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Polarimetry as a Probe of Magnetic Fields in AGN Jets
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Polarimetry as a Probe of Magnetic Fields in AGN Jets

A special issue of Galaxies (ISSN 2075-4434).

Deadline for manuscript submissions: closed (31 March 2021) | Viewed by 28712

Special Issue Editors

Dr. Margo Aller

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Guest Editor
Department of Astronomy, University of Michigan, Ann Arbor, MI 48109-1042, USA
Interests: blazars; relativistic jets; radio band polarization
Dr. Jose L. Gómez

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Guest Editor
Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía S/N, 18008 Granada, Spain
Interests: black holes; active galactic nuclei; relativistic jets; blazars; RMHD simulations
Special Issues, Collections and Topics in MDPI journals
Prof. Dr. Eric Perlman

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Guest Editor
Department of Physics & Space Sciences, Florida Institute of Technology, Melbourne, FL 32901, USA
Interests: AGN physics; jet physics; observational tests
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues, 

The relativistic jets produced by active galactic nuclei are, according to unified schemes, created very close to the central black hole as an integral part of the accretion process. Magnetic fields are believed to be essential to the processes that create and launch these jets, and they contribute to the processes by which emitting particles are accelerated. One of the few ways that these magnetic fields can be traced is through polarimetry.

This special issue of Galaxies entitled Polarimetry as a Probe of Magnetic Fields in AGN Jets, motivated in part by presentations and discussion at a special session on polarimetry at the conference “The 3C Extragalactic Radio Sky” held in Torino, Italy in September 2019, is dedicated to papers describing progress in understanding magnetic fields in AGN jets using polarization as a tool. The quality and number of observations have increased dramatically in the past few years with the advent of new monitoring programs in the optical and radio bands; technical upgrades in instruments such as the JVLA and VLBA and the participation of ALMA in mm-VLBI arrays such as the EHT and GMVA, providing dramatically-improved sensitivity and resolution; and the extension of the spectral coverage to new and unexplored domains (IXPE). Concurrently, theoretical explorations of the temporal and spatial evolution of jet magnetic field structure extending from the black hole/accretion disk to kiloparsec scales and radiative transfer modeling have provided new insights and challenges to our understanding of magnetic field geometry and its evolution. This issue is dedicated to describing these observational and theoretical advances and will include both solicited and contributed papers. We invite you to submit articles and reviews addressing these topics.

Invited contributing authors include Ivan Agudo and Clemens Thum, Denise Gabuzda, Ciriaco Goddi, Talvikki Hovatta,  Alan Marscher, Ivan Marti-Vidal, Ken Nishikawa, Alice Pasetto, Claudia Raiteri, Lerato Sebokolodi, and Fabrizio Tavecchio.

Dr. Margo F. Aller
Prof. José L. Gómez
Prof. Eric S. Perlman
Guest Editors

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Galaxiesis an international peer-reviewed open access quarterly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • active galaxies;
  • jets;
  • quasars;
  • polarimeters;
  • polarimetry.

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Published Papers (10 papers)

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Research

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8 pages, 1880 KiB  
Article
High-Frequency Polarization Variability from Active Galactic Nuclei
by Iván Martí-Vidal and Ciriaco Goddi
Galaxies 2021, 9(3), 51; https://doi.org/10.3390/galaxies9030051 - 15 Jul 2021
Viewed by 2534
Abstract
The linear polarization of non-thermal emission encodes information about the structure of the magnetic fields, either from the region where the emission is produced (i.e., the intrinsic polarization angle) and/or from the screens of magnetized plasma that may be located on its way [...] Read more.
The linear polarization of non-thermal emission encodes information about the structure of the magnetic fields, either from the region where the emission is produced (i.e., the intrinsic polarization angle) and/or from the screens of magnetized plasma that may be located on its way towards Earth (i.e., the effect of Faraday rotation). In addition, the variability timescale of the polarized emission, or its Faraday rotation, can be used to estimate the size of the region where the emission (or the Faraday rotation) originates. The observation of polarized emission from active galactic nuclei (AGN) and, in particular, its time evolution, also provides information about the critical role that magnetic fields may play in the process of jet launching and propagation. In this paper, we review some recent results about polarization variability from the cores of AGN jets, including observations at high spatial resolutions and/or at high radio frequencies. Full article
(This article belongs to the Special Issue Polarimetry as a Probe of Magnetic Fields in AGN Jets)
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17 pages, 9296 KiB  
Article
Frequency and Time Dependence of Linear Polarization in Turbulent Jets of Blazars
by Alan P. Marscher and Svetlana G. Jorstad
Galaxies 2021, 9(2), 27; https://doi.org/10.3390/galaxies9020027 - 27 Apr 2021
Cited by 37 | Viewed by 2864
Abstract
Time-variable polarization is an extremely valuable observational tool to probe the dynamical physical conditions of blazar jets. Since 2008, we have been monitoring the flux and linear polarization of a sample of gamma-ray bright blazars at optical frequencies. Some of the observations were [...] Read more.
Time-variable polarization is an extremely valuable observational tool to probe the dynamical physical conditions of blazar jets. Since 2008, we have been monitoring the flux and linear polarization of a sample of gamma-ray bright blazars at optical frequencies. Some of the observations were performed on nightly or intra-night time-scales in four optical bands, providing information on the frequency and time dependence of the polarization. The observed behavior is similar to that found in simulations of turbulent plasma in a relativistic jet that contains a standing shock and/or a helical background magnetic field. Similar simulations predict the characteristics of X-ray synchrotron polarization of blazars that will be measured in the future by the Imaging X-ray Polarimetry Explorer (IXPE). Full article
(This article belongs to the Special Issue Polarimetry as a Probe of Magnetic Fields in AGN Jets)
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9 pages, 3526 KiB  
Article
Unraveling the Physics of Quasar Jets: Optical Polarimetry and Implications for the X-ray Emission Process
by Eric S. Perlman, Devon Clautice, Sayali Avachat, Mihai Cara, William B. Sparks, Markos Georganopoulos and Eileen Meyer
Galaxies 2020, 8(4), 71; https://doi.org/10.3390/galaxies8040071 - 27 Sep 2020
Cited by 9 | Viewed by 3022
Abstract
Since the launch of Chandra twenty years ago, one of the greatest mysteries surrounding Quasar Jets is the production mechanism for their extremely high X-ray luminosity. Two mechanisms have been proposed. In the first view, the X-ray emission is inverse-Comptonized CMB photons. This [...] Read more.
Since the launch of Chandra twenty years ago, one of the greatest mysteries surrounding Quasar Jets is the production mechanism for their extremely high X-ray luminosity. Two mechanisms have been proposed. In the first view, the X-ray emission is inverse-Comptonized CMB photons. This view requires a jet that is highly relativistic (bulk Lorentz factor >20–40) on scales of hundreds of kiloparsecs, and a jet that is comparably or more powerful than the black hole’s Eddington luminosity. The second possibility is synchrotron emission from a high-energy population of electrons. This requires a much less powerful jet that does not need to be relativistically beamed, but it imposes other extreme requirements, namely the need to accelerate particles to >100 TeV energies at distances of hundreds of kiloparsecs from the active nucleus. We are exploring these questions using a suite of observations from a diverse group of telescopes, including the Hubble Space Telescope (HST), Chandra X-ray Observatory (CXO), Fermi Gamma-ray Space Telescope and various radio telescope arrays. Our results strongly favor the hypothesis that the X-ray emission is synchrotron radiation from a separate, high-energy electron population. We discuss the observations, results and new questions brought up by these surprising results. We investigate the physical processes and magnetic field structure that may help to accelerate particles to such extreme energies. Full article
(This article belongs to the Special Issue Polarimetry as a Probe of Magnetic Fields in AGN Jets)
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19 pages, 1131 KiB  
Article
Diagnosing Magnetic Field Geometry in Blazar Jets Using Multi-Frequency, Centimeter-Band Polarimetry and Radiative Transfer Modeling
by Margo Aller, Philip Hughes, Hugh Aller and Talvikki Hovatta
Galaxies 2020, 8(1), 22; https://doi.org/10.3390/galaxies8010022 - 10 Mar 2020
Cited by 1 | Viewed by 2400
Abstract
We use multi-frequency linear polarization observations from the University of Michigan blazar program (UMRAO), in combination with radiative transfer simulations of emission from a relativistic jet, to investigate the time-dependent flow conditions, including magnetic field geometry, in an example blazar OT 081. We [...] Read more.
We use multi-frequency linear polarization observations from the University of Michigan blazar program (UMRAO), in combination with radiative transfer simulations of emission from a relativistic jet, to investigate the time-dependent flow conditions, including magnetic field geometry, in an example blazar OT 081. We adopt a scenario incorporating relativistic shocks during flaring, and both ordered axial and helical magnetic field components and magnetic turbulence in the underlying flow; these constituents are consistent with the observed periods of ordered behavior in the polarization intermixed with stochastic variations. The simulations are able to reproduce the global features of the observed light curves, including amplitude and spectral evolution of the linear polarization, during four time periods spanning 25 years. From the simulations, we identify the signature of a weak-to-strong helical magnetic field on the polarization, but conclude that a dominant helical magnetic field is not consistent with the UMRAO polarization data. The modeling identifies time-dependent changes in the ratio of the ordered-to-turbulent magnetic field, and changes in the flow direction and Lorentz factor. These suggest the presence of jet-like structures within a broad envelope seen at different orientations. Full article
(This article belongs to the Special Issue Polarimetry as a Probe of Magnetic Fields in AGN Jets)
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Review

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12 pages, 317 KiB  
Review
The Polarized Emission of AGN at Millimeter Wavelengths as Seen by POLAMI
by Iván Agudo and Clemens Thum
Galaxies 2022, 10(4), 87; https://doi.org/10.3390/galaxies10040087 - 4 Aug 2022
Cited by 3 | Viewed by 1897
Abstract
We review results from the POLAMI program, which monitors the polarization properties of 36 blazars at the IRAM 30 m telescope. We found that the variability of the degree of linear polarization is faster and of higher amplitude at 1 mm than at [...] Read more.
We review results from the POLAMI program, which monitors the polarization properties of 36 blazars at the IRAM 30 m telescope. We found that the variability of the degree of linear polarization is faster and of higher amplitude at 1 mm than at 3 mm and that the linear polarization is also more variable than the total flux. The linear polarization angle is highly variable in all sources with excursions > 180°; and for the case of the polarization angle, also the 1 mm variations appear to be faster than those at 3 mm. These results are fully compatible with recent multi-zone turbulent jet models, and they definitively rule out the popular single-zone models for blazars. They also further confirm that the short-wavelength (inner) emitting regions have better ordered magnetic fields than the long-wavelength ones (further downstream). Moreover, the POLAMI program has shown statistical evidence that, for most of the monitored sources, circular polarization emission is displayed the majority of the time. The circular polarization detection rate and the maximum degree of circular polarization found are comparable with previous surveys at much longer wavelengths, thus, opening a new window for circular polarization and jet composition studies in the mm range. The process generating circular polarization must not be strongly wavelength-dependent. The widespread presence of circular polarization in the POLAMI sample is likely due to Faraday conversion of the linearly polarized synchrotron radiation in the helical magnetic field of the jets. The peculiar behavior of circular polarization in 3C 66A, which we consider a hallmark of circular polarization generation by Faraday conversion in helical fields, is discussed. Full article
(This article belongs to the Special Issue Polarimetry as a Probe of Magnetic Fields in AGN Jets)
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21 pages, 5697 KiB  
Review
Inherent and Local Magnetic Field Structures in Jets from Active Galactic Nuclei
by Denise C. Gabuzda
Galaxies 2021, 9(3), 58; https://doi.org/10.3390/galaxies9030058 - 25 Aug 2021
Cited by 20 | Viewed by 4309
Abstract
In theoretical models for the electromagnetic launching of astrophysical jets, a helical magnetic (B)-field component is generated through the winding up of an initial longitudinal field component by the rotation of the cental black hole and accretion disk. This helical field [...] Read more.
In theoretical models for the electromagnetic launching of astrophysical jets, a helical magnetic (B)-field component is generated through the winding up of an initial longitudinal field component by the rotation of the cental black hole and accretion disk. This helical field component travels outward with the jet plasma. There is now abundant evidence that the jets of active galactic nuclei carry helical B fields, and the presence of such fields has been invoked to explain a wide range of phenomena observed in these jets. However, distinguishing between features associated with this inherent jet B field and with B fields generated by local phenomena such as shocks and shear can be challenging. There is now evidence that the field that is accreted is dipolar like, giving rise to a current distribution with inward currents along both jet axes and outward currents in a more extended region around the jets. Striking limb brightening has been observed for several relatively nearby active galactic nuclei; it is argued that this must be due to some intrinsic property of the jet, which is independent of the viewing angle, such as its helical B field, or mass loading and/or particle acceleration at the jet edges. Circular-polarization observations may make it possible to reconstruct the full three-dimensional B field of jets carrying a helical B-field component, and to correctly infer the direction of rotation of the central black hole and its accretion disk. Full article
(This article belongs to the Special Issue Polarimetry as a Probe of Magnetic Fields in AGN Jets)
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17 pages, 13212 KiB  
Review
Message in a Bottle: Unveiling the Magneto-Ionic Complexity of AGNs through the Stokes QU-Fitting Technique
by Alice Pasetto
Galaxies 2021, 9(3), 56; https://doi.org/10.3390/galaxies9030056 - 11 Aug 2021
Cited by 2 | Viewed by 2255
Abstract
Here, I overview one of the available techniques for the analysis of broad-band spectropolarimetric data, the Stokes QU-fitting. Since broad-band receivers have been installed at most radio facilities, the collection of radio data, both the total intensity and the linear polarization, is revealing [...] Read more.
Here, I overview one of the available techniques for the analysis of broad-band spectropolarimetric data, the Stokes QU-fitting. Since broad-band receivers have been installed at most radio facilities, the collection of radio data, both the total intensity and the linear polarization, is revealing interesting features in their spectra. The polarized light, and therefore its properties, i.e. the fractional polarization p and the polarized angle χ, are now finally well sampled in wide wavelength ranges. The new complex behaviors revealed by the data can be studied using the Stokes QU-fitting, which consists of modeling the Stokes parameters Q and U using wavelength-dependent analytical models, available in the literature. This technique provides a very good diagnostic of the nature and structure of the magnetized plasma, with the possibility to identify complex structures, internal or external, of the source of study. A summary of the available and most used models describing the polarization behavior, is presented. Moreover, some of the most significant observational works which use this technique are also summarized. Full article
(This article belongs to the Special Issue Polarimetry as a Probe of Magnetic Fields in AGN Jets)
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16 pages, 2585 KiB  
Review
An Unofficial Account of the Beginnings of VLBI Polarimetry: From Jodrell Bank to the Event Horizon Telescope
by John Wardle
Galaxies 2021, 9(3), 52; https://doi.org/10.3390/galaxies9030052 - 19 Jul 2021
Cited by 1 | Viewed by 2610
Abstract
I offer a brief and personal history of the development of polarization sensitive observations with widely separated antennas. The story starts at Jodrell Bank in the late 1960s with a 24 km baseline radio linked (but not phase stable) interferometer and reaches to [...] Read more.
I offer a brief and personal history of the development of polarization sensitive observations with widely separated antennas. The story starts at Jodrell Bank in the late 1960s with a 24 km baseline radio linked (but not phase stable) interferometer and reaches to the present Event Horizon Telescope (with global span and independent atomic clocks) which has just published an image of the linearly polarized radiation surrounding the black hole shadow of M87*. I was privileged to be witness to many of the developments along the way, either as an instigator, a bystander, or an unindicted co-conspirator. I am most interested in the technical developments that enabled these increasingly sophisticated observations, and in the ideas that advanced the data analysis and imaging. Full article
(This article belongs to the Special Issue Polarimetry as a Probe of Magnetic Fields in AGN Jets)
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9 pages, 404 KiB  
Review
Polarimetric Properties of Blazars Caught by the WEBT
by Claudia M. Raiteri and Massimo Villata
Galaxies 2021, 9(2), 42; https://doi.org/10.3390/galaxies9020042 - 11 Jun 2021
Cited by 9 | Viewed by 2529
Abstract
Active galactic nuclei come in many varieties. A minority of them are radio-loud, and exhibit two opposite prominent plasma jets extending from the proximity of the supermassive black hole up to megaparsec distances. When one of the relativistic jets is oriented closely to [...] Read more.
Active galactic nuclei come in many varieties. A minority of them are radio-loud, and exhibit two opposite prominent plasma jets extending from the proximity of the supermassive black hole up to megaparsec distances. When one of the relativistic jets is oriented closely to the line of sight, its emission is Doppler beamed and these objects show extreme variability properties at all wavelengths. These are called “blazars”. The unpredictable blazar variability, occurring on a continuous range of time-scales, from minutes to years, is most effectively investigated in a multi-wavelength context. Ground-based and space observations together contribute to give us a comprehensive picture of the blazar emission properties from the radio to the γ-ray band. Moreover, in recent years, a lot of effort has been devoted to the observation and analysis of the blazar polarimetric radio and optical behaviour, showing strong variability of both the polarisation degree and angle. The Whole Earth Blazar Telescope (WEBT) Collaboration, involving many tens of astronomers all around the globe, has been monitoring several blazars since 1997. The results of the corresponding data analysis have contributed to the understanding of the blazar phenomenon, particularly stressing the viability of a geometrical interpretation of the blazar variability. We review here the most significant polarimetric results achieved in the WEBT studies. Full article
(This article belongs to the Special Issue Polarimetry as a Probe of Magnetic Fields in AGN Jets)
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17 pages, 3474 KiB  
Review
Probing Magnetic Fields and Acceleration Mechanisms in Blazar Jets with X-ray Polarimetry
by Fabrizio Tavecchio
Galaxies 2021, 9(2), 37; https://doi.org/10.3390/galaxies9020037 - 18 May 2021
Cited by 20 | Viewed by 2948
Abstract
X-ray polarimetry promises us an unprecedented look at the structure of magnetic fields and on the processes at the base of acceleration of particles up to ultrarelativistic energies in relativistic jets. Crucial pieces of information are expected from observations of blazars (that are [...] Read more.
X-ray polarimetry promises us an unprecedented look at the structure of magnetic fields and on the processes at the base of acceleration of particles up to ultrarelativistic energies in relativistic jets. Crucial pieces of information are expected from observations of blazars (that are characterized by the presence of a jet pointing close to the Earth), in particular of the subclass defined by a synchrotron emission extending to the X-ray band (so-called high synchrotron peak blazars, HSP). In this review, I give an account of some of the models and numerical simulations developed to predict the polarimetric properties of HSP at high energy, contrasting the predictions of scenarios assuming particle acceleration at shock fronts with those that are based on magnetic reconnection, and I discuss the prospects for the observations of the upcoming Imaging X-ray Polarimetry Explorer (IXPE) satellite. Full article
(This article belongs to the Special Issue Polarimetry as a Probe of Magnetic Fields in AGN Jets)
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