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.
Highlights
IM Pegasi is a close binary RS Canum Venaticorum (RS CVn) star (Hall 1976). This star was chosen as the reference star for Gravity Probe B (GP-B), the space-borne relativity experiment developed by NASA and Stanford University; for an introduction to GP-B, see Shapiro et al (2012), hereafter Paper I
What can be said about the nature of the variability of IM Peg’s radio emission? Since IM Peg is a known binary and its orbit is well known from optical spectroscopy (Marsden et al 2005; Berdyugina et al 1999b) and from our very long baseline interferometry (VLBI) astrometry (Papers IV and V), we investigate whether the radio emission varies significantly with the orbital phase of the binary or in a secular fashion
We discuss the unprecedented series of 35 VLBI radio images of the RS CVn star IM Peg at 8.4 GHz, taken over a period of 8.5 years
Summary
IM Pegasi (hereafter IM Peg; HR 8703, HD 216489, FK5 3829) is a close binary RS Canum Venaticorum (RS CVn) star (Hall 1976). This star was chosen as the reference star for Gravity Probe B (GP-B), the space-borne relativity experiment developed by NASA and Stanford University; for an introduction to GP-B, see Shapiro et al (2012), hereafter Paper I. The radio structure and astrometric stability of the reference sources are described in Papers II (Ransom et al 2012a) and III (Bartel et al 2012), respectively. Results on radio imaging and astrometry of IM Peg were presented in Ransom et al (2005) and a brief overview was given in Bartel et al (2008)
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