67 research outputs found
VLBI for Gravity Probe B. V. Proper Motion and Parallax of the Guide Star, IM Pegasi
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.Comment: Accepted for publication in the Astrophysical Journal Supplement
Serie
VLBI for Gravity Probe B. VII. The Evolution of the Radio Structure of IM Pegasi
We present measurements of the total radio flux density as well as
very-long-baseline interferometry (VLBI) 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 (wavelength = 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.Comment: Accepted for publication in the Astrophysical Journal Supplement
Serie
Testing general relativity by micro-arcsecond global astrometry
The global astrometric observations of a GAIA-like satellite were modeled
within the PPN formulation of Post-Newtonian gravitation. An extensive
experimental campaign based on realistic end-to-end simulations was conducted
to establish the sensitivity of global astrometry to the PPN parameter \gamma,
which measures the amount of space curvature produced by unit rest mass. The
results show that, with just a few thousands of relatively bright,
photometrically stable, and astrometrically well behaved single stars, among
the ~10^9 objects that will be observed by GAIA, \gamma can be estimated after
1 year of continuous observations with an accuracy of ~10^{-5} at the 3\sigma
level. Extrapolation to the full 5-year mission of these results based on the
scaling properties of the adjustment procedure utilized suggests that the
accuracy of \simeq 2x10^{-7}, at the same 3\sigma level, can be reached with
\~10^6 single stars, again chosen as the most astrometrically stable among the
millions available in the magnitude range V=12-13. These accuracies compare
quite favorably with recent findings of scalar-tensor cosmological models,
which predict for \gamma a present-time deviation, |1-\gamma|, from the General
Relativity value between 10^{-5} and 10^{-7}.Comment: 7 pages, 2 figures, to be published in A&
VLBI for Gravity Probe B. I. Overview
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.Comment: Accepted for publication in the Astrophysical Journal Supplement
Serie
Recommended from our members
VLBI for Gravity Probe B. VII. The Evolution of the Radio Structure of IM Pegasi
We present measurements of the total radio flux density as well as very-long-baseline interferometry (VLBI) 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 (wavelength = 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.Astronom
VLBI for Gravity Probe B. III. A Limit on the Proper Motion of the "Core" of the Quasar 3C 454.3
We made VLBI 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. 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 1-sigma upper limits of 15 and 30
micro-arcsec/yr in RA and decl., 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 1-sigma upper limits on its proper motion of 39
micro-arcsec/yr (1.0c) and 30 micro-arcsec/yr (0.8c) in RA and decl.,
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
micro-arcsec/yr in RA and decl., respectively. Some of 3C 454.3's jet
components show significantly superluminal motion with speeds of up to ~200
micro-arcsec/yr or 5c in the CRF. The core of 3C 454.3 thus provides for
Gravity Probe B a sufficiently stable reference in the distant universe.Comment: Accepted for publication in the Astrophysical Journal Supplement
Serie
VLBI for Gravity Probe B. VI. The Orbit of IM Pegasi and the Location of the Source of Radio Emission
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 VLBI 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) deg, which is closely aligned with
the sky projections of the orbit normal (p.a. = -49.5 +- 8.6 deg) 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| >= 70 deg) than
near the equator (|latitude| <= 20 deg), 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.Comment: Accepted for publication in the Astrophysical Journal Supplement
Serie
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