A short-term VLBI analysis of selected IDV sources

V. Impellizzeri, T. Krichbaum, U. Bach, L. Fuhrmann, A. Kraus, A. Witzel, 
A. Zensus

Four IDV sources were selected for VLBI observations in December 2000 
over a total of two weeks. The sources belong to the class of blazars 
and are subsample of the S5 catalogue. The VLBI observations were 
divided into four epochs and run in parallel to Effelsberg single dish 
flux-density monitoring for the whole duration of the VLBI session. Aim 
of our observation was to detect IDV related changes in the VLBI 
structure of the sources and possibly locate the origin of the 
variability (jet-core). Three of the four sources were found to vary in 
both total intensity, polarisation and EVPA on timescales of one day to 
two weeks. Most of the variation was found to occur in the core region 
of the sources. Most notably, in 0716+714 a 40% increase in polasation 
in the core of the source within one day was followed by a factor three 
drop two weeks later accompanied by a swing in the EVPA by 30 degrees. 
No changes were detected in the position of the jet components in any of 
the sources.