Journal of Geodesy

, Volume 88, Issue 6, pp 575–586 | Cite as

The effects of frequency-dependent quasar variability on the celestial reference frame

  • Stanislav S. Shabala
  • Jonathan G. Rogers
  • Jamie N. McCallum
  • Oleg A. Titov
  • Jay Blanchard
  • James E. J. Lovell
  • Christopher S. Watson
Original Article

Abstract

We examine the relationship between source position stability and astrophysical properties of radio-loud quasars making up the International Celestial Reference Frame (ICRF2). Understanding this relationship is important for improving quasar selection and analysis strategies, and therefore reference frame stability. We construct flux density time series, known as light curves, for 95 of the most frequently observed ICRF2 quasars at both the 2.3 and 8.4 GHz geodetic very long baseline interferometry (VLBI) observing bands. Because the appearance of new quasar components corresponds to an increase in quasar flux density, these light curves alert us about potential changes in source structure before they appear in VLBI images. We test how source position stability depends on three astrophysical parameters: (1) flux density variability at X band; (2) time lag between flares in S and X bands; (3) spectral index root-mean-square (rms), defined as the variability in the ratio between S and X band flux densities. We find that the time lag between S and X band light curves provides a good indicator of position stability: sources with time lags \(<\)0.06 years are significantly more stable (\(>\)20 % improvement in weighted rms) than sources with larger time lags. A similar improvement is obtained by observing sources with low \((<\)0.12) spectral index variability. On the other hand, there is no strong dependence of source position stability on flux density variability in a single frequency band. These findings can be understood by interpreting the time lag between S and X band light curves as a measure of the size of the source structure. Monitoring of source flux density at multiple frequencies therefore appears to provide a useful probe of quasar structure on scales important to geodesy. The observed astrometric position of the brightest quasar component (the core) is known to depend on observing frequency. We show how multi-frequency flux density monitoring may allow the dependence on frequency of the relative core positions along the jet to be elucidated. Knowledge of the position–frequency relation has important implications for current and future geodetic VLBI programs, as well as the alignment between the radio and optical celestial reference frames.

Keywords

Very long baseline interferometry (VLBI) Astrometry  Celestial reference frame (CRF) Source structure Quasar variability Light curves Core shift International VLBI Service for Geodesy and Astrometry (IVS) 

Notes

Acknowledgments

SS and JM thank the Australian Research Council for Super Science Fellowships (FS100100037 and FS110200045). We are grateful to Harald Schuh, Simon Ellingsen and John Dickey for useful discussions, and Chris Jacobs, Richard Porcas and two other anonymous referees for thorough and constructive comments that have significantly improved the manuscript. This research has made use of the United States Naval Observatory (USNO) Radio Reference Frame Image Database (RRFID).

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Copyright information

© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  • Stanislav S. Shabala
    • 1
  • Jonathan G. Rogers
    • 1
  • Jamie N. McCallum
    • 1
  • Oleg A. Titov
    • 3
  • Jay Blanchard
    • 1
    • 2
  • James E. J. Lovell
    • 1
  • Christopher S. Watson
    • 4
  1. 1.School of Physical SciencesUniversity of TasmaniaHobartAustralia
  2. 2.Departamento de AstronomíaUniversidad de ConcepciónConcepciónChile
  3. 3.Geoscience AustraliaCanberraAustralia
  4. 4.School of Land and FoodUniversity of TasmaniaHobart Australia

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