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.
Highlights
This paper is the fifth in a series describing the astronomical effort undertaken in support of the NASA/Stanford Gravity Probe B (GP-B) relativity mission, an Earth-orbiting mission to test the geodetic and frame-dragging predictions of general relativity
Since the proper motion of no bright star was known with such accuracy, we undertook a dedicated program of astrometry to determine this proper motion for the chosen guide star, IM Pegasi (HR 8703)
The uncertainties of these IM Peg positions are dominated by poorly characterized systematic errors, whose size we can estimate from the scatter in the differential positions we found for our pair of reference sources with the smallest separation on the sky, B2252+172 and 3C 454.3
Summary
This paper is the fifth in a series describing the astronomical effort undertaken in support of the NASA/Stanford Gravity Probe B (GP-B) relativity mission, an Earth-orbiting mission to test the geodetic and frame-dragging predictions of general relativity. As described in Paper I (Shapiro et al 2012), the rotating GP-B spacecraft monitored the precessions of four ultrahigh accuracy onboard gyroscopes with respect to the spacecraft To transform these precessions to a reference frame that is not rotating with respect to the distant universe, the mission team required both the star-tracking data recorded by the spacecraft’s telescope and independent knowledge of the proper motion of an adequately bright “guide” star. Since the proper motion of no bright star was known with such accuracy, we undertook a dedicated program of astrometry to determine this proper motion for the chosen guide star, IM Pegasi (HR 8703) This star is an RS Canum Venaticorum (RS CVn) spectroscopic binary star, with an orbital period of ∼25 days and variable radio emission at centimeter wavelengths. Throughout, we use the words “images” and “maps” almost interchangeably
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