VLBI for Gravity Probe B. V. Proper Motion and Parallax of the Guide Star, IM Pegasi

Abstract

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 in right ascension and -27.27 +- 0.03 +- 0.09 mas/yr 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 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.

Accepted for publication in the Astrophysical Journal Supplement Series

References in corpus (9)

• Gravity Probe B: Final Results of a Space Experiment to Test General Relativity
• Tertiary companions to close spectroscopic binaries
• Astrometric accuracy of phase-referenced observations with the VLBA and EVN
• VLBI for Gravity Probe B. III. A Limit on the Proper Motion of the "Core" of the Quasar 3C 454.3
• VLBI for Gravity Probe B. I. Overview
• VLBI for Gravity Probe B. IV. A New Astrometric Analysis Technique and a Comparison with Results from Other Techniques
• VLBI for Gravity Probe B. VI. The Orbit of IM Pegasi and the Location of the Source of Radio Emission
• VLBI for Gravity Probe B. II. Monitoring of the Structure of the Reference Sources 3C 454.3, B2250+194, and B2252+172
• VLBI for Gravity Probe B. VII. The Evolution of the Radio Structure of IM Pegasi

Cited by in corpus (9)

• Micro-Arcsecond Radio Astrometry
• Comparison of Gaia DR2 Parallaxes of Stars with VLBI Astrometry
• VLBI for Gravity Probe B: The Guide Star IM Pegasi
• VLBI for Gravity Probe B. III. A Limit on the Proper Motion of the "Core" of the Quasar 3C 454.3
• VLBI for Gravity Probe B. I. Overview
• VLBI for Gravity Probe B. II. Monitoring of the Structure of the Reference Sources 3C 454.3, B2250+194, and B2252+172
• VLBI for Gravity Probe B. VII. The Evolution of the Radio Structure of IM Pegasi
• Parameters of the Link between the Optical and Radio Frames from Gaia DR2 Data and VLBI Measurements
• Residual HCRF Rotation relative to the Inertial Coordinate System