[1] Chinese Acad Sci, Natl Astron Observ, 20A Datun Rd, Beijing 100012, Peoples R China
[2] Univ Chinese Acad Sci, Sch Astron & Space Sci, Beijing 100049, Peoples R China
[3] Harvard Smithsonian Ctr Astrophys, 60 Garden St, Cambridge, MA 02138 USA
[4] Max Planck Inst Radioastron, Aufdem Hugel 69, D-53121 Bonn, Germany