* Astronomy

Title: VLBI for Gravity Probe B. VI. The Orbit of IM Pegasi and the Location of the Source of Radio Emission
Authors: R. R. Ransom, N. Bartel, M. F. Bietenholz, D. E. Lebach, J.-F. Lestrade, M. I. Ratner, I. I. Shapiro

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.

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