MOJAVE: MONITORING OF JETS IN ACTIVE GALACTIC NUCLEI WITH VLBA EXPERIMENTS. VIII. FARADAY ROTATION IN PARSEC-SCALE AGN JETS

Abstract

We report observations of Faraday rotation measures (RMs) for a sample of 191\nextragalactic radio jets observed within the MOJAVE program. Multifrequency\nVLBA observations were carried out over twelve epochs in 2006 at four\nfrequencies between 8 and 15 GHz. We detect parsec-scale Faraday RMs in 149\nsources and find the quasars to have larger RMs on average than BL Lac objects.\nThe median core RMs are significantly higher than in the jet components. This\nis especially true for quasars where we detect a significant negative\ncorrelation between the magnitude of the RM and the de-projected distance from\nthe core. We perform detailed simulations of the observational errors of total\nintensity, polarization and Faraday rotation, and concentrate on the errors of\ntransverse Faraday RM gradients in unresolved jets. Our simulations show that\nthe finite image restoring beam size has a significant effect on the observed\nRM gradients, and spurious gradients can occur due to noise in the data if the\njet is less than two beams wide in polarization. We detect significant\ntransverse RM gradients in four sources (0923+392, 1226+023, 2230+114 and\n2251+158). In 1226+023 the RM is for the first time seen to change sign from\npositive to negative over the transverse cuts, which supports the presence of a\nhelical magnetic field in the jet. In this source we also detect variations in\nthe jet RM over a time scale of three months, which are difficult to explain\nwith external Faraday screens and suggest internal Faraday rotation. By\ncomparing fractional polarization changes in jet components between the four\nfrequency bands to depolarization models we find that an external purely random\nFaraday screen viewed through only a few lines of sight can explain most of our\npolarization observations but in some sources, such as 1226+023 and 2251+158,\ninternal Faraday rotation is needed.\n