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.

Item Type:	Refereed Article
Keywords:	very long baseline interferometry, quasar structure, quasar evolution, celestial reference frame, source structure, light curves, core shift, international VLBI service for geodesy and astronomy
Research Division:	Physical Sciences
Research Group:	Astronomical sciences
Research Field:	Cosmology and extragalactic astronomy
Objective Division:	Expanding Knowledge
Objective Group:	Expanding knowledge
Objective Field:	Expanding knowledge in the physical sciences
UTAS Author:	Shabala, SS (Associate Professor Stas Shabala)
UTAS Author:	Rogers, J (Mr Jonathan Rogers)
UTAS Author:	McCallum, JN (Dr Jamie McCallum)
UTAS Author:	Blanchard, J (Mr Jay Blanchard)
UTAS Author:	Lovell, JEJ (Dr Jim Lovell)
UTAS Author:	Watson, CS (Dr Christopher Watson)
ID Code:	91832
Year Published:	2014
Web of Science® Times Cited:	14
Deposited By:	Mathematics and Physics
Deposited On:	2014-05-30
Last Modified:	2017-11-03
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