M. Rioja, R. Porcas, S. Garrington, A. Alberdi, D. Saikia
Relative astrometry and phase-reference mapping in the 1636+473 system using 
MERLIN and Global VLBI
 
                   ABSTRACT
 
 
We present astrometric results and maps from simultaneous MERLIN
and Global VLBI observations of the very weak core of the double-lobed
radio source 1636+47B, using as reference the bright, compact, flat-spectrum qua
1636+473 A, 20 arcsec away. The observations were made in May 1995 at 5 GHz.
The phase-referenced VLBI map of the weak core, of
resolution 1.5 mas, contains a high percentage of the core flux density seen in
the MERLIN map. Further hybrid mapping iterations reveal a
faint, one-sided core extension on the same side as the MERLIN jet.
We investigate the effect of the stucture in the strong quasar core on
the astrometric separation measurement, and
explore the effect of both temporal and other coherence losses
on the phase-reference map. We relate our results to the structural asymmetry
in weak AGN cores within the context of unification models.
 
-----------------------------------------------------------------------
In Proceedings of the 13th Working Meeting on European VLBI for Geodesy
and Astrometry, Viechtach, 1999
 
Keywords: VLBI, astrometry, phase referencing, quasars