Serendipitous VLBI observations of polarization intraday variability in three BL Lacertae objects

Global lambda = 6 cm polarization very long baseline interferometry (VLBI) observations of 18 BL Lacertae objects were obtained in 1992 March. The pha sed Very Large Array (VLA) was included among the antennas in the VLB array , and provided measurements of the integrated total flux and polarization o f each of the sources during each 6.5-min VLBI scan. For nearly all the sou rces, these measurements were consistent with the fluxes and polarizations being constant during the 24-h VLBI experiment. However, in three objects ( 1334-127, 2131-021, 2155-152), the VLA measurements indicated that the inte grated polarizations varied during the course of the VLBI observations. Sub sequent analysis demonstrated that the variations in all three sources occu rred in the milliarcsecond-scale compact radio structure. In the case of 21 55-152, only one of two polarized milliarcsecond-scale features varied, whi le the polarization of the other feature was constant. These results sugges t that the observed 'intraday' polarization variability was dominated by va riations intrinsic to the sources.