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VLBI for Gravity Probe B. III. A Limit on the Proper Motion of the "Core" of the Quasar 3C 454.3

We made VLBI observations at 8.4 GHz between 1997 and 2005 to estimate the coordinates of the "core" component of the superluminal quasar, 3C 454.3, the ultimate reference point in the distant universe for the NASA/Stanford Gyroscope Relativity Mission, Gravity Probe B. These coordinates are determined relative to those of the brightness peaks of two other compact extragalactic sources, B2250+194 and B2252+172, nearby on the sky, and within a celestial reference frame (CRF), defined by a large suite of compact extragalactic radio sources, and nearly identical to the International Celestial Reference Frame 2 (ICRF2). We find that B2250+194 and B2252+172 are stationary relative to each other, and also in the CRF, to within 1-sigma upper limits of 15 and 30 micro-arcsec/yr in RA and decl., respectively. The core of 3C 454.3 appears to jitter in its position along the jet direction over ~0.2 mas, likely due to activity close to the putative supermassive black hole nearby, but on average is stationary in the CRF within 1-sigma upper limits on its proper motion of 39 micro-arcsec/yr (1.0c) and 30 micro-arcsec/yr (0.8c) in RA and decl., respectively, for the period 2002 - 2005. Our corresponding limit over the longer interval, 1998 - 2005, of more importance to GP-B, is 46 and 56 micro-arcsec/yr in RA and decl., respectively. Some of 3C 454.3's jet components show significantly superluminal motion with speeds of up to ~200 micro-arcsec/yr or 5c in the CRF. The core of 3C 454.3 thus provides for Gravity Probe B a sufficiently stable reference in the distant universe.