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Paper: VLBI Imaging and Astrometry of the Gravity Probe B Guide Star HR 8703
Volume: 340, Future Directions in High Resolution Astronomy: The 10th Anniversary of the VLBA
Page: 506
Authors: Ransom, R.R.; Bartel, N.; Bietenholz, M.F.; Ratner, M.I.; Lebach, D.E.; Shapiro, I.I.; Lestrade, J.-F.
Abstract: We have made multi-epoch VLBI observations at 3.6 cm (8.4 GHz) of the RS CVn binary star HR 8703 (IM Pegasi) in support of the NASA-Stanford relativity gyroscope experiment, Gravity Probe B (GP-B). We present a selection of phase-referenced images of HR 8703 produced from 28 sets of observations made between December 1991 and November 2002. The images show radio source structures that vary in size from ∼0.5 mas to ∼2 mas. Moreover, images from temporal subsets of several observing sessions exhibit both structural evolution in the radio emission and motions of the radio centroid of up to ∼1 mas on hour time scales. Based upon an astrometric analysis of the phase-referenced positions obtained at each epoch, we have (1) made accurate determinations of HR 8703's parallax and proper motion, and (2) apparently detected the major axis of the orbit of HR 8703's K2 III primary.
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