Searching For Helical Magnetic Fields in Active Galactic Nuclei

TL;DR

This study provides new evidence of helical magnetic fields in the jets of active galactic nuclei by detecting systematic Faraday Rotation gradients across multiple BL Lac objects using VLBA polarization data.

Contribution

It extends previous work by analyzing eight additional BL Lac objects, confirming the presence of helical magnetic fields through observed RM gradients in jets and cores.

Findings

RM gradients are observed across jets, consistent with helical B fields.

Gradients are also detected in the cores of several sources.

Reversals in RM gradients are observed between cores and jets in some sources.

Abstract

Previously, a multi-wavelength (2cm, 4cm and 6cm) polarization study by Gabuzda, Murray and Cronin (2004) showed systematic Faraday Rotation gradients across the parsec-scale jets of several BL Lac objects, interpreted as evidence for helical magnetic (B) fields - the gradients were taken to be due to the systematic variation of the line-of-sight B field across the jet. We present here new results for the parsec-scale Faraday Rotation distributions for eight additional BL Lac objects, based on polarization data obtained with the Very Long Baseline Array (VLBA) at two wavelengths near each of the 2cm, 4cm and 6cm bands. The Rotation Measure (RM) maps for all these sources indicate gradients across their jets, as expected if these jets have helical B fields. Such gradients are also detected in the cores of several sources. This provides evidence that these gradients are present in appreciable regions of the jets and are not isolated phenomena. We also observe reversals in the RM gradient in the core region as compared to the gradient in the jet in at least three sources.