Polarization Observations of AGN Jets Past and Future

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Citation: Park, J.; Algaba, J.C.

Polarization Observations of AGN

Jets: Past and Future. Galaxies 2022,

10, 102. https://doi.org/10.3390/

galaxies10050102

Academic Editor: Dario Gasparrini

Received: 31 August 2022

Accepted: 18 October 2022

Published: 20 October 2022

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galaxies

Review

Polarization Observations of AGN Jets: Past and Future

Jongho Park 1,2,* and Juan Carlos Algaba 3

1Korea Astronomy and Space Science Institute, Yuseong-gu, Daejeon 34055, Korea

2Institute of Astronomy & Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building,

AS/NTU No. 1, Taipei 10617, Taiwan

3Department of Physics, Faculty of Science, Universiti Malaya, Kuala Lumpur 50603, Malaysi

*Correspondence: jpark@kasi.re.kr

Abstract:

The magnetic ﬁeld is believed to play a critical role in the bulk acceleration and propagation of

jets produced in active galactic nuclei (AGN). Polarization observations of AGN jets provide valuable

information about their magnetic ﬁelds. As a result of radiative transfer, jet structure, and stratiﬁcation,

among other factors, it is not always straightforward to determine the magnetic ﬁeld structures from

observed polarization. We review these effects and their impact on polarization emission at a variety of

wavelengths, including radio, optical, and ultraviolet wavelengths in this paper. It is also possible to

study the magnetic ﬁeld in the launching and acceleration regions of AGN jets by using very long baseline

interferometry (VLBI), which occurs on a small physical scale. Due to the weak polarization of the jets in

these regions, probing the magnetic ﬁeld is generally difﬁcult. However, recent VLBI observations have

detected signiﬁcant polarization and Faraday rotation in some nearby sources. We present the results of

these observations as well as prospects for future observations. Additionally, we brieﬂy discuss recently

developed polarization calibration and imaging techniques for VLBI data, which enable more in-depth

analysis of the magnetic ﬁeld structure around supermassive black holes and in AGN jets.

Keywords:

galaxies: active; galaxies: jets; polarization; magnetic ﬁelds; techniques: interferometric; black

hole physics; accretion; accretion disks; radiation mechanisms: non-thermal

1. Introduction

In some active galactic nuclei (AGN), collimated outﬂows are observed, known as jets (e.g.,

[

]). They often move at relativistic speeds (e.g., [

]) and contribute to the evolution of the

interstellar and intergalactic medium by transferring momentum and energy (e.g., [

–

]). As

one of the most energetic phenomena in the universe, AGN jets also emit high-energy photons

at X-ray and γ-ray wavelengths (e.g., [7–9]) and even at very high energies (e.g., [10–12]).

It is believed that magnetic ﬁelds are important in the formation of relativistic jets in AGN.

They are twisted as a result of the frame-dragging effect of spinning black holes or differential

rotation of the accretion disk, which may produce jets (e.g., [

–

]). The magnetic ﬁeld is

also crucial for the acceleration of AGN jets to relativistic speeds. Magnetohydrodynamic

(MHD) models predict that jet acceleration can efﬁciently occur as a result of the Poynting ﬂux

to kinetic energy ﬂux conversion through the magnetic nozzle effect (e.g., [

–

]).

Furthermore, magnetic ﬁelds, through, for example, magnetic reconnection, may contribute

to strong ﬂares that are often observed in AGN across a wide range of the electromagnetic

spectrum (e.g., [27–29]).

Because these phenomena generally occur at very small spatial scales, polarization obser-

vations using very long baseline interferometry (VLBI) are well suited to study the magnetic

ﬁelds. A good example is the linear polarization image taken with the event horizon telescope

Galaxies 2022,10, 102. https://doi.org/10.3390/galaxies10050102 https://www.mdpi.com/journal/galaxies

arXiv:2210.13819v1 [astro-ph.HE] 25 Oct 2022

Galaxies 2022,10, 102 2 of 23

(EHT) of the ring-like structure surrounding the dark shadow of the supermassive black hole

in M87 [

]. Due to the very high angular resolution of the new millimeter VLBI arrays, it is

now possible to directly observe the magnetic ﬁeld in the vicinity of supermassive black holes.

A number of polarization observations of blazars and nearby radio galaxies using centimeter

VLBI arrays have also provided valuable insights into magnetic ﬁeld structures and their role

in jet collimation, bulk acceleration, and particle acceleration.

In this review, we brieﬂy describe some of the results of recent polarization observations

of AGN jets, particularly the VLBI observations of nearby AGN jets. Readers are referred to

recent reviews by Blandford et al. [

], Boccardi et al. [

], and Hada et al. [

] for the latest

progress of observational studies of AGN jets, including results from total intensity imaging,

multiwavelength studies, theoretical modeling efforts, etc. In Section 2, we summarize early

attempts to correlate observed polarization properties across the electromagnetic spectrum, in

particular between optical and radio bands, and explain these observations in terms of emission

and propagation effects. Our Section 3brieﬂy summarizes recent polarimetric studies using

VLBI for nearby AGN jets, M87, 3C 84, and 3C 273. In Section 4, we review recent progress in

developing new algorithms for instrumental polarization calibration and polarimetric imaging

of VLBI data, which could be crucial for future observations of VLBI polarization. Section 6

summarizes and concludes the paper.

2. Polarization Studies in Blazar Jets

The electromagnetic spectrum, in particular from radio to optical and ultraviolet bands, of

most radio-loud AGN is dominated by synchrotron radiation, arising from presumably the

same electron population or, at least, populations with similar physical properties. It would

thus be naïve to expect that observations would indicate similarities and a close connection in

the properties at the different bands.

In this section, we review efforts to correlate optical and radio polarization observables in

blazars, such as total (

) and fractional (

) polarization, or the electric vector position angle

(EVPA,

), as well as their variability, and how understanding where the differences arise has

helped to understand both the dynamics as well as the radiative processes of blazar jets.

2.1. Early Optical–Radio Polarization Correlations

Kinman et al. [

] studied the OJ 287 optical and radio polarization and found that the

source was strongly polarized at optical wavelengths, falling to only one or two percent at

short centimeter wavelengths and then rising to nearly 5% at 11 cm. Furthermore, the strong

variability found did not appear to show any correlations among the bands. They pointed

out that even when considering Faraday and depolarization effects, the observations could

only be explained if the source was inhomogeneous, and the emission at 11 cm was arising

at a different region with a more ordered different magnetic ﬁeld conﬁgurations, or with a

much lower thermal electron density. Such an inhomogeneous model was supported by their

analysis of centimeter wavelength spectral indices.

The study of Rudnick et al. [

] included quasi-simultaneous observations at cm, mm,

infrared, and optical for OJ 287, BL Lac, and 0735+17. They observed a very small or negligible

Faraday rotation on radio bands, and a general increase of the degree of polarization toward

shorter wavelengths. For OJ 287, the EVPA appear to be very similar on all observing bands,

including optical with the exception of 11 cm, which was ascribed to an optical depth transition

at that wavelength. These suggest that, contrary to previous observations, there is a strong

relationship among the emitting region from radio to optical in this source. The case of BL Lac

seemed quite similar, with EVPA

∼

◦

from optical to cm wavelengths, whereas at longer

wavelengths, the EVPA change could not be simply explained by opacity. On 0735+17, on the

other hand, a difference of ∼60◦was found between optical and radio EVPA.

Galaxies 2022,10, 102 3 of 23

2.2. Resolving the Polarization Structure

Based on the observations described above, it seemed that correlation between optical

and radio polarization properties occurred only in certain cases. These correlations would

not only be subject to the source considered, but would also be time dependent. The initial

consideration would be that such correlations would be observed only while the source would

be homogeneous, the emission co-spatial, and opacity effects properly accounted for. In the

most general case, however, different physical properties of the regions, such as magnetic

ﬁeld strength, morphology or entanglement, size of the emitting region, or opacity, or other

frequency-dependent radiative transfer effects, would generally lead to different optical and

radio emission properties. Therefore, no correlations with optical and radio variability and

polarization properties may be found a priori in all sources. The obvious question is, of course,

what is the mechanism behind these special cases where a clear correlation can be found? Is

the correlation particular to some sources with very particular or special characteristics, or is it

rather a common feature to most AGNs, only hidden by external optical, geometric, and/or

observational effects?

In some theoretical models, it is indeed possible that, even considering general inhomo-

geneous synchrotron source models, the observed radio and UV optical infrared (UVOIR)

emission can be co-spatial. Ghisellini et al. [

] considered the emission from an inhomoge-

neous axially symmetric region with general assumptions on geometry and radial dependences

of physical quantities. In this model, the total intensity at a given frequency is dominated by

the contribution from all the regions that are optically thin. It is seen that the factor governing

whether the inner or the outer regions dominate the emission is purely geometrical (see their

Figure 2). For example, for a jet with conical geometry, the emission above the self-absorption

spectral frequency

νbr

will be dominated by the inner regions, whereas emission below

νbr

will have contributions of larger radii with lower frequency. On the contrary, for a jet with

parabolic geometry, the whole UVOIR spectrum may be dominated by the outer regions. This

is remarkably interesting because recent ﬁndings [

–

] suggest that many sources may have

a jet geometry break, with the upstream regions of the jet with a parabolic and the downstream

with a conical shape.

This suggests that a powerful tool to investigate if UVOIR emission is co-spatial would be

to investigate the polarization on resolved structures, so that we can have better chances of

separating the emission from different regions. Because many of the sources appear resolved

only on milli-arcsecond scales, it was only possible to study the co-spatiality of the emission

with the emergence of the VLBI technique. In general, the milli-arcsecond structure of most

radio-loud AGN jets will be that of a compact and typically optically thick “core” and an

optically thin jet appearing as a series of components or knots. Because the polarization of the

core and/or these components may differ both in

and

, they may contribute differently to

the integrated polarization properties, and when comparing with the optical properties we

may be able to identify one (or more) of these components to which the optical polarization is

associated.

Works in this direction were ﬁrst performed by Gabuzda et al. [

] who compared the

6 cm VLBI and optical EVPA of a sample of sources. They found that

χopt

and

χVLBI

of the core

or (in some cases) a prominently polarized jet component near the core were either aligned or

perpendicular for the BL Lac objects 0735+178, 1147+245, 1219+285, 1418+546, and 1749+096,

OJ 287, and BL Lac, and the QSO 3C 279. They found the probability of this happening by

chance to be very small small. They therefore argued for a connection between the optical and

the VLBI EVPAs.

To explain this connection, they indicated that polarization of BL Lacs at centimeter

wavelengths appear to be dominated by newly emerging VLBI components. In this case, the

observed bimodal distribution could arise due to the presence of an unresolved emerging VLBI

Galaxies 2022,10, 102 4 of 23

component. The VLBI core radio EVPA

χVLBI

would then be initially perpendicular to optical

χopt

due to opacity effects, as long as it appears blended within an optically thick region. As the

component evolves in time and reaches an optically think regime, the

χVLBI

and

χopt

would

become aligned with

χopt

. It is worth noting, however, that the ﬂip in the EVPA would only

happen in regions with very large opacity (

τ&

5 [

]), and in general, contributions from the

most optically thin regions would dominate the radio emission, and therefore an opacity-based

90◦EVPA ﬂip would occur only in rare cases.

This work was later expanded upon by Algaba et al. [

]. They included corrections

for Faraday rotation effects and a larger sample of sources, including not only BL Lac objects,

but also low and high polarized radio quasars. These studies put a much tighter constraint on

the probability of the optical and the radio VLBI EVPAs to be either parallel or perpendicular

to each other in BL Lacs. They also found that this relationship did not appear to be that simple

in the case of quasars. For the latter, no obvious correlation with the degree of polarization,

depolarization between optical and radio bands or magnetic ﬁeld strength was found (see

Figure 1). This suggested that either BL Lacs were a particular class of objects with a peculiar

emission geometry, or there were other additional factors that had not been taken into account.

Figure 1.

Histogram of the difference between the radio-core and optical polarization angles

∆χ

, including

BL Lac objects (white), HPQs (dark grey), and LPQs (light grey). Reprinted with permission from Algaba

The core RM may nonetheless depend on the observing frequency as

RMcore,ν∝νa

. This

is due to the frequency-dependent shift of the core location owing to opacity effects together

with a gradient of electron density and magnetic ﬁeld from the central engine [

]. Park et al.

[

] studied the rotation measure (RM) trend in more detail with the Korean VLBI Network

(KVN) by using simultaneous observations at 22, 43, and 86 GHz, hereby obtaining resolved

VLBI RM analyses at one of the highest frequencies to date. Their results showed that the

suggested systematic RM increase and its frequency dependence continued at larger radio

frequencies. Interestingly, when comparing with optical bands, they found indications of a

saturation frequency of a few hundreds GHz for the majority of sources, where the RM value is

expected to reach a plateau and remain constant over higher frequencies. This suggests that

we could pinpoint to an actual structure, such as a recollimation shock, causing the emission.

Therefore, once we observe at large frequencies, or correct for such frequency-dependent RM

up to the saturation frequency, we should be able to connect the radio, optical and high-energy

polarization properties.

Galaxies 2022,10, 102 5 of 23

2.3. A Blazar Jet Polarization Model

Another hint in the phenomenology of the connection in the UVOIR polarization arises

from D’Arcangelo et al. [

], who studied the variation in the polarization characteristics of the

quasar 0420–014 during an 11-day monitoring campaign in late 2005. They found a rotation of

the 43 GHz VLBI EVPA in the VLBI core by an amount of more than 80

◦

, a trend that was also

followed by the optical EVPA. Furthermore, once the Faraday correction was considered, the

43 GHz and the optical EVPAs agreed well. Their observations matched well with a model

where the bulk of the polarized emission could be associated with a conical shock, whereas the

rest of the emission would be associated with mostly unpolarized regions.

Observations that captured a large EVPA rotation together with superluminal knots pass-

ing through the core strengthen the idea that polarization is dominated by shock components

in the jets. In Marscher et al. [

], a rotation of the EVPA by about 240

◦

in a ﬁve-day interval

was observed for BL Lac in the optical R band, whereas the degree of polarization dropped to a

minimum in the middle of the rotation. Interestingly, simultaneous 7-mm VLBI observations

show the appearance of a new superluminal component emerging from the core moving down-

ward from the jet with a position angle of about 190

◦

, parallel to the jet and in good agreement

with the ﬁnal value that the optical EVPA reached after the rotation (see Figure 2).

Figure 2.

Time evolution of BL Lac. (

) Optical ﬂux, electric vector position angle and polarization.

(b) A

43 GHz VLBI component. A clear ﬂare together with a polarization decrease and a rotation of about

240

◦

of the optical EVPA is seen together with the emergence from the core of a new VLBI component.

(Reprinted by permission from Springer Nature, 2008 [52] with permission of the authors.)

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Citation:Park,J.;Algaba,J.C.PolarizationObservationsofAGNJets:PastandFuture.Galaxies2022,10,102.https://doi.org/10.3390/galaxies10050102AcademicEditor:DarioGasparriniReceived:31August2022Accepted:18October2022Published:20October2022Publisher'sNote:MDPIstaysneutralwithregardtojurisdictionalclaimsinpu...

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