Astronomy & Astrophysics manuscript no. Gabuzda-AA-Dec2017 c
ESO 2017
December 25, 2017
The jets of AGN as giant coaxial cables.
Denise C. Gabuzda,1 Matt Nagle1 and Naomi Roche1
Dept. of Physics, University College Cork, Cork, Ireland.
Email: d.gabuzda@ucc.ie
Received ; accepted
ABSTRACT
Context. 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 field, which will give rise to a systematic change in the RM
across the jet.
Aims. The aim of this study is to identify new AGNs displaying statistically significant transverse RM gradients across their parsecscale
jets, in order to determine how often helical magnetic fields occur in AGN jets, and to look for overall patterns in the implied
directions for the toroidal field components and jet currents.
Methods. 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 Very Long Baseline Array (VLBA). In a number of key ways, our procedures were identical
to those of the original authors, but the new imaging and analysis differs from the original methods in several ways: the technique
used to match the resolutions at the different frequencies, limits on the widths spanned by the RM gradients analyzed, treatment of
core-region RM gradients, approach to estimation of the significances of the gradients analyzed, and inclusion of a supplementary
analysis using circular beams with areas equal to those of the corresponding elliptical naturally weighted beams.
Results. This new analysis has substantially increased the number of AGNs known to display transverse RM gradients that may
reflect the presence of a toroidal magnetic-field component. 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 co-axial cable, accompanied by a self-consistent system of nested helical magnetic fields, whose toroidal components
give rise to the observed transverse Faraday rotation gradients.
Conclusions. The new results presented here make it possible for the first time to conclusively demonstrate the existence of a preferred
direction for the toroidal magnetic-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
magnetic field, which likely plays an important role in the propagation and collimation of the jets; one possibility is the action of a
“cosmic battery”.
Key words. accretion, accretion disks—galaxies: active—galaxies: jets—galaxies: magnetic fields—magnetic fields
1. Introduction
Active galactic nuclei (AGNs) release vast amounts of energy,
whose ultimate source is a supermassive black hole in the galactic
nucleus. In so-called radio-loud AGNs, two relativistic jets
of plasma emanate from the nucleus, presumably along the rotational
axis of the black hole. The radio emission is synchrotron
radiation, and can be linearly polarized up to about 75% in optically
thin regions with uniform magnetic fields, with the polarization
angle χ orthogonal to the projection of the magnetic field
B onto the plane of the sky (Pacholczyk 1970).
Very Long Baseline Interferometry (VLBI) yields radio images
with very high resolution, corresponding to linear sizes of
the order of a parsec at the typical distances of AGNs. A structure
with a compact “core” at one end and a jet extending away
from it predominates for radio-loud AGNs. The VLBI jets are
virtually always one-sided, due to the relativistic aberration of
the radiation in the forward direction of the jets’ motion: one jet
approaches the Earth and is highly boosted, while the receding
jet is highly de-boosted.
A theoretical picture of the basic nature of this core–jet structure
was proposed by Blandford & K¨onigl (1979), in which the
“core” observed with VLBI corresponds to the “photosphere”
of the jet, where the optical depth is near unity, τ ≈ 1, and the
jet material makes a transition from optically thick to optically
thin. Although the orientation of the observed polarization angle
rotates 90◦ to become parallel to the synchrotron B field in
sufficiently optically thick regions, this transition does not occur
until an optical depth of τ ≈ 6, so that the polarized emission
observed in all regions, including the VLBI core, is effectively
expected to be optically thin (Cobb 1993,Wardle 2018).
Multi-frequency VLBI polarization observations provide information
about the wavelength dependence of the parsec-scale
polarization. One example is Faraday rotation occurring along
the line of sight between the emitting region and the observer.
Faraday rotation is a rotation of the observed linear polarization
that arises when the associated electromagnetic wave passes
through a region with free electrons and a non-zero B field component
along the line of sight. The simplest case corresponds to
the situation when this mechanism operates in regions of “thermal”
(non-relativistic or only mildly relativistic) plasma outside
the emitting region, when the rotation is given by
χobs − χo =
e3λ2
8π2ǫom2c3 Z neB · dl ≡ RMλ2, (1)
ESO 2017
December 25, 2017
The jets of AGN as giant coaxial cables.
Denise C. Gabuzda,1 Matt Nagle1 and Naomi Roche1
Dept. of Physics, University College Cork, Cork, Ireland.
Email: d.gabuzda@ucc.ie
Received ; accepted
ABSTRACT
Context. 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 field, which will give rise to a systematic change in the RM
across the jet.
Aims. The aim of this study is to identify new AGNs displaying statistically significant transverse RM gradients across their parsecscale
jets, in order to determine how often helical magnetic fields occur in AGN jets, and to look for overall patterns in the implied
directions for the toroidal field components and jet currents.
Methods. 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 Very Long Baseline Array (VLBA). In a number of key ways, our procedures were identical
to those of the original authors, but the new imaging and analysis differs from the original methods in several ways: the technique
used to match the resolutions at the different frequencies, limits on the widths spanned by the RM gradients analyzed, treatment of
core-region RM gradients, approach to estimation of the significances of the gradients analyzed, and inclusion of a supplementary
analysis using circular beams with areas equal to those of the corresponding elliptical naturally weighted beams.
Results. This new analysis has substantially increased the number of AGNs known to display transverse RM gradients that may
reflect the presence of a toroidal magnetic-field component. 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 co-axial cable, accompanied by a self-consistent system of nested helical magnetic fields, whose toroidal components
give rise to the observed transverse Faraday rotation gradients.
Conclusions. The new results presented here make it possible for the first time to conclusively demonstrate the existence of a preferred
direction for the toroidal magnetic-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
magnetic field, which likely plays an important role in the propagation and collimation of the jets; one possibility is the action of a
“cosmic battery”.
Key words. accretion, accretion disks—galaxies: active—galaxies: jets—galaxies: magnetic fields—magnetic fields
1. Introduction
Active galactic nuclei (AGNs) release vast amounts of energy,
whose ultimate source is a supermassive black hole in the galactic
nucleus. In so-called radio-loud AGNs, two relativistic jets
of plasma emanate from the nucleus, presumably along the rotational
axis of the black hole. The radio emission is synchrotron
radiation, and can be linearly polarized up to about 75% in optically
thin regions with uniform magnetic fields, with the polarization
angle χ orthogonal to the projection of the magnetic field
B onto the plane of the sky (Pacholczyk 1970).
Very Long Baseline Interferometry (VLBI) yields radio images
with very high resolution, corresponding to linear sizes of
the order of a parsec at the typical distances of AGNs. A structure
with a compact “core” at one end and a jet extending away
from it predominates for radio-loud AGNs. The VLBI jets are
virtually always one-sided, due to the relativistic aberration of
the radiation in the forward direction of the jets’ motion: one jet
approaches the Earth and is highly boosted, while the receding
jet is highly de-boosted.
A theoretical picture of the basic nature of this core–jet structure
was proposed by Blandford & K¨onigl (1979), in which the
“core” observed with VLBI corresponds to the “photosphere”
of the jet, where the optical depth is near unity, τ ≈ 1, and the
jet material makes a transition from optically thick to optically
thin. Although the orientation of the observed polarization angle
rotates 90◦ to become parallel to the synchrotron B field in
sufficiently optically thick regions, this transition does not occur
until an optical depth of τ ≈ 6, so that the polarized emission
observed in all regions, including the VLBI core, is effectively
expected to be optically thin (Cobb 1993,Wardle 2018).
Multi-frequency VLBI polarization observations provide information
about the wavelength dependence of the parsec-scale
polarization. One example is Faraday rotation occurring along
the line of sight between the emitting region and the observer.
Faraday rotation is a rotation of the observed linear polarization
that arises when the associated electromagnetic wave passes
through a region with free electrons and a non-zero B field component
along the line of sight. The simplest case corresponds to
the situation when this mechanism operates in regions of “thermal”
(non-relativistic or only mildly relativistic) plasma outside
the emitting region, when the rotation is given by
χobs − χo =
e3λ2
8π2ǫom2c3 Z neB · dl ≡ RMλ2, (1)