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 for a sample of 191 extragalactic radio jets observed within the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE) program. Multifrequency Very Long Baseline Array (VLBA) observations were carried out over twelve epochs in 2006 at four frequencies between 8 and 15 GHz. We detect parsec-scale Faraday rotation measures in 149 sources and find the quasars to have larger rotation measures on average than BL Lac objects. The median core rotation measures are significantly higher than in the jet components. This is especially true for quasars where we detect a significant negative correlation between the magnitude of the rotation measure and the de-projected distance from the core. We perform detailed simulations of the observational errors of total intensity, polarization and Faraday rotation, and concentrate on the errors of transverse Faraday rotation measure gradients in unresolved jets. Our simulations show that the finite image restoring beam size has a significant effect on the observed rotation measure gradients, and spurious gradients can occur due to noise in the data if the jet is less than two beams wide in polarization. We detect significant transverse rotation measure gradients in four sources (0923+392, 1226+023, 2230+114 and 2251+158). In 1226+023 the rotation measure is for the first time seen to change sign from positive to negative over the transverse cuts, which supports the presence of a helical magnetic field in the jet. In this source we also detect variations in the jet rotation measure over a time scale of three months, which are difficult to explain with external Faraday screens and suggest internal Faraday rotation. By comparing fractional polarization changes in jet components between the four frequency bands to depolarization models we find that an external purely random Faraday screen viewed through only a few lines of sight can explain most of our polarization observations but in some sources, such as 1226+023 and 2251+158, internal Faraday rotation is needed.

18–22 cm VLBA Observational Evidence for Toroidal B-Field Components in Six AGN Jets

“…It was pointed out by Blandford [1] that, in the presence of a helical B-field, transverse Faraday rotation measure gradients should be observed across AGN jets, due to the systematic change in the line-of-sight component of the B-field. The detection of these gradients was first reported by Asada et al [2], and more recently in the studies [3][4][5][6][7][8], providing evidence for the presence of an azimuthal (toroidal) field component which may be associated with helical fields threading these jets.…”

“…The polarization angle maps used to make the RM maps were all convolved with the lowest-frequency beam to match their resolutions. We removed the effect of integrated Galactic foreground RM [15,16] before constructing the Faraday RM maps, and applied the results of [3] when estimating the RM uncertainties.…”

“…However, their analysis was subject to uncertainty due to the inability at that time to derive accurate estimates of the statistical significances of these transverse RM gradients. A number of recent studies have been aimed at reanalyzing the previously published RM maps considered in [18] using the approach developed in [3] to more accurately and reliably estimate the uncertainties in the measured RM values and the statistical significances of detected transverse RM gradients [6,7]. The recent analysis of Christodoulou et al [19], based on available RM maps on relatively large scales exceeding about 20 pc, has provided strong evidence for a preference for CCW RM gradients (or outward jet currents) on these larger scales.…”

On the Time Variable Rotation Measure in the Core Region of Markarian 421

“…For example, in the jet region of the radio source 3C273, Asada et al (2008) [10] found a time-variable RM, and in some cases RM sign reversals are observed with distance from the radio core and within different frequency intervals [4,5,11]. Another important finding is the detection of RM gradient transverse to the jet axis, suggesting the presence of helical magnetic fields in relativistic jets (e.g., [5,[11][12][13]). …”

“…For the RM estimate, multi-frequency polarimetric observations with the Very Long Baseline Interferometry (VLBI) technique represent the best tool, allowing us to investigate the magnetic field configuration in the innermost regions of the AGN jets (e.g., [2][3][4][5][6][7][8]). …”