High Energy Astrophysics 12 JAN, 2013

MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. VIII. Faraday rotation in parsec-scale AGN jets

MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. VIII. Faraday rotation in parsec-scale AGN jets

We report observations of Faraday rotation measures (RMs) for a sample of 191 extragalactic radio jets observed within the MOJAVE program. Multifrequency VLBA observations were carried out over twelve epochs in 2006 at four frequencies between 8 and 15 GHz. We detect parsec-scale Faraday RMs in 149 sources and find the quasars to have larger RMs on average than BL Lac objects. The median core RMs 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 RM 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 RM gradients in unresolved jets. Our simulations show that the finite image restoring beam size has a significant effect on the observed RM 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 RM gradients in four sources (0923+392, 1226+023, 2230+114 and 2251+158). In 1226+023 the RM 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 RM 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.

💡 Research Summary

The paper presents a comprehensive study of Faraday rotation measures (RMs) in a large sample of active‑galactic‑nucleus (AGN) jets observed as part of the MOJAVE (Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments) program. Using the Very Long Baseline Array (VLBA), the authors obtained multi‑frequency polarization data for 191 extragalactic radio jets over twelve epochs in 2006, covering four frequencies between 8 GHz and 15 GHz (8.0, 8.4, 12.0, and 15.0 GHz). By fitting the linear dependence of the observed electric‑vector position angle (EVPA) on λ², they derived RM values for both the compact core regions and the downstream jet components.

Out of the 191 sources, 149 (≈78 %) yielded reliable, parsec‑scale RM detections. The authors find systematic differences between source classes: quasars exhibit significantly larger absolute RMs than BL Lac objects, and the median RM in the core is substantially higher than in the extended jet. For quasars, a clear negative correlation is observed between the magnitude of the RM and the de‑projected distance from the core, indicating that the Faraday‑rotating medium becomes less dense or less magnetised as the jet expands.

A major part of the work is devoted to quantifying observational uncertainties. The authors perform Monte‑Carlo simulations of total intensity, linear polarization, and RM, explicitly modelling the effect of the finite restoring beam. They demonstrate that when a jet is narrower than roughly two beam widths in polarized intensity, noise can produce spurious transverse RM gradients. Consequently, they establish a practical resolution criterion: only jets broader than two beams can yield trustworthy transverse RM gradient measurements.

Applying this criterion to the data, the authors identify statistically significant transverse RM gradients in four sources: 0923+392, 1226+023 (3C 273), 2230+114, and 2251+158 (3C 454.3). In 3C 273, the gradient not only changes magnitude across the jet but also reverses sign from positive to negative, providing the first direct evidence of a helical magnetic field geometry on parsec scales in this object. Moreover, the RM pattern in 3C 273 varies on a three‑month timescale, a rapid change that is difficult to reconcile with a purely external Faraday screen and instead points to internal Faraday rotation within the jet plasma.

To interpret the depolarization behavior, the authors compare the observed frequency dependence of fractional polarization with several depolarization models. For the majority of sources, a simple external screen composed of a few random cells along the line of sight adequately reproduces the data, implying that the Faraday‑rotating medium is largely external and patchy. However, for 3C 273 and 3C 454.3 the external‑screen model fails; internal Faraday rotation must be invoked to explain the observed low fractional polarization and the complex RM structure.

In summary, this study combines high‑resolution VLBI polarization imaging with rigorous error analysis to map the magnetic‑field‑related Faraday rotation in AGN jets on parsec scales. It establishes that (1) cores are surrounded by denser, more magnetised plasma than the downstream jet, (2) quasars tend to have stronger Faraday rotation than BL Lacs, (3) transverse RM gradients—when reliably measured—provide compelling evidence for ordered (helical) magnetic fields, and (4) time‑variable RM in some jets indicates that internal Faraday rotation can be significant. These findings place strong constraints on models of jet launching, collimation, and interaction with the surrounding medium, and they highlight the importance of multi‑epoch, multi‑frequency VLBI polarimetry for probing the magneto‑ionic environment of relativistic jets.