2013MNRAS.429.3551A

We have studied very long baseline array (VLBA) polarimetric observations of eight sources, including quasars and BL Lacertae objects, at 12, 15, 22, 24 and 43 GHz. We present high-frequency rotation-measure maps. We find typical values for the rotation measure in the very long baseline interferometry core of several thousand rad/m2. These values are higher than the values given in the literature at lower frequencies. Assuming a dependence of the form RM ∝ νa, where RM is the rotation measure, we obtain an average value of a = 3.6±1.3, which is larger than that expected using theoretical considerations. Rotation measures are detected in the jets of only two sources, and we find that only 0906+430 (and possibly 1633+382) show indications of a robust gradient. We discuss the Faraday-corrected polarization properties of the sources. Our interpretation supports the presence of helical magnetic fields with new, unresolved, components affecting the intrinsic direction of polarization close to the base of the jet of some objects.