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tokens: We thank the anonymous referee for careful reading and offering constructive comments for improving the paper. S.K. thanks Masanori Nakamura and Keiichi Asada for helpful comments and encouragement. This paper is based on observations carried out with the VLBA. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This work made use of the Swinburne University of Technology software correlator (Deller et al. 2011 ), developed as part of the Australian Major National Research Facilities Programme and operated under license. This work was done with the contribution of the Italian Ministry of Foreign Affairs and University and Research for the collaboration project between Italy and Japan. This work is partially supported by JSPS KAKENHI grant Nos. 18K03656 (M.K.), 18H03721 (K.N., M.K., K.H.), 24340042 (A.D.), and 25120007 (M.H., K.H., K.A.). K.A. is a Jansky Fellow of the National Radio Astronomy Observatory. The Black Hole Initiative at Harvard University is financially supported by a grant from the John Templeton Foundation. K.A. is financially supported in part by a grant from the National Science Foundation (AST-1614868). G.Y.Z is supported by the Korea Research Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2015H1D3A1066561). Facility: VLBA.