The Jets of AGN as giant co-axial cables (1712.08414v1)

Abstract: The currents carried by the jets of active galactic nuclei (AGNs) can be probed using maps of the Faraday rotation measure (RM), since a jet current will be accompanied by a toroidal magnetic (B) field, which will give rise to a systematic change in the RM across the jet. The aim of this study is to identify new AGNs displaying statistically significant transverse RM gradients across their parsec-scale jets, and to look for overall patterns in the implied directions for the toroidal B-field components and jet currents. We have carried out new analyses of Faraday RM maps derived from previously published 8.1, 8.4, 12.1 and 15.3 GHz data obtained in 2006 on the NRAO VLBA. In a number of important ways, our procedures were identical to those of the original authors, but several other key aspects of the new imaging and analysis differ from the original methods. Our new analysis has substantially increased the number of AGNs known to display transverse RM gradients. The collected data on parsec and kiloparsec scales indicate that the current typically flows inward along the jet axis and outward in a more extended region surrounding the jet, typical to the current structure of a coaxial cable, accompanied by a self-consistent system of nested helical B fields, whose toroidal components give rise to the observed transverse RM gradients. These new results make it possible for the first time to conclusively demonstrate the existence of a preferred direction for the toroidal B-field components - and therefore of the currents - of AGN jets. Discerning the origin of this current-field system is of cardinal importance for understanding the physical mechanisms leading to the formation of the intrinsic jet B field, which likely plays an important role in the propagation and collimation of the jets; one possibility is the action of a "cosmic battery".

• The paper demonstrates that transverse Faraday rotation gradients indicate a nested helical magnetic field structure with inward currents along the jet axis.
• It employs multi-frequency VLBA data and rigorous statistical methods to detect 18 firm cases among 191 AGNs, enhancing the reliability of the findings.
• The results support theoretical models such as the 'cosmic battery' by linking the observed magnetic configurations to processes in jet launching and collimation.

Overview of "The Jets of AGN as Giant Coaxial Cables"

The paper titled "The Jets of AGN as Giant Coaxial Cables," conducted by Gabuzda, Nagle, and Roche, investigates the nature of magnetic fields and currents in the jets of Active Galactic Nuclei (AGNs). This research employs Faraday rotation measure (RM) maps derived from high-resolution radio observations to identify and analyze transverse RM gradients in AGN jets. These gradients serve as indicators of toroidal magnetic field components and the associated jet currents.

Goals and Methodology

The primary objective of this paper is to expand the understanding of helical magnetic field structures in AGN jets by identifying statistically significant transverse RM gradients across parsec-scale jet structures. Leveraging previously published data obtained at 8.1, 8.4, 12.1, and 15.3 GHz via the NRAO Very Long Baseline Array (VLBA), this analysis builds on earlier work with new imaging methodologies and statistical assessments. In contrast to previous studies, this analysis matches resolutions differently across frequencies and performs a rigorous statistical treatment of RM gradient significances.

Results

The analysis reveals a substantive increase in the number of AGNs with detected transverse RM gradients, elucidating a clear tendency for jet currents to flow inward along the jet's axis and outward in a surrounding region. This consistent pattern resembles the structure of coaxial cables and indicates a nested helical magnetic field system. Such configurations potentially carry inward currents in the central axis and outward currents in a surrounding sheath, dictated by the observed RM gradients indicative of toroidal magnetic field components.

The paper details 18 firm detections of transverse RM gradients from a sample of 191 AGNs, significantly enhancing the statistical dataset for AGNs with observed RM gradients. This increase paves the way for a more comprehensive statistical analysis, showing a preference for inward currents among these jets, while acknowledging potential biases such as unresolvable small-scale magnetic fields or observational limitations.

Implications and Future Research

The identified asymmetry in the directions of jet currents (predominance of inward currents on parsec scales) is statistically significant, suggesting a non-random distribution of magnetic field orientations. This supports theories that propose nested helical fields driven by the action of the central supermassive black hole's rotation in conjunction with its accretion disk, potentially validating models such as the "cosmic battery." These findings indicate that AGN jets are electromagnetic structures shaped by complex interactions between inward and outward currents.

Future research could focus on further exploring the origins and implications of these magnetic configurations. Understanding the mechanisms driving these inward and outward currents, and the possible influence of rotational energy extraction (e.g., via the Blandford-Znajek mechanism), will be critical. Additionally, examining potential variability in these magnetic configurations over time, as observed in some AGNs, could uncover dynamic processes influencing jet propagation and collimation mechanisms.

Overall, this paper contributes a significant advancement in the understanding of AGN jet magnetic fields and currents, suggesting that the interplay of these elements is fundamental to comprehending jet launching, stability, and their broader cosmic impact as electromagnetic structures.