Dc. Gabuzda et al., Intraday polarization variability outside the VLBI core of the active galactic nucleus 0716+714, M NOT R AST, 313(3), 2000, pp. 627-636
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.