Journal of the Geodetic Society of Japan

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The flux variation of extra-galactic radio sources is an important scientific theme in astrophysical fields. Since the typical timescale of the variations at radio frequency is a month-a decade, ‘long-term’ light curves are usually required. In spite of the long archiving since 1979, the geodetic VLBI data are generally insufficient in their exposure time to draw a VLBI image, and have been rarely utilized for astrophysical purpose so far. We established a procedure to obtain such long-term light curves from the geodetic VLBI archival data at 8 GHz, and evaluate its reliability. We estimate typical error of derived VLBI fluxes by comparing with published single dish fluxes. Consequently, the total error is estimated to be ∼15%, which is sufficiently small for astronomical studies. As a result, 700 light curves were obtained from all public geodetic VLBI data. 81 objects out of 700 ones provide light curves of longer than 8 years, and 35 of them have not been appeared in literature to date. We show an example of astrophysical study of the variability of the AGNs using these statistically enhanced data sets. We conclude that the long-term light curves of the AGNs can be obtained from the geodetic VLBI data, even if the exposure time of each VLBI observation is too short to draw a VLBI image. Future geodetic VLBI system which has a wider observing frequency band ranging 2-15 GHz will allow us to obtain multi-frequency light curves.

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