Intraday polarization variability outside the VLBI core of the active galactic nucleus 0716+714

Rapid ('intraday') cm-wavelength variations in both total and polarized flu x density have been observed in a number of strong extragalactic radio sour ces. It is difficult to explain these variations purely as propagation effe cts, but if they are intrinsic to the sources, implausibly high brightness temperatures are required. We discuss here rapid polarization variability d uring our lambda=6 cm global very long-baseline interferometry (VLBI) obser vations of the active galactic nucleus 0716+714. Measurements made with the Very Large Array (VLA) during the VLBI observations indicate a similar or equal to 50 degrees swing in the position angle chi of the VLA core polariz ation in 12 hours. Corresponding variations were observed only for short VL BI baselines, so that they could not have occurred in the VLBI core (the on ly feature detected in our VLBI polarization map). The fact that the variat ions appear both in the VLA data and in the VLBI data for short baselines m akes it difficult to explain them as instrumental effects. This leads us to conclude that the rapid variations occurred outside the area covered by ou r VLBI map; we estimate that the variability occurred in some compact featu re roughly 25 milliarcseconds from the nucleus. It is clear that compact st ructures on a wide range of scales must be taken into account in studies of intraday variability in AGN.