1. Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China.
2. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
3. Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany.
4. Mizusawa VLBI (Very Long Baseline Interferometry) Observatory, National Astronomical Observatory of Japan, Japan.
5. INAF (Istituto Nazionale di Astrofisica)–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy.
6. Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China.
7. Nanjing University, Nanjing 210093, China.