VLBI for Gravity Probe B: The Guide Star IM Pegasi
N. Bartel, M. F. Bietenholz, D. E. Lebach, R. R Ransom, M. I Ratner, and I. I. Shapiro

TL;DR
This paper details VLBI observations of the guide star IM Pegasi, providing precise measurements of its proper motion and parallax to support the Gravity Probe B mission, and reports on the radio emission characteristics of the star.
Contribution
It presents the first precise VLBI-based proper motion and parallax measurements of IM Pegasi, supporting the accuracy of the Gravity Probe B experiment.
Findings
Proper motion of IM Pegasi measured with high precision
Parallax of IM Pegasi determined as 10.37 mas
Upper limits on core component proper motion of 3C 454.3
Abstract
We review the radio very long baseline interferometry (VLBI) observations of the guide star, IM Peg, and three compact extragalactic reference sources, 3C 454.3, B2250+194, and B2252+172, made in support of the NASA/Stanford gyroscope relativity mission, GP-B. The main goal of the observations was the determination of the proper motion of IM Peg relative to the distant universe. VLBI observations made between 1997 and 2005 yield a proper motion of IM Peg of -20.83 0.09 mas yr in RA and -27.27 0.09 mas yr in dec, in a celestial reference frame of extragalactic radio galaxies and quasars virtually identical to the International Celestial Reference Frame 2 (ICRF2). They also yield a parallax for IM Peg of 10.37 0.07 mas, corresponding to a distance of 96.4 0.7 pc. The uncertainties are standard errors with statistical and estimated systematic…
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