Table of contents

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 very long baseline interferometry 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⁻¹ and −27.267 ± 0.095 mas yr⁻¹ 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.

We used 8.4 GHz very long baseline interferometry images obtained at up to 35 epochs between 1997 and 2005 to examine the radio structures of the main reference source, 3C 454.3, and two secondary reference sources, B2250+194 and B2252+172, for the guide star for the NASA/Stanford relativity mission Gravity Probe B (GP-B). For one epoch in 2004 May, we also obtained images at 5.0 and 15.4 GHz. The 35 8.4 GHz images for quasar 3C 454.3 confirm a complex, evolving, core-jet structure. We identified at each epoch a component, C1, near the easternmost edge of the core region. Simulations of the core region showed that C1 is located, on average, 0.18 ± 0.06 mas west of the unresolved "core" identified in 43 GHz images. We also identified in 3C 454.3 at 8.4 GHz several additional components that moved away from C1 with proper motions ranging in magnitude between 0.9 c and 5 c. The detailed motions of the components exhibit two distinct bends in the jet axis located ∼3 and ∼5.5 mas west of C1. The spectra between 5.0 and 15.4 GHz for the "moving" components are steeper than those for C1. The 8.4 GHz images of B2250+194 and B2252+172, in contrast to those of 3C 454.3, reveal compact structures. The spectrum between 5.0 and 15.4 GHz for B2250+194 is inverted while that for B2252+172 is flat. Based on its position near the easternmost edge of the 8.4 GHz radio structure, close spatial association with the 43 GHz core, and relatively flat spectrum, we believe 3C 454.3 component C1 to be the best choice for the ultimate reference point for the GP-B guide star. The compact structures and inverted-to-flat spectra of B2250+194 and B2252+172 make these objects valuable secondary reference sources.

We made very long baseline interferometry 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 (GP-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 1σ upper limits of 15 and 30 μas yr⁻¹ in α and δ, 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 1σ upper limits on its proper motion of 39 μas yr⁻¹ (1.0c) and 30 μas yr⁻¹ (0.8c) in α and δ, 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 μas yr⁻¹ in α and δ, respectively. Some of 3C 454.3's jet components show significantly superluminal motion with speeds of up to ∼200 μas yr⁻¹ or 5c in the CRF. The core of 3C 454.3 thus provides for GP-B a sufficiently stable reference in the distant universe.

When very long baseline interferometry (VLBI) observations are used to determine the position or motion of a radio source relative to reference sources nearby on the sky, the astrometric information is usually obtained via (1) phase-referenced maps or (2) parametric model fits to measured fringe phases or multiband delays. In this paper, we describe a "merged" analysis technique which combines some of the most important advantages of these other two approaches. In particular, our merged technique combines the superior model-correction capabilities of parametric model fits with the ability of phase-referenced maps to yield astrometric measurements of sources that are too weak to be used in parametric model fits. We compare the results from this merged technique with the results from phase-referenced maps and from parametric model fits in the analysis of astrometric VLBI observations of the radio-bright star IM Pegasi (HR 8703) and the radio source B2252+172 nearby on the sky. In these studies we use central-core components of radio sources 3C 454.3 and B2250+194 as our positional references. We obtain astrometric results for IM Peg with our merged technique even when the source is too weak to be used in parametric model fits, and we find that our merged technique yields astrometric results superior to the phase-referenced mapping technique. We used our merged technique to estimate the proper motion and other astrometric parameters of IM Peg in support of the NASA/Stanford Gravity Probe B mission.

We present the principal astrometric results of the very long baseline interferometry (VLBI) program undertaken in support of the Gravity Probe B (GP-B) relativity mission. VLBI observations of the GP-B guide star, the RS CVn binary IM Pegasi (HR 8703), yielded positions at 35 epochs between 1997 and 2005. We discuss the statistical assumptions behind these results and our methods for estimating the systematic errors. We find the proper motion of IM Peg in an extragalactic reference frame closely related to the International Celestial Reference Frame 2 (ICRF2) to be −20.83 ± 0.03 ± 0.09 mas yr⁻¹ in right ascension and −27.27 ± 0.03 ± 0.09 mas yr⁻¹ in declination. For each component, the first uncertainty is the statistical standard error and the second is the total standard error (SE) including plausible systematic errors. We also obtain a parallax of 10.37 ± 0.07 mas (distance: 96.4 ± 0.7 pc), for which there is no evidence of any significant contribution of systematic error. Our parameter estimates for the ∼25 day period orbital motion of the stellar radio emission have SEs corresponding to ∼0.10 mas on the sky in each coordinate. The total SE of our estimate of IM Peg's proper motion is ∼30% smaller than the accuracy goal set by the GP-B project before launch: 0.14 mas yr⁻¹ for each coordinate of IM Peg's proper motion. Our results ensure that the uncertainty in IM Peg's proper motion makes only a very small contribution to the uncertainty of the GP-B relativity tests.

We present a physical interpretation for the locations of the sources of radio emission in IM Pegasi (IM Peg, HR 8703), the guide star for the NASA/Stanford relativity mission Gravity Probe B. This emission is seen in each of our 35 epochs of 8.4 GHz very long baseline interferometry observations taken from 1997 to 2005. We found that the mean position of the radio emission is at or near the projected center of the primary to within about 27% of its radius, identifying this active star as the radio emitter. The positions of the radio brightness peaks are scattered across the disk of the primary and slightly beyond, preferentially along an axis with position angle, P.A. = −38° ± 8°, which is closely aligned with the sky projections of the orbit normal (P.A. = −495 ± 86) and the expected spin axis of the primary. Comparison with simulations suggests that brightness peaks are 3.6^+0.4_{− 0.7} times more likely to occur (per unit surface area) near the pole regions of the primary (latitude, |λ| ⩾ 70°) than near the equator (|λ| ⩽ 20°), and to also occur close to the surface with ∼2/3 of them at altitudes not higher than 25% of the radius of the primary.

We present measurements of the total radio flux density as well as very long baseline interferometry images of the star, IM Pegasi, which was used as the guide star for the NASA/Stanford relativity mission Gravity Probe B. We obtained flux densities and images from 35 sessions of observations at 8.4 GHz (λ = 3.6 cm) between 1997 January and 2005 July. The observations were accurately phase-referenced to several extragalactic reference sources, and we present the images in a star-centered frame, aligned by the position of the star as derived from our fits to its orbital motion, parallax, and proper motion. Both the flux density and the morphology of IM Peg are variable. For most sessions, the emission region has a single-peaked structure, but 25% of the time, we observed a two-peaked (and on one occasion perhaps a three-peaked) structure. On average, the emission region is elongated by 1.4 ± 0.4 mas (FWHM), with the average direction of elongation being close to that of the sky projection of the orbit normal. The average length of the emission region is approximately equal to the diameter of the primary star. No significant correlation with the orbital phase is found for either the flux density or the direction of elongation, and no preference for any particular longitude on the star is shown by the emission region.