Dominance of outflowing electric currents on decaparsec to kiloparsec scales in extragalactic jets

Abstract

Context. Helical magnetic fields embedded in the jets of active galactic nuclei (AGNs) are required by the broad range of theoretical models that advocate for electromagnetic launching of the jets. In most models, the direction of the magnetic field is random, but if the axial field is generated by a Cosmic Battery generated by current in the direction of rotation in the accretion disk, there is a correlation between the directions of the spin of the AGN accretion disk and of the axial field, which leads to a specific direction for the axial electric current, azimuthal magnetic field, and the resulting observed transverse Faraday-rotation (FR) gradient across the jet, due to the systematic change in the line-of-sight magnetic field. Aims. We consider new observational evidence for the presence of a nested helical magnetic-field structure such as would be brought about by the operation of the Cosmic Battery, and make predictions about the expected behavior of transverse FR gradients observed on decaparsec and kiloparsec scales. Methods. We have jointly considered 27 detections of transverse FR gradients on parsec scales, four reports of reversals in the directions of observed transverse FR gradients observed on parsec-decaparsec scales, and five detections of transverse FR gradients on decaparsec-kiloparsec scales, one reported here for the first time. We also consider seven tentative additional examples of transverse FR gradients on kiloparsec scales, based on an initial visual inspection of published Very Large Array FR maps of 85 extragalactic radio sources, for three of which we have carried out quantitative analyses in order to quantitatively estimate the significances of the gradients. Results. The data considered indicate a predominance of transverse FR gradients in the clockwise direction on the sky (i.e., net axial current flowing inward in the jet) on parsec scales and in the counter-clockwise direction on the sky (i.e., net axial current flowing outward) on scales greater than about 10 pc, consistent with the expectations for the Cosmic Battery. The predominance of counter-clockwise FR gradients on larger scales has been established at the 3σ confidence level. Conclusions. The collected results provide evidence for a reversal in the direction of the net azimuthal magnetic field determining the ordered component of the observed FR images, with distance from the jet base. This can be understood if the dominant azimuthal field on parsec scales corresponds to an axial electric current flowing inward along the jet, whereas the (weaker) dominant azimuthal field on kiloparsec scales corresponds to a outward-flowing current in the outer sheath of the jet and/or an extended disk wind. This is precisely the current/magnetic field structure that should be generated by the Cosmic Battery.

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

“…Thus, there is no evidence in our results for the small number of sources we have considered for a preferential direction for the observed transverse RM gradients, and consequently for the currents flowing along the jets. This may be consistent with the model proposed by [19], but 18-22 cm RM maps for more AGN are required before any firm conclusions can be drawn.…”

“…The scales probed by our observations fall near the transition between the scales considered in [18] and in [19], making it of interest to see whether our results provide any evidence for a preferred direction for the observed transverse RM gradients on these scales. The fourth column in Table 1 lists the directions on the sky of the detected 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. They suggest that the claim of [18] for a preference of CW RM gradients (inward jet currents) on parsec scales and their finding of a preference of CCW RM gradients (outward jet currents) on larger scales can be understood in terms of a single self-consistent system of fields and currents, with an inward current along the jet axis surrounded by inner and outer regions of helical field with opposite directions for their azimuthal components, with these two regions separated by a current sheet carrying outward current.…”

Determining the Jet Poloidal B Field and Black-Hole Rotation Directions in AGNs

“…Such a correlation is actually predicted by the "cosmic battery" mechanism promoted in [28][29][30]. In this picture, Poynting-Robertson drag generates currents in the accretion disk in the direction of the rotation, coupling the directions of rotation and B p .…”

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

“…Thermal electrons within the jet sheath can act as a foreground Faraday screen. In this framework, we interpret the observed RM sign reversals reported in the upper panel of Figure 1 within the context of the magnetic tower models (e.g., [24,25]) by means of the Poynting-Robertson (PR) cosmic battery effect [26][27][28][29]. Because of the accretion disc differential rotation, through the PR effect, two nested helical fields are produced in the jet, anchored to the inner and outer accretion disk.…”