Connecting magnetic towers with Faraday rotation gradients in active galactic nuclei jets

Abstract

The idea that systematic Faraday Rotation gradients across the parsec-scale jets of Active Galactic Nuclei (AGNs) can reveal the presence of helical magnetic (B) fields has been around since the early 1990s, although the first observation of this phenomenon was about ten years later. These gradients are taken to be due to the systematic variation of the line-of-sight B field across the jet. We present here the parsec-scale Faraday Rotation distributions for the BL Lac objects 0716+714 and 1749+701, based on polarization data obtained with the Very Long Baseline Array (VLBA) at two wavelengths near each of the 2cm, 4cm and 6cm bands (0716+714) and at four wavelengths in the range 18-22 cm (1749+701). The Rotation Measure (RM) maps for both these sources indicate systematic gradients across their jets, as expected if these jets have helical B fields. The significance of these transverse RM gradients is > 3σ in all cases. We present the results of Monte Carlo simulations directly demonstrating the possibility of observing such transverse RM gradients even if the intrinsic jet structure is much narrower than the observing beam. We observe an intriguing new feature in these sources, a reversal in the direction of the gradient in the jet as compared to the gradient in the core region. This provides new evidence to support models in which field lines emerging from the central region of the accretion disk and closing in the outer region of the accretion disk are both "wound up" by the differential rotation of the disk. The net observed RM gradient will essentially be the sum effect of two regions of helical field, one nested inside the other. The direction of the net RM gradient will be determined by whether the inner or outer helix dominates the RM integrated through the jet, and RM gradient reversals will be observed if the inner and outer helical fields dominate in different regions of the jet. This potentially provides new insights about the overall configuration of the jet B fields.

Magnetic fields and polarization in AGN jets

“…They also state "Attempting to detect RM gradients in jets less than two beams wide is discouraged as the probability to detect false positives exceeds 0.02 even when a 3σ limit is used." This statement is not quite in keeping with their own Figure 30, which, as quoted in [9] and [64], seems to indicate that for a 3σ limit, the false positive rate is only 0.01 even at one and a half jet beams. Hovatta (private communication) has confirmed that the figure is correct and that the "0.02" in the text was a typographical error, and also points out that a 1% false positive rate becomes more significant in the context of large surveys like MOJAVE where well over 100 sources were observed.…”

“…This point has now been extensively investigated with Monte-Carlo simulations by Hovatta et al [8], and also by Gabuzda and co-workers [63], [9], [64]. Hovatta et al emphasize that thermal noise can generate high RMs and spurious gradients if the jet is less than two beams wide in polarization.…”

“…Murphy et al [64] and Mahmud et al [9] demonstrated through Monte-Carlo simulations that a RM gradient in a jet whose width is even somewhat smaller than the beam can still be reliably detected, even though the magnitude of the gradient is reduced. Presumably this result must depend on the SNR, though that dependence is not quite clear in these papers.…”

“…This is based directly on the "magnetic tower" model of Lynden-Bell [70]. The idea is further developed in [9]. However, Broderick & McKinney [62] criticize this model both on physical grounds, and because their GRMHD simulations reveal no such structures.…”

“…These have been investigated in detail both analytically and by extensive simulations in largely unsung appendices to observational papers e.g. [6], [7], [8], [9]. They should be read by every practitioner in the field.…”