Detection of the VLBI-scale Counter Jet in NGC 6251
  Mapping the central 5 pc region of the nearby radio galaxy NGC 6251 with a 0.2 pc resolution using VLBI at two radio frequencies, 5 GHz and 15 GHz, we have found the sub-pc-scale counter jet for the first time in this radio galaxy. This discovery allows us to investigate the jet acceleration based on the relativistic beaming model (Ghisellini et al. 1993). 
H. Sudou (1), Y. Taniguchi (1), Y. Ohyama (2), S. Kameno (2), S.   Sawada-Satoh(2), M. Inoue(2), O. Kaburaki(1), and S. Sasa...