4 Summary

We present 8.4 GHz VLBA maps of the thirteen extragalactic radio sources from the S5 polar cap sample. For two epochs, all sources of the sample have been imaged (Figs. 2-16) with mas-resolution to the mJy level (Table 1). In all cases, we have modeled the interferometric visibilities with elliptical Gaussian models (Table 2).

We can report some morphological changes for selected radio sources. The sources with more evident variations are QSO0016+731, QSO0836+710, QSO1928+738 and BL2007+777. For QSO0016+731 the variations have been surprising, since the source is rather compact and does not show a prominent jet structure larger than 2-3 mas at 8.4 GHz. Its change in morphology is associated with the strong decrease in flux density of one of the components from the first to the second epoch. QSO0836+710 and QSO1928+738 show the familiar changes in structure produced by emerging components in a core-jet system, as reported by Otterbein et al. (1998) for the former, and Hummel et al. (1992a) for the latter. The BLLac object 2007+777 displays an interesting, and hard to explain change in position angle towards north for components near to the core, a fact also reported by Krichbaum et al. (2000) from 5 GHz VSOP observations.

The images are the result of first and second epochs of a phase-delay astrometric program intended to check the absolute kinematics of all radio sources of the sample with precisions better than 100$\mu$as. Such accurate results will be obtained after the mapping and astrometric reduction of more observing epochs is carried out. This multi-source astrometric approach provides a large number of constraints for all the relative source pairs, which allow a precise registration of the maps through the observing epochs (as shown by Ros et al. 1999 for triplets of radio sources).

We have recently extended our astrometric programme to 15 and 43 GHz. At these frequencies, and based on test observations, we expect to attain astrometric precisions of 50 and 20 $\mu$as, respectively. The two-fold and five-fold improved resolution of those observations with respect to the observations presented here, combined with the expected astrometric precisions, will allow precise registrations of all the sources at each and all wavelengths for all epochs, and will provide unprecedented spectral information of components of milliarcsecond sources. The determination of the detailed kinematics and spectral content of the compact components of a complete sample of radio sources should then turn out to be a decisive element in our understanding of the activity around the cores of these objects and a definitive test of the standard jet model.

Acknowledgements

This work has been partially financed by Grant PB96-0782 of the Spanish DGICYT. This research has made use of data from the University of Michigan Radio Astronomy Observatory which is supported by the National Science Foundation and by funds from the University of Michigan. NRAO is operated under license by Associated Universities Inc., under cooperative agreement with NSF.