Here we summarize our results from a detailed multi-frequency study of the
QSO 3C309.1 based on the Very Long Baseline Array (VLBA) observations made in
mid 1998. From our images, we find a curved jet extending up to 100
milliarcseconds (mas) to the east at low frequencies with two main components,
A and B. A preliminary astrometric analysis (Ros and Lobanov 2001) provides a
determination of the core position at different frequencies by
phase-referencing to a nearby radio source, QSO S5 1448+76. The changes of the
core position with frequency suggest high opacity close to the core caused by
synchrotron self-absorption. Due to the large astrometric uncertainties we
cannot draw any conclusions about the values of the opacity gradients at high
frequencies. We believe that a detailed analysis of the frequency depedence of
the core position will reveal the profile of the matter distribution in the
broad line region, as was initially suggested by Lobanov (1998).Comment: To be published in the volume "Highlights of Spanish Astrophysics
(III), Proceedings of the 5th Scientific Meeting of the Spanish Astronomical
Society" of the Astrophysics and Space Science Library (Kluwer), J. Gallego,
J. Zamorano, N. Cardiel (eds.), 1 page, 1 figure, no abstract, needs
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