Structural changes of the quasar 3C454.3 used as an extragalactic reference source for the Gravity Probe B Mission
Abstract
April 20, 2004, marked the launch of the NASA/Stanford Gravity Probe B (GP-B) Mission, a project over forty years in the making. For approximately 12 months, the GP-B spacecraft tested two predictions of Einstein's theory of general relativity (GR) within an accuracy previously unattainable. The GP-B spacecraft orbited Earth at an altitude of approximately 650 km and used custom engineered, essentially free falling gyroscopes to measure the geodetic and frame dragging effects predicted by GR. The geodetic and frame dragging effects describe how a massive rotating body warps and twists spacetime around it. The geodetic effect is a precession of the gyroscopes of 6.6 arcseconds per year in a north - south direction, while the frame dragging effect is a precession of 41 milliarcseconds (mas) per year in the direction of Earth's rotation. Gyro data is currently being analyzed by GP-B's science team at Stanford University and will later be combined with astrophysical data gathered by groups at the Harvard-Smithsonian Centre for Astrophysics (CfA) and York University, in order to meet GP-B's mission objective of measuring geodetic and frame dragging effects with a standard error of 0.01% and 1%, respectively, or better. This means that the gyro precessions have to be measured with a standard error of 0.5 mas per year or better.
The measurements of precessions are made relative to a guide star. The guide star is a radio and optically bright RS CVn binary system, known as HR 8703, or IM Pegasi. The motions of IM Pegasi on the sky are being measured with the radio astronomical technique of Very-Long-Baseline Interferometry (VLBI) relative to three extragalactic reference sources nearby on the sky so that changes in the gyros' spin axes can be related to the distant universe. One of these sources is the quasar 3C454.3, a bright radio source with a redshift of z 0.859. 3C454.3 displays a condensed region to the east with two relatively compact components, C1 and C2, a low brightness component to the southwest, E1, and, an elongated region to the west, J1. It is important for the GP-B mission to determine possible structure changes and component motions, or upper limits on them, in order to provide an estimate of the stability of this reference source. More precisely, a fiducial point in the structure of 3C454.3 needs to be identified which is stationary relative to other quasars and the motion of the guide star, HR 8703. With a view toward determining the stability of the structure of 3C454.3, or any structural changes, previously obtained VLBI images of 3C454.3 from 31 epochs between 1997 and 2004 were convolved with the same statistically determined optimal beam. Then these images were displayed with a consistent contouring scale to allow direct epoch to epoch comparisons. For the comparisons, several procedures were chosen. First, the images were compared visually and structure changes identified. Then, relative positions of the maxima of the components, C1, C2, E1, and J1, were determined. Further, elliptical Gaussian models were fit to these components and their relative positions determined. From this set of data, relative proper motions of these components were estimated with a weighted least squares fitting program. It was found that the eastern most component, C1, is the most compact component and likely most stationary component and therefore most closely related to the supermassive black hole and the center of activity of the quasar. The nearby westerly component, C2, moves away from C1 to the south-southwest at a rate of (-42 ± 9) [mu]as/yr and (17 ±1 6) [mu]as/yr, in R.A. and Decl., respectively. The component E1 is largely amorphous and is the least well defined component. It moves away from C1, also to the south-southwest at a rate of (35 ± 17) [mu]as and ( 118 ± 65) in R.A. and Decl., respectively. The component J1 of the extended jet moves away from C1 the fastest, to the north, at a rate of (-3 ± 23) [mu]as/yr and (77 ± 18) [mu]as/yr, in R.A. and Decl., respectively. This latter motion corresponds to a superluminal motion of about 2 c for a distance of approximately 1600 Mpc. The result is important for the selection of the reference point in the structure of 3C454.3 relative to which the motion of the guide star is measured. In a later analysis, the motion of component C1 will be measured relative to the other two extragalactic reference sources and the degree of C1's stationarity at the sky determined. This determination will help in defining the fiducial point to be used in 3C454.3 as a reference source for the GP-B mission.- Publication:
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Masters Thesis
- Pub Date:
- 2006
- Bibcode:
- 2006MsT..........4L
- Keywords:
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- Astronomy, Astrophysics