VLBI for Gravity Probe B. VII. The Evolution of the Radio Structure of IM Pegasi

TL;DR

This study uses VLBI observations over several years to analyze the radio structure and variability of IM Pegasi, a star used as a guide star in a NASA relativity experiment, revealing its emission characteristics and morphology.

Contribution

It provides the first detailed VLBI imaging and analysis of IM Pegasi's radio structure evolution over multiple years, including flux variability and morphology without orbital phase correlation.

Findings

Radio flux density and morphology are variable over time.

Emission regions are mostly single-peaked but sometimes show multiple peaks.

The average emission elongation aligns with the orbit normal and matches the star's diameter.

Abstract

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.