Milliarcsecond Change of IM Pegasi Radio Position in 1 Hour Coincident with Sharp Rise in Flux Density

Lebach, D. E.; Ratner, M. I.; Shapiro, I. I.; Ransom, R. R.; Bietenholz, M. F.; Bartel, N.; Lestrade, J. F.

doi:10.1086/312026

Cited by 18 publications

(26 citation statements)

References 16 publications

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“…On occasion, time-resolved images show variations in morphology over several hours. As we have mentioned, apparent motion of IM Peg's emission region, correlated with evolution of its flux density on timescales of 1 hr, is definitely seen in our 1997 January 16 session (Lebach et al 1999), and may be present in at least two of our other observing sessions.…”

Section: Discussionsupporting

confidence: 70%

“…A rapid change in the apparent position accompanied by a large change in flux density is seen in several sessions. The most dramatic example was the already mentioned change in apparent position of ∼0.9 mas over a period of 1.4 hr on 1997 January 16 (see Lebach et al 1999). Such a change in apparent position could be due either to (1) fast motion at ∼1000 km s −1 of flare-energized 11 As we mention in Section 3.5 above, the systematic uncertainty on the average angle of elongation of the radio emission likely dominates the statistical one of ±7 • .…”

Section: Rapid Evolution Of the Radio Structurementioning

confidence: 91%

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VLBI FOR GRAVITY PROBE B . VII. THE EVOLUTION OF THE RADIO STRUCTURE OF IM PEGASI

Bietenholz

Bartel

Lebach

et al. 2012

ApJS

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We present measurements of the total radio flux density as well as very long baseline interferometry 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 (λ = 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.

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Section: Discussionsupporting

confidence: 70%

Section: Rapid Evolution Of the Radio Structurementioning

confidence: 91%

VLBI FOR GRAVITY PROBE B . VII. THE EVOLUTION OF THE RADIO STRUCTURE OF IM PEGASI

Bietenholz

Bartel

Lebach

et al. 2012

ApJS

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“…We suspect that the two radio components seen in all three maps correspond to two regions near the 1M Peg primary, which has of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0074180900221359 an estimated angular diameter of 1.3 mas (Lebach et al 1999). Alternatively, one of the radio components could correspond to a region near the primary and the other to a region near the secondary, which to date is optically undetected.…”

Section: Resultsmentioning

confidence: 99%

“…Our first session of VLBI observations, made on 1997 January 16-17, revealed an apparent motion in the radio position of 1M Peg of about 0.9 mas over VLBI Imaging and A strometry of 1M Peg 319 a 1.4 h period in which there was also a dramatic rise in flux density (Lebach et al 1999). Such position changes are more rapid than can be explained by proper motion, parallax, and orbital motion and thus suggest physical evolution of the stellar radio emission on hour and even sub hour time scales.…”

Section: Introductionmentioning

confidence: 90%

VLBI Imaging and Astrometry of the RS CVn Binary Star IM Pegasi

Lebach

Ratner

Shapiro

et al. 2001

Symp. - Int. Astron. Union

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Abstract. We have made 3.6 em VLBI observations of the RS CVn binary star 1M Pegasi (HR 8703) approximately four times per year since 1997 in support of the NASA/Stanford Relativity Gyroscope Experiment (Gravity Probe B). Phase-referenced maps reveal structural changes in the radio emission of the star on hour time scales during several of the sessions. Analyses of the VLBI phase delays with a Kalman-filter estimator reveal submilliarcsecond motions of the radio centroid of the star on hour and even subhour time scales. The observed structural changes and centroid motions often coincide with rapid changes in the star's flux density, as measured with the VLA. We report on our latest results and summarize our findings to date.

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“…The largest source of scatter is likely a highly variable spatial offset between the stellar radio emission and the center of the primary component of the IM Peg binary. The strongest evidence for this assertion is that, for some of those VLBI sessions marked by emission strong enough to be detectable in single scans, our VLBI astrometry reveals changes in position of up to ∼1 mas occurring in synchrony with changes in the brightness of the emission (Lebach et al 1999). In addition, it is plausible that the radio emission is both powered and loosely confined by the stellar magnetic field (Franciosini & Chiuderi Drago 1995).…”

Section: Estimated Positions and Their Errorsmentioning

confidence: 87%

VLBI FOR GRAVITY PROBE B . V. PROPER MOTION AND PARALLAX OF THE GUIDE STAR, IM PEGASI

Ratner¹,

Bartel²,

Bietenholz³

et al. 2012

ApJS

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We present the principal astrometric results of the very long baseline interferometry (VLBI) program undertaken in support of the Gravity Probe B (GP-B) relativity mission. VLBI observations of the GP-B guide star, the RS CVn binary IM Pegasi (HR 8703), yielded positions at 35 epochs between 1997 and 2005. We discuss the statistical assumptions behind these results and our methods for estimating the systematic errors. We find the proper motion of IM Peg in an extragalactic reference frame closely related to the International Celestial Reference Frame 2 (ICRF2) to be −20.83 ± 0.03 ± 0.09 mas yr −1 in right ascension and −27.27 ± 0.03 ± 0.09 mas yr −1 in declination. For each component, the first uncertainty is the statistical standard error and the second is the total standard error (SE) including plausible systematic errors. We also obtain a parallax of 10.37 ± 0.07 mas (distance: 96.4 ± 0.7 pc), for which there is no evidence of any significant contribution of systematic error. Our parameter estimates for the ∼25 day period orbital motion of the stellar radio emission have SEs corresponding to ∼0.10 mas on the sky in each coordinate. The total SE of our estimate of IM Peg's proper motion is ∼30% smaller than the accuracy goal set by the GP-B project before launch: 0.14 mas yr −1 for each coordinate of IM Peg's proper motion. Our results ensure that the uncertainty in IM Peg's proper motion makes only a very small contribution to the uncertainty of the GP-B relativity tests.

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Milliarcsecond Change of IM Pegasi Radio Position in 1 Hour Coincident with Sharp Rise in Flux Density

Cited by 18 publications

References 16 publications

VLBI FOR GRAVITY PROBE B . VII. THE EVOLUTION OF THE RADIO STRUCTURE OF IM PEGASI

VLBI FOR GRAVITY PROBE B . VII. THE EVOLUTION OF THE RADIO STRUCTURE OF IM PEGASI

VLBI Imaging and Astrometry of the RS CVn Binary Star IM Pegasi

VLBI FOR GRAVITY PROBE B . V. PROPER MOTION AND PARALLAX OF THE GUIDE STAR, IM PEGASI

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