VLBI and VLA observations of intraday polarization variability in 0917+624and 0954+658

Total intensity and polarization lambda = 6 cm Very Large Array (VLA) and g lobal very long baseline interferometry (VLBI) images of the quasar 0917+62 4 and the BL Lacertae object 0954+658 (both at epoch 1991.43) are analysed. Integrated measurements using the VLA during the VLBI observations indicat ed that, although there were no substantial total intensity variations, the re were significant polarization variations for both sources during the 24- h VLBI experiment. The VLBI data were divided into 2-3 h segments in order to try to identify corresponding rapid variability in the VLBI structure. T his analysis revealed intraday variability (IDV) in the VLBI core of 0917+6 24: both the polarized flux and the polarization position angle varied subs tantially on time-scales of similar to 5-10 h. There is evidence that the V LA polarization variations for 0954+658 occurred in an inner VLBI jet compo nent, where the polarized flux varied by similar to 30-40 per cent on time- scales of similar to 2 h. 0917+624 and 0954+658 were observed together with 0716+714, an object that also displayed IDV in the polarized flux density measured during our experiment (analysed in a separate paper). These three sources were targeted for the VLBI observations since they had been previou sly identified as intraday variables, but we had no way of knowing whether they would vary during our observations. The fact that all three exhibited IDV in polarization (but not in total intensity) during our experiment sugg ests that polarization IDV occurs frequently in at least some IDV sources.