| Gravity Probe B (GP-B) is the spaceborne relativity experiment developed by NASA and Stanford University to test two predictions of general relativity (GR). The experiment will use four super-conducting gyroscopes, contained in a low-earth, polar orbiting spacecraft, to precisely measure the geodetic effect and the much smaller frame-dragging effect. According to GR, each of the effects will induce precessions in the gyroscopes. For the frame-dragging effect, the predicted precession is ∼42 mas/yr (mas ≡ milliarcsecond). The precessions will be measured with respect to a “guide star,” namely the RS CVn binary star HR 8703 (IM Pegasi). The goal of the GP-B experiment is to measure the precessions with a standard error of about 0.5 mas/yr or better. To achieve this level of precision, the proper motion of the guide star must be determined in an inertial reference frame with a standard error ≤0.15 mas/yr.; Nineteen sets of very-long-baseline interferometry (VLBI) observations at 8.4 GHz between January 1997 and June 2001 were made of HR 8703 and two extragalactic reference sources, 3C454.3 and B2250+194, in support of GP-B. We produced VLBI images of 3C454.3 and B2250+194 for each observing session, and VLBI images of HR 8703 for all but one of the observing sessions. The images of HR 8703 show a variety of radio source structures which range from compact single-emission-region structures <1 mas in angular diameter to complex double-lobe structures with lobe separations of ∼1.5 mas. Moreover, images from temporal subsets of several observing sessions show on hour time scales both structural evolution in the emission source and motions of the radio centroid of up to ∼1 mas. This is the first time that hourly activity on or close to a star has been observed directly, apart from the activity on the Sun. Based upon an astrometric analysis of the phase-referenced positions obtained at each epoch, we have (1) made precise determinations of HR 8703's parallax and proper motion, and (2) apparently detected the major axis of the orbit of the primary giant star in the HR 8703 binary system. The proper motion determination demonstrates that the GP-B astrometric goal will be met with continued observations. The identification of the primary as the spatial origin of the radio emission in the binary system makes HR 8703 only the second close binary for which such an identification has unambiguously been made. |
