Opacity in the jet of 3C 309.1,
E. Ros, A.P. Lobanov
Proceedings of the 15th Working Meeting on
European VLBI for Geodesy and Astrometry, Rius A, Behrend D. (eds.),
IEEC, CSIC, Barcelona, Spain, in press (2001)
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The "core" of a radio source is believed to mark the
frequency-dependent location where the optical depth to
synchrotron self-absorption tau_s~1. The frequency
dependence can be used for derive physical conditions of the
radio emitting region and the ambient environment near the central engine
of the radio source.
In order to test and improve the models to derive this
information, we made multi-frequency
dual-polarization observations of 3C 309.1 in 1998.6,
phase-referenced to the QSOs
S5 1448+76 and 4C 72.20 (S5 1520+72) (4o26' and
1o49' away, respectively).
We present here preliminary results from these observations:
total and polarized intensity maps, spectral information
deduced from these images, and the relative position
of 3C&nbbsp;309.1 with respect to S5 1448+76 at different
frequencies. Finally, we discuss briefly the observed shift
of the core position.