High sensitivity VLBI study of the quasar 3C454.3

The quasar 3C454.3 is the primary extragalactic reference source for the NASA/Stanford space-borne relativity experiment, Gravity Probe B. Thirty-five sets of previous very-long-baseline interferometry (VLBI) observations at 8.4 GHz between January 1997 and July 2005 were used for a sensitive search for the counter jet of 3C454.3 and components in the outer region of the jet. High dynamic range VLBI images of 3C454.3 are presented for each observing session, each image showing the structure as a core region with a jet extending to the northwest.; Averaging 21 of the best images of 3C454.3 reduced the rms background noise. The result is a final image with a dynamic range of 22500:1, one of the highest ever achieved in VLBI, with no counter jet detected. On the basis of the relativistic beaming model, it was found that the observations can be interpreted in terms of a jet approaching us at an angle < 11° with a velocity of 0.85c. A new component in the outer regions of the jet was detected for the first time at 8.4 GHz having a flux density of 23.7 +/- 2.1 mJy/beam and a spectral index of alpha = -0.87 +/- 0.09. The averaging procedure was also applied to images of two other extragalactic reference sources also used for Gravity Probe B: B2250+194 and B2252+172. No additional components were revealed for these two sources.