Wednesday, June 5, 2013

1306.0696 (Stanislav Shabala et al.)

The effects of frequency-dependent quasar evolution on the celestial reference frame    [PDF]

Stanislav Shabala, Jonathan Rogers, Jamie McCallum, Oleg Titov, Jay Blanchard, Jim Lovell, Christopher Watson
We examine the relationship between source position stability and astrophysical properties of radio-loud quasars making up the International Celestial Reference Frame. We construct light curves for 95 most frequently observed ICRF2 quasars at both the geodetic VLBI observing bands. Because the appearance of new quasar components corresponds to an increase in quasar flux density, these light curves allow us to probe source structure on sub-100 microarcsecond scales, much smaller than conventional VLBI imaging. Flux density monitoring also allows us to trace the evolution of quasar structure. We test how source position stability depends on three astrophysical parameters: (1) Flux density variability at X-band; (2) Time lag between S and X-band light curves; (3) Spectral index rms, defined as the variability in the ratio between S and X-band flux densities. We find that small (<0.15 years) time lags between S and X-band light curves and low (<0.10) spectral index variability are excellent 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 source structure. Monitoring of source flux density at multiple frequencies therefore provides a probe of quasar structure on scales important to geodesy, but below the resolution limit of VLBI imaging. We show how multi-frequency flux density monitoring allows the exact dependence of core position on frequency (the core shift) 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.
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