea.