1. VLBI Imaging and Astrometry of the Gravity Probe B Guide Star HR 8703 Jean-Francois Lestrade Observatoire de Paris/DEMIRM Ryan Ransom, Norbert Bartel, Michael Bietenholz York University Michael Ratner, Daniel Lebach, Irwin Shapiro Harvard-Smithsonian Center for Astrophysics June 12, 2003

2. Outline Gravity Probe B Mission VLBI Observations Astrometric Results Light Curves and Images Summary

4. 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%

5. Global VLBI Array

6. Phase-Reference Mapping

7. J. Lederman et al. See Poster:

8. Flux-Density History at 8.4 GHz 1997 Jan 16 2002 Nov 20

9. 02 Nov 20 97 Jan 16

10. Δμ α = ± 0.17 mas/yr Δμ δ = ± 0.17 mas/yr Δπ = ± 0.13 mas 97 Jan 16 02 Nov 20

12. a 1 = 0.9 ± 0.2 mas Ω = 40° ± 15° i = 80° ± 10°

13. K2 III K0 V (?)

15. 12345

19. 1 2 3 4 5 6 7 8 9 10 11 12 13

22. Summary 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.