VLBI for Gravity Probe B. I. Overview

TL;DR

This paper reports on VLBI radio observations over 8.5 years to precisely measure the proper motion and distance of the GP-B mission's guide star, supporting accurate relativistic precession measurements.

Contribution

It provides highly accurate astrometric measurements of IM Pegasi's proper motion and parallax, crucial for the Gravity Probe B experiment's precision.

Findings

Proper motion of IM Pegasi determined with ~0.09 mas/yr accuracy

Parallax measurement of 10.370 mas, corresponding to 96.43 pc

Results do not significantly affect GP-B's relativistic precession estimates

Abstract

We describe the NASA/Stanford gyroscope relativity mission, Gravity Probe B (GP-B), and provide an overview of the following series of six astrometric and astrophysical papers that report on our radio observations and analyses made in support of this mission. The main goal of this 8.5 year program of differential VLBI astrometry was to determine the proper motion of the guide star of the GP-B mission, the RS CVn binary IM Pegasi (IM Peg; HR 8703). This proper motion is determined with respect to compact, extragalactic reference sources. The results are: -20.833 +- 0.090 mas/yr and -27.267 +- 0.095 mas/yr for, respectively, the right ascension and declination, in local Cartesian coordinates, of IM Peg's proper motion, and 10.370 +- 0.074 mas (i.e., 96.43 +- 0.69 pc) for its parallax (and distance). Each quoted uncertainty is meant to represent an ~70% confidence interval that includes the estimated contribution from systematic error. These results are accurate enough not to discernibly degrade the GP-B estimates of its gyroscopes' relativistic precessions: the frame-dragging and geodetic effects.