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

We examine the relationship between source position stability and astrophysical properties of radio-loud quasars making up the International Celestial Reference Frame. Understanding this relationship is important for improving quasar selection and analysis strategies, and therefore reference frame stability. We construct light curves for 95 of the most frequently observed ICRF2 quasars at both the 2.3 and 8.4 GHz geodetic VLBI observing bands. Because the appearance of new quasar components corresponds to an increase in quasar flux density, these light curves alert us to 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 rms, defined as the variability in the ratio between S and X-band flux densities. We find that small time lags between S and X-band light curves, and low spectral index variability, are good indicators of position stability. 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. 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. (Abridged)