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

Total intensity and polarization l 6 cm Very Large Array (VLA) and global very long baseline interferometry (VLBI) images of the quasar 09171624 and the BL Lacertae object 09541658 (both at epoch 1991.43) are analysed. Integrated measurements using the VLA during the VLBI observations indicated that, although there were no substantial total intensity variations, there were significant polarization variations for both sources during the 24-h VLBI experiment. The VLBI data were divided into 23 h segments in order to try to identify corresponding rapid variability in the VLBI structure. This analysis revealed intraday variability (IDV) in the VLBI core of 09171624: both the polarized flux and the polarization position angle varied substantially on time-scales of ,510 h. There is evidence that the VLA polarization variations for 09541658 occurred in an inner VLBI jet component, where the polarized flux varied by ,3040 per cent on time-scales of ,2 h. 09171624 and 09541658 were observed together with 07161714, 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 previously 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 suggests that polarization IDV occurs frequently in at least some IDV sources.

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