VLBI Imaging and Astrometry of the Gravity Probe B Guide Star HR 8703 Jean-Francois Lestrade Observatoire de Paris/DEMIRM Ryan Ransom, Norbert Bartel,

VLBI Imaging and Astrometry of the Gravity Probe B

Guide Star HR 8703

Jean-Francois LestradeObservatoire de Paris/DEMIRM

Ryan Ransom, Norbert Bartel, Michael BietenholzYork University

Michael Ratner, Daniel Lebach, Irwin ShapiroHarvard-Smithsonian Center for Astrophysics

June 12, 2003

OutlineOutline

• Gravity Probe B Mission

• VLBI Observations

• Astrometric Results

• Light Curves and Images

• Summary

GRAVITY PROBE B

GOAL: Measure precessions with standard error < 0.5 mas/yr

ASTROMETRY: Measure proper motion with standard error 0.15 mas/yr

~1%

Global VLBI Array

Phase-Reference Mapping

J. Lederman et al.See Poster:

Flux-Density History at 8.4 GHz

1997 Jan 16

2002 Nov 20

02 Nov 20

97 Jan 16

Δμα = ± 0.17 mas/yrΔμδ = ± 0.17 mas/yrΔπ = ± 0.13 mas

97 Jan 16

02 Nov 20

a1 = 0.9 ± 0.2 masΩ = 40° ± 15°i = 80° ± 10°

K2 III

K0 V (?)

1 2 3 4 5

1 2 3 4 5 6 7 8 9 10 11 12 13

SummarySummary• A very precise measurement of the proper motion of

HR 8703 has been made based on 23 VLBI observations.

• The orbit of the primary K giant star has been detected.

• Motion of the centroid of the radio emission has been detected on hour time scales and likely represents the rapid flow of energetic electrons along closed magnetic field lines attached to the giant star.

• Double-lobe radio source structures have been observed and may be a signature of emission from the north and south poles of the giant star.