The Need For High Resolution In Studies Of The 3-D Magnetic Field Structure Of AGN Jets

TL;DR

This study uses multi-frequency VLBA polarization observations to investigate the 3-D magnetic field structure of AGN jets, revealing evidence of helical magnetic fields and their complex configurations.

Contribution

It demonstrates the effectiveness of broadband multi-frequency polarization observations in mapping the 3-D magnetic field structure of AGN jets, including detection of transverse RM gradients.

Findings

Detection of transverse RM gradients indicating helical B fields

Different signs of RM in core regions suggest jet bends affecting magnetic field orientation

Future high-resolution polarization observations will further elucidate jet magnetic structures

Abstract

We are using "broadband" (4.6 to 43 GHz) multi-frequency VLBA polarization observations of compact AGN to investigate the 3-D structure of their jet magnetic (B) fields. Observing at several frequencies, separated by short and long intervals, enables reliable determination of the distribution of Faraday Rotation, and thereby the intrinsic B field structure. Transverse Rotation Measure (RM) gradients were detected in the jets of 0954+658 and 1418+546, providing evidence for the presence of a helical B field surrounding the jet. The RM in the core regions of 2200+420 (BL Lac), 0954+658 and 1418+546 display different signs in different frequency-intervals (on different spatial scales); we suggest an explanation for this in terms of modest bends in a helical B field surrounding their jets. In future, polarization observations with a combination of VSOP-2 at 8, 22 and 43 GHz and ground arrays at frequencies with corresponding resolution will help map out the distributions of Faraday rotation, spectral index and the 3-D B field structure both across the jet and closer to the central engine, providing strong constraints for any jet B field models.