Source structure and VLBI position instabilities
Résumé
Most of the extragalactic radio sources that materialize the International Celestial Reference Frame (ICRF) exhibit spatially - extended structures on milliarcsecond scales which limit the accuracy of the source positions estimated from Very Long Baseline Interferometry (VLBI) data. These structures when varying with time may also lead to instabilities in the source positions. Structural variations are generally due to ejection of material from the VLBI core in a recurrent although unpredictable manner. Based on the available VLBI astrometric data and source images, we compare position instabilities and structural variations and conclude about the level of correlation between the two phenomena.