Sub-Parsec-Scale Acceleration of the Radio Jet in the Powerful Radio Galaxy NGC 6251

In order to investigate the genesis of a powerful radio jet, we have mapped the central 10 pc region of the nearby radio galaxy, NGC 6251, with a 0.2 pc resolution using Very Long Baseline Interferometer (VLBI) at two radio frequencies, 5 GHz and 15 GHz. We have found the sub-parsec-scale counterjet for the first time in this radio galaxy. This discovery allows us to investigate the jet acceleration based on the relativistic beaming model.