Special Issue "Blazars through Sharp Multi-wavelength Eyes"

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

Deadline for manuscript submissions: closed (5 September 2016)

Special Issue Editors

Guest Editor
Dr. Jose L. Gómez

Instituto de Astrofísica de Andalucía–CSIC, Glorieta de la astronomía s/n, Granada 18008, Spain
Website | E-Mail
Interests: active galactic nuclei; relativistic jets; blazars; RMHD simulations
Guest Editor
Dr. Alan P. Marscher

Insitute for Astrophysical Research, Boston University, 725 Commonwealth Ave., Boston, MA 02215, USA
Website | E-Mail
Interests: active galactic nuclei; relativistic jets; blazars; black holes; non-thermal emission
Guest Editor
Dr. Svetlana G. Jorstad

Insitute for Astrophysical Research, Boston University, 725 Commonwealth Ave., Boston, MA 02215, USA
Website | E-Mail
Interests: active galactic nuclei; relativistic jets; blazars; polarization

Special Issue Information

Dear Colleagues,

Blazars, the most extreme variety of active galactic nuclei (AGN) and the most luminous long-lived individual objects in the universe, continue to pose major astrophysical puzzles. Blazars are powered by accretion of gas onto supermassive black holes, and possess a pair of oppositely directed jets of magnetized, high-energy plasma continuously flowing outward at speeds close to that of the light. These systems radiate across the electromagnetic spectrum from radio to gamma-ray frequencies.

Observations of blazars involve the most modern instruments and techniques developed, while theories of jet formation and mechanisms for high-energy particle and photon production attempt to solve one of the most prominent problems of modern astrophysics.

This Special Issue of Galaxies is based on the contributions presented during the international symposium “Blazars through Sharp Multi-Wavelength Eyes” held in Málaga, Spain, between 30 May and 3 June, 2016. Gathering the leading theorists and observers around the world, along with young scientists and students, the conference analyzed the blazar phenomenon, focusing on the study of jet formation, probes of blazar jets with instruments providing the finest angular resolution, multi-wavelength observations, magnetic fields and polarization, and non-blazar AGN jets.

The timing of the conference is well matched to the availability of premier high-energy facilities, such as the Fermi Gamma-ray Space Telescope, the Chandra, Swift, NuSTAR, and Astrosat X-ray satellites, new high-resolution arrays such as the Event Horizon Telescope, RadioAstron, and ALMA, sensitive very high-energy gamma-ray Cherenkov detectors, and long-term monitoring with the Very Long Baseline Array, Global mm-VLBI Array, and optical-IR telescopes.

An extra motivation of the conference was to celebrate Prof. Alan Marscher's contributions to blazar research.

Dr. Jose L. Gómez
Dr. Alan P. Marscher
Dr. Svetlana G. Jorstad
Guest Editors

Manuscript Submission Information

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Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 350 CHF (Swiss Francs). 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.

Published Papers (73 papers)

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Editorial

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Open AccessEditorial The Conference Blazars through Sharp Multi-Wavelength Eyes
Galaxies 2016, 4(3), 21; doi:10.3390/galaxies4030021
Received: 30 August 2016 / Accepted: 31 August 2016 / Published: 1 September 2016
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Abstract
Blazars are active, galactic nuclei with jets of magnetized, high-energy plasma that flow outward from the center of the host galaxy at near-light speeds.[...] Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)

Research

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Open AccessArticle The Disk-Driven Jet of Cygnus A
Galaxies 2017, 5(2), 22; doi:10.3390/galaxies5020022
Received: 5 September 2016 / Revised: 4 April 2017 / Accepted: 18 April 2017 / Published: 28 April 2017
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Abstract
Recently published results from VLBI observations at 3 and 7 millimeters of the radio galaxy Cygnus A are reviewed in this article, and discussed within the model of a prominently stratified jet outflow. At the source redshift (z= 0.056), mm-VLBI allows
[...] Read more.
Recently published results from VLBI observations at 3 and 7 millimeters of the radio galaxy Cygnus A are reviewed in this article, and discussed within the model of a prominently stratified jet outflow. At the source redshift ( z = 0.056), mm-VLBI allows a spatial resolution down to 200 Schwarzschild radii to be achieved, providing an extremely detailed view of the two-sided jet base. Through a study of the kinematic properties of the flow and of its transverse structure, it is shown that the radio emission is produced by an accelerating, mildly relativistic, parabolically expanding disk-wind. The observed transverse stratification, both of the flux density and of the bulk speed, supports the presence of an invisible faster spine close to the jet axis, powered either by the inner regions of the accretion disk or by the spinning black hole. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Gaussian Processes for Blazar Variability Studies
Galaxies 2017, 5(1), 19; doi:10.3390/galaxies5010019
Received: 5 September 2016 / Revised: 22 February 2017 / Accepted: 7 March 2017 / Published: 17 March 2017
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Abstract
This article briefly introduces Gaussian processes as a new approach for modelling time series in the field of blazar physics. In the second part of the paper, recent results from an application of GP modelling to the multi-wavelength light curves of the blazar
[...] Read more.
This article briefly introduces Gaussian processes as a new approach for modelling time series in the field of blazar physics. In the second part of the paper, recent results from an application of GP modelling to the multi-wavelength light curves of the blazar PKS 1502+106 are discussed. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Long-Term Monitoring of Bright Blazars in the Multi-GeV to TeV Range with FACT
Galaxies 2017, 5(1), 18; doi:10.3390/galaxies5010018
Received: 14 July 2016 / Revised: 2 January 2017 / Accepted: 2 February 2017 / Published: 20 February 2017
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Abstract
Blazars like Markarian 421 or Markarian 501 are active galactic nuclei (AGN), with their jets orientated towards the observer. They are among the brightest objects in the very high energy (VHE) gamma ray regime (>100 GeV). Their emitted gamma-ray fluxes are extremely variable,
[...] Read more.
Blazars like Markarian 421 or Markarian 501 are active galactic nuclei (AGN), with their jets orientated towards the observer. They are among the brightest objects in the very high energy (VHE) gamma ray regime (>100 GeV). Their emitted gamma-ray fluxes are extremely variable, with changing activity levels on timescales between minutes, months, and even years. Several questions are part of the current research, such as the question of the emission regions or the engine of the AGN and the particle acceleration. A dedicated longterm monitoring program is necessary to investigate the properties of blazars in detail. A densely sampled and unbiased light curve allows for observation of both high and low states of the sources, and the combination with multi-wavelength observation could contribute to the answer of several questions mentioned above. FACT (First G-APD Cherenkov Telescope) is the first operational telescope using silicon photomultiplier (SiPM, also known as Geigermode—Avalanche Photo Diode, G-APD) as photon detectors. SiPM have a very homogenous and stable longterm performance, and allow operation even during full moon without any filter, leading to a maximal duty cycle for an Imaging Air Cherenkov Telescope (IACT). Hence, FACT is an ideal device for such a longterm monitoring of bright blazars. A small set of sources (e.g., Markarian 421, Markarian 501, 1ES 1959+650, and 1ES 2344+51.4) is currently being monitored. In this contribution, the FACT telescope and the concept of longterm monitoring of bright blazars will be introduced. The results of the monitoring program will be shown, and the advantages of densely sampled and unbiased light curves will be discussed. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle The Statistical Study of the Daily Fermi Light Curves of 130 Sources
Galaxies 2017, 5(1), 14; doi:10.3390/galaxies5010014
Received: 15 July 2016 / Revised: 9 January 2017 / Accepted: 2 February 2017 / Published: 11 February 2017
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Abstract
Blazars show rapid and high amplitude variability. It is interesting to question what kind of process the variability corresponds to. Maybe it is a result of the instability of the accretion flows. In this work, Fermi daily light curves of 130 sources are
[...] Read more.
Blazars show rapid and high amplitude variability. It is interesting to question what kind of process the variability corresponds to. Maybe it is a result of the instability of the accretion flows. In this work, Fermi daily light curves of 130 sources are analyzed, and the distributions of daily variability are compared by using a Kolmogorov–Smirnov (K-S) test. The results can be summarized as follows:(1) in most cases, the distributions are not Gaussian; (2) some pairs of the distributions are similar. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle OJ 287 as a Rotating Helix
Galaxies 2017, 5(1), 12; doi:10.3390/galaxies5010012
Received: 6 September 2016 / Revised: 9 January 2017 / Accepted: 16 January 2017 / Published: 8 February 2017
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Abstract
We present preliminary data from high-cadence 15-GHz VLBA images of OJ 287 from 1995 to 2015. The ridgelines suggest that the jet is rotating, perhaps with a period of∼30 years. The EVPA of the core rotated by 240° in 2001–2002 and decreased slowly
[...] Read more.
We present preliminary data from high-cadence 15-GHz VLBA images of OJ 287 from 1995 to 2015. The ridgelines suggest that the jet is rotating, perhaps with a period of∼30 years. The EVPA of the core rotated by 240° in 2001–2002 and decreased slowly after that. The inner jet apparently moved to a new direction after the rotation,as shown by the emergence of a new component at a new PA at 43 GHz, in 2004. This was presaged by a strong rise in the flux density of the core, and then its sudden fall as the new component was identified. The equivalent sequence of events took place about 5 years later at 15 GHz, but in addition the core EVPA had a step in 2006 and moved to be aligned with the new 43-GHz component. The 15-GHz core became optically thin in 2006, but the angular resolution was insufficient to separate the new component from the core until 2010 Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Study of the Time-Series of Microvariability in Kepler Blazar W2R 1926+42
Galaxies 2017, 5(1), 13; doi:10.3390/galaxies5010013
Received: 31 August 2016 / Revised: 20 January 2017 / Accepted: 31 January 2017 / Published: 8 February 2017
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Abstract
One of the remarkable features of blazars is violent variability over a wide wavelength range. The variation mechanism causing the observed complex behavior is still under debate. The variability timescales range from less than a day to decades. Variation on timescales less than
[...] Read more.
One of the remarkable features of blazars is violent variability over a wide wavelength range. The variation mechanism causing the observed complex behavior is still under debate. The variability timescales range from less than a day to decades. Variation on timescales less than a day is known as “microvariability.” Such short-term variations can provide insights regarding the origin of the variability after they are distinguished from longer-term variational components. We select about 195 microvariability events from the continuous light curve of blazar W2R 1926+42 with 1-min time resolution obtained by the Kepler spacecraft, and estimate the timescale and amplitude of each event. The rise and decay timescales of the events reveal random variations over short timescales less than a day, but they indicate systematic variations on timescales longer than several days. This result implies that the events are not independent, but rather mutually correlated. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle The Origin and Structure of the Magnetic Fields and Currents of AGN Jets
Galaxies 2017, 5(1), 11; doi:10.3390/galaxies5010011
Received: 17 October 2016 / Accepted: 3 January 2017 / Published: 2 February 2017
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Abstract
This paper reviews observational evidence obtained to date about the overall structure of the magnetic fields in the jets of Active Galactic Nuclei (AGN). Because they are sensitive to the line-of-sight magnetic-field component, Faraday rotation observations of AGN jets provide an effective tool
[...] Read more.
This paper reviews observational evidence obtained to date about the overall structure of the magnetic fields in the jets of Active Galactic Nuclei (AGN). Because they are sensitive to the line-of-sight magnetic-field component, Faraday rotation observations of AGN jets provide an effective tool for searching for toroidal jet magnetic fields, whose line-of-sight component changes systematically across the jet. Transverse Faraday rotation measure (RM) gradients providing direct evidence for helical/toroidal magnetic fields have been reliably detected in nearly 40 AGN on parsec scales. Helical magnetic fields are believed to form due to the combined action of the rotation of the central black hole and accretion disk, and these observations demonstrate that at least some of this helical field survives to distances well beyond the Very Long Baseline Interferometry (VLBI) core. Observations of reversals in the direction of the transverse RM gradients in a number of AGN provide evidence for a“return”magnetic field forming a nested helical-field structure with oppositely directed azimuthal components in the inner and outer regions of the helical magnetic field. The collected data now provide firm evidence for a predominance of inward jet currents on parsec scales and outward currents on scales greater than a few tens of parsecs. This suggests a global pattern of magnetic fields and currents with an inward current near the jet axis and an outward current farther from the jet axis, with these currents closing in the accretion disk and far out in the radio lobes, forming a self-consistent set of fields and currents together with the implied nested helical-field structure. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle The Power of (Near) Simultaneous Multi-Frequency Observations for mm-VLBI and Astrometry
Galaxies 2017, 5(1), 9; doi:10.3390/galaxies5010009
Received: 23 December 2016 / Revised: 16 January 2017 / Accepted: 17 January 2017 / Published: 24 January 2017
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Abstract
Simultaneous or near-simultaneous observations at multiple frequency bands have the potential to overcome the fundamental limitation imposed by the atmospheric propagation in mm-VLBI observations. The propagation effects place a severe limit in the sensitivity achievable in mm-VLBI, reducing the time over which the
[...] Read more.
Simultaneous or near-simultaneous observations at multiple frequency bands have the potential to overcome the fundamental limitation imposed by the atmospheric propagation in mm-VLBI observations. The propagation effects place a severe limit in the sensitivity achievable in mm-VLBI, reducing the time over which the signals can be coherently combined, and preventing the use of phase referencing and astrometric measurements. We present two demonstrations of the power of (near) simultaneous multi-frequency observations with the KVN and VLBA, and our recently developed analysis strategies to enable new measurements at mm-VLBI. The first case comprises simultaneous observations at 22, 43, 87 and 130 GHz of a group of five AGNs, the weakest of which is ∼200 mJy at 130 GHz, with angular separations ranging from 3.6 to 11 degrees, using the KVN. We analysed this data using the Frequency Phase Transfer (FPT) and the Source Frequency Phase Referencing (SFPR) techniques, which use the observations at a lower frequency to correct those at a higher frequency. The results of the analysis provide an empirical demonstration of the increase in the coherence times at 130 GHz from a few tens of seconds to about twenty minutes, with FPT, and up to many hours with SFPR. Moreover the astrometric analysis provides high precision relative position measurements between two frequencies, including, for the first time, astrometry at 130 GHz. The second case is a variation of the above, whereby adding dedicated wide-band cm-wavelength observations to measure the ionosphere eliminates the need for a second, calibrator, source. This addresses the scarcity of calibrators at mm-VLBI. We dubbed this technique Multi Frequency Phase Referencing (MFPR). We present bona fide astrometrically aligned VLBA images of BL Lacertae at 22 and 43 GHz using MFPR, which, combined with results from conventional phase referencing at cm-wavelengths, suggests the VLBI core has a recollimation shock that is revealed at mm-wavelengths. These shocks could be responsible for the γ-ray emission in blazar jets. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Proper Motions of Jets on the Kiloparsec Scale: New Results with HST
Galaxies 2017, 5(1), 8; doi:10.3390/galaxies5010008
Received: 13 September 2016 / Revised: 13 January 2017 / Accepted: 16 January 2017 / Published: 20 January 2017
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Abstract
The Hubble Space Telescope recently celebrated 25 years of operation. Some of the first images of extragalactic optical jets were taken by HST in the mid-1990s; with time baselines on the order of 20 years and state-of-the-art astrometry techniques, we are now able
[...] Read more.
The Hubble Space Telescope recently celebrated 25 years of operation. Some of the first images of extragalactic optical jets were taken by HST in the mid-1990s; with time baselines on the order of 20 years and state-of-the-art astrometry techniques, we are now able to reach accuracies in proper-motion measurements on the order of a tenth of a milliarcsecond per year. We present the results of a recent HST program to measure the kiloparsec-scale proper motions of eleven nearby optical jets with Hubble, the first sample of its kind. When paired with VLBI proper-motion measurements on the parsec scale, we are now able to map the full velocity profile of these jets from near the black hole to the final deceleration as they extend out into and beyond the host galaxy. We see convincing evidence that weak-flavor jets (i.e., FR Is) have a slowly increasing jet speed up to 100 pc from the core, where superluminal components are first seen. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Optical and Gamma-Ray Variability of the vRL NLSy1 Galaxy, 1H 0323+342
Galaxies 2017, 5(1), 7; doi:10.3390/galaxies5010007
Received: 2 September 2016 / Revised: 10 November 2016 / Accepted: 11 November 2016 / Published: 20 January 2017
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Abstract
1H 0323+342 was one of the first vRLNLSy1 galaxies detected at gamma-rays with the Fermi-LAT and is one of the brightest of this class observed at optical wavelengths. We report the results of monitoring the optical flux, polarization and the gamma-ray flux of
[...] Read more.
1H 0323+342 was one of the first vRLNLSy1 galaxies detected at gamma-rays with the Fermi-LAT and is one of the brightest of this class observed at optical wavelengths. We report the results of monitoring the optical flux, polarization and the gamma-ray flux of 1H 0323+342 during the past ~5 years. In some cases, the optical flux has been monitored on timescales as short as ~minutes simultaneously with two telescopes, demonstrating, for the first time, the reality of microvariability events with durations as short as ~15 min for this object. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Multifrequency Study of the Blazar 3C 454.3
Galaxies 2017, 5(1), 3; doi:10.3390/galaxies5010003
Received: 15 July 2016 / Revised: 26 November 2016 / Accepted: 15 December 2016 / Published: 30 December 2016
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Abstract
This work is devoted to multi-frequency studying of the blazar 3C 454.3. The study includes spectroscopic and photometric observations in the optical, IR, and gamma-ray bands. We investigate whether a correlation exists in the light curves at different wavelengths. We have carried out
[...] Read more.
This work is devoted to multi-frequency studying of the blazar 3C 454.3. The study includes spectroscopic and photometric observations in the optical, IR, and gamma-ray bands. We investigate whether a correlation exists in the light curves at different wavelengths. We have carried out observations of the optical spectrum (from 4000 to 7000 Angstroms) between 2007 and 2009, and identified MgII [2800 Angstroms] and FeII emission line features. We have obtained optical magnitudes and color indices of the quasar and performed a correlation between the optical, IR, and gamma-ray light curves. We have found statistically significant correlations between the light curves at different wavelengths. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle A New Statistical Approach to the Optical Spectral Variability in Blazars
Galaxies 2017, 5(1), 1; doi:10.3390/galaxies5010001
Received: 29 September 2016 / Revised: 14 December 2016 / Accepted: 15 December 2016 / Published: 28 December 2016
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Abstract
We present a spectral variability study of a sample of about 25 bright blazars, based on optical spectroscopy. Observations cover the period from the end of 2008 to mid 2015, with an approximately monthly cadence. Emission lines have been identified and measured in
[...] Read more.
We present a spectral variability study of a sample of about 25 bright blazars, based on optical spectroscopy. Observations cover the period from the end of 2008 to mid 2015, with an approximately monthly cadence. Emission lines have been identified and measured in the spectra, which permits us to classify the sources into BL Lac-type or FSRQs, according to the commonly used EW limit. We have obtained synthetic photometry and produced colour-magnitude diagrams which show different trends associated with the object classes: generally, BL Lacs tend to become bluer when brighter and FSRQs become redder when brighter, although several objects exhibit both trends, depending on brightness. We have also applied a pattern recognition algorithm to obtain the minimum number of physical components which can explain the variability of the optical spectrum. We have used NMF (Non-Negative Matrix Factorization) instead of PCA (Principal Component Analysis) to avoid un-realistic negative components. For most targets we found that 2 or 3 meta-components are enough to explain the observed spectral variability. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle The Structure and Propagation of the Misaligned Jet M87
Galaxies 2017, 5(1), 2; doi:10.3390/galaxies5010002
Received: 18 July 2016 / Revised: 10 December 2016 / Accepted: 14 December 2016 / Published: 26 December 2016
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Abstract
Due to its proximity, M87 is a prime target for next-generation high-resolution VLBI at short millimeter wavelengths, by which the jet launching region and the black hole shadow are expected to be resolved and imaged sometime soon. Along with this situation, high-quality VLBI
[...] Read more.
Due to its proximity, M87 is a prime target for next-generation high-resolution VLBI at short millimeter wavelengths, by which the jet launching region and the black hole shadow are expected to be resolved and imaged sometime soon. Along with this situation, high-quality VLBI imaging and monitoring at lower frequencies play an important role in complementing the high-frequency data. Here, we present our recent and ongoing observational studies of the M87 jet on pc-to-subpc scales based on ultra-deep VLBI imaging programs at 86 GHz and 15 GHz. The high-dynamic-range images have allowed us to obtain some remarkably improved views on this jet. We also introduce the KVN and VERA Array (KaVA), a new regularly-operating VLBI network in East Asia, which is quite suitable for studying the structure and propagation of relativistic jets. Some early results from our pilot study for M87—including the detection of superluminal motions near the jet base—implying an efficient magnetic-to-kinetic conversion at these scales, are reported. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Conical Stream of the Two-Sided Jets in NGC 4261 over the Range of 103–109 Schwarzschild Radii
Galaxies 2016, 4(4), 80; doi:10.3390/galaxies4040080
Received: 6 September 2016 / Revised: 23 November 2016 / Accepted: 14 December 2016 / Published: 20 December 2016
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Abstract
We report the jet width profile of of the nearby (30Mpc) AGN NGC 4261 for both the approaching jet and the counter jet at radial distances ranging from 103109 Schwarzschild radius (RS
[...] Read more.
We report the jet width profile of of the nearby ( 30 Mpc ) AGN NGC 4261 for both the approaching jet and the counter jet at radial distances ranging from 10 3 10 9 Schwarzschild radius ( R S ) from the central engine. Our Very Large Array (VLA) and Very Long Baseline Array (VLBA) observations reveal that the jets maintain a conical structure on both sides over the range 10 3 10 9 R S without any structural transition (i.e., parabolic to conical) like in the approaching jet in M87. Thus, NGC 4261 will provide a unique opportunity to examine the conical jet hypothesis in blazars, while it may require some additional consideration on the acceleration and collimation process in AGN jets. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Emission Knots and Polarization Swings of Swinging Jets
Galaxies 2016, 4(4), 75; doi:10.3390/galaxies4040075
Received: 26 September 2016 / Revised: 21 November 2016 / Accepted: 23 November 2016 / Published: 7 December 2016
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Abstract
Knots (emission features in jets of active galactic nuclei) often show non-ballistic dynamics and variable emission/polarization properties. We model these features as emission pattern propagating in a jet that carries a helical magnetic field and is launched along a changing direction. The model
[...] Read more.
Knots (emission features in jets of active galactic nuclei) often show non-ballistic dynamics and variable emission/polarization properties. We model these features as emission pattern propagating in a jet that carries a helical magnetic field and is launched along a changing direction. The model can reproduce a wide range of phenomena observed in the motion of knots: non-ballistic motion (both smooth and occasional sudden change of direction, and/or oscillatory behavior), variable brightness, and confinement of knots’ motion within an overlaying envelope. The model also reproduces smooth large polarization angle swings, and at the same time allows for the seemingly random behavior of synchrotron fluxes, polarization fraction, and occasional π / 2 polarization jumps. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Multi-Frequency Monitoring of the Flat Spectrum Radio Quasar PKS 1222+216 in 2008–2015
Galaxies 2016, 4(4), 72; doi:10.3390/galaxies4040072
Received: 15 July 2016 / Revised: 20 November 2016 / Accepted: 21 November 2016 / Published: 28 November 2016
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Abstract
We analyze the broadband activity of the flat spectrum radio quasar PKS 1222+216 from 2008 to 2015 using multi-frequency monitoring which involves γ-ray data from the Fermi Large Area Telescope, total intensity and linear polarization observations from different optical telescopes in R
[...] Read more.
We analyze the broadband activity of the flat spectrum radio quasar PKS 1222+216 from 2008 to 2015 using multi-frequency monitoring which involves γ-ray data from the Fermi Large Area Telescope, total intensity and linear polarization observations from different optical telescopes in R band, and imaging of the inner jet structure with the Very Long Baseline Array (VLBA) at 43 GHz. During the observations, the source showed several dramatic flares at γ rays and optical bands, with the rising branch of a γ-ray flare accompanied by a rapid rotation of the polarization position angle (EVPA), a fast increase of the degree of polarization in the optical band, brightening of the VLBI core, and appearance of a new superluminal component in the parsec-scale jet. The rapid variability of the optical linear polarization may be explained by a strong turbulence in the jet plasma. We find a correlation between the γ rays, optical R band, and 43 GHz variability on a long-term scale (months and years), and a good general alignment between EVPAs in R band and at 43 GHz, while the correlation between short-term variations (days and weeks) is weaker. Synchronous activity across the bands supports the idea that the emission regions responsible for the γ-ray and optical flares are co-spatial and located in the vicinity of the mm-wave core of the parsec-scale jet. However, these connections do not completely explain the challenging behaviour of PKS 1222+216, since there are some γ-ray flares which are not accompanied by jet events, and vice versa. We need a continuation of multi-frequency monitoring along with high resolution imaging of the parsec-scale jet to understand in detail the origin of high energy emission in blazars. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle JVLA Wideband Polarimetry Observations on a Sample of High Rotation Measure Sources
Galaxies 2016, 4(4), 66; doi:10.3390/galaxies4040066
Received: 17 July 2016 / Revised: 9 November 2016 / Accepted: 12 November 2016 / Published: 24 November 2016
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Abstract
We present preliminary results of JVLA wideband full polarization observations of a sample of Active Galactic Nuclei (AGN) with very high Rotation Measure (RM) values, a sign of extreme environment. Polarization properties show a complex behaviour such that the polarization angle (PA) and
[...] Read more.
We present preliminary results of JVLA wideband full polarization observations of a sample of Active Galactic Nuclei (AGN) with very high Rotation Measure (RM) values, a sign of extreme environment. Polarization properties show a complex behaviour such that the polarization angle (PA) and fractional polarization (fp) change dramatically within the wide band. The measured RM is not constant within the wide band. Its complex behaviour reflects the complexity of the medium with the presence of several Faraday components. The depolarization has been studied by modelling the variations of the Stokes parameters Q and U together with the polarization parameters (PA and fp) with wavelength using combinations of the simplest existing depolarization models. With this JVLA study we could spectrally resolve multiple polarized components of unresolved AGN. These preliminary results reveal the complexity of these objects, but improvements to the depolarization modelling are needed to better understand the polarization structure of these sources. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Multiwavelength Picture of the Blazar S5 0716+714 during Its Brightest Outburst
Galaxies 2016, 4(4), 69; doi:10.3390/galaxies4040069
Received: 15 July 2016 / Revised: 11 November 2016 / Accepted: 17 November 2016 / Published: 24 November 2016
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Abstract
S5 0716+714 is a well known BL Lac object, and one of the brightest and most active blazars. The discovery in the Very High Energy band (VHE, E > 100 GeV) by MAGIC happened in 2008. In January 2015, the source went through
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S5 0716+714 is a well known BL Lac object, and one of the brightest and most active blazars. The discovery in the Very High Energy band (VHE, E > 100 GeV) by MAGIC happened in 2008. In January 2015, the source went through the brightest optical state ever observed, triggering MAGIC follow-up and a VHE detection with ∼ 13 σ significance (ATel 6999 ). Rich multiwavelength coverage of the flare allowed us to construct the broad-band spectral energy distribution of S5 0716+714 during its brightest outburst. In this work, we will present the preliminary analysis of MAGIC and Fermi-LAT data of the flaring activity in January and February 2015 for the HE (0.1 < HE < 300 GeV) and VHE band, together with radio (Metsähovi, OVRO, VLBA, Effelsberg), sub-millimeter (SMA), optical (Tuorla, Perkins, Steward, AZT-8+ST7, LX-200, Kanata), X-ray and UV (Swift-XRT and UVOT), in the same time-window and discuss the time variability of the multiwavelength light curves during this impressive outburst. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Direct Imaging of a Toroidal Magnetic Field in the Inner Jet of NRAO 150
Galaxies 2016, 4(4), 70; doi:10.3390/galaxies4040070
Received: 15 July 2016 / Revised: 15 November 2016 / Accepted: 16 November 2016 / Published: 24 November 2016
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Abstract
Most formation models and numerical simulations cause a helical magnetic field to form, accelerate and collimate jets in active galactic nuclei (AGN). For this reason, observational direct evidence for the existence of these helical magnetic fields is of special relevance. In this work,
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Most formation models and numerical simulations cause a helical magnetic field to form, accelerate and collimate jets in active galactic nuclei (AGN). For this reason, observational direct evidence for the existence of these helical magnetic fields is of special relevance. In this work, we present ultra- high-resolution observations of the innermost regions of the jet in the quasar NRAO150. We study the polarization structure and report evidence of a helical magnetic field. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Correlation Analysis of Delays between Variations of Gamma-Ray and Optical Light Curves of Blazars
Galaxies 2016, 4(4), 64; doi:10.3390/galaxies4040064
Received: 14 July 2016 / Revised: 11 November 2016 / Accepted: 15 November 2016 / Published: 23 November 2016
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Abstract
We have been performing multi-wavelength monitoring of a sample of γ-ray blazars since the launch of the Fermi Gamma-ray Space Telescope in 2008. We present γ-ray and optical light curves for several quasars and BL Lac objects from the sample to
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We have been performing multi-wavelength monitoring of a sample of γ -ray blazars since the launch of the Fermi Gamma-ray Space Telescope in 2008. We present γ -ray and optical light curves for several quasars and BL Lac objects from the sample to illustrate different patterns of variability. We investigate correlations between γ -ray and R-band light curves and, if these are statistically significant, determine delays between variations at the two wavebands. Such time delays can reveal the relative locations of the emitting regions in AGN jets and the origin of the high-energy photons. We present preliminary results of this analysis. Of the 29 blazars with sufficient time coverage, 17 display a significant, singular, correlated time lag when tested over the entire 7-year period. Of these sources, the six that exhibit a consistent time lag across a majority of epochs of high activity have lags of 0 ± 7 days; the 11 without consistency across epochs of high activity generally display longer mean lags, with γ -ray leading optical. Eleven sources display no significant singular correlation over either the entire 7-year period or across shorter intervals. No significant difference is apparent between the BL Lac objects and FSRQs. Even after 7 years of monitoring, our correlation analysis remains plagued with uncertainties due to insufficient data. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Recent Progress in Understanding the Large Scale Jets of Powerful Quasars
Galaxies 2016, 4(4), 65; doi:10.3390/galaxies4040065
Received: 7 September 2016 / Revised: 8 November 2016 / Accepted: 10 November 2016 / Published: 23 November 2016
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Abstract
Our understanding of the physics of kpc-scale quasar jets had seemed to converge to a paradigm in which these jets are as highly relativistic on the kpc scale as they are on sub-pc scales close to the central black hole. Retaining bulk Lorentz
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Our understanding of the physics of kpc-scale quasar jets had seemed to converge to a paradigm in which these jets are as highly relativistic on the kpc scale as they are on sub-pc scales close to the central black hole. Retaining bulk Lorentz factors (Γ) on the order of 10–20 at these distances implies a jet power comparable to or higher than their Eddington luminosity. We recently started challenging this paradigm, which was put in place to explain the surprisingly bright X-ray emission of the knots of many quasar jets as inverse Compton scattering off the cosmic microwave background (IC/CMB). We have shown that the knot X-ray emission of the archetypical jets 3C 273 and PKS 0637-752 is not due to IC/CMB. With IC/CMB disfavored, an alternative interpretation for the X-rays is synchrotron radiation from a second population of electrons accelerated in situ up to ∼100 TeV. These results are the first step towards resolving the long-standing issue of the nature of the X-ray emission in powerful quasar jets. Comprehensive observational and theoretical work on essentially all X-ray-detected large-scale quasar jets to test the IC/CMB model over a much larger population needs to be done to examine the implications of slower jets that are extremely efficient accelerators. A fascinating case can be made that—contrary to popular belief—the total radiative power of the large-scale jet of these sources is comparable to that of the quasar core. Even more so, the angle-integrated TeV output of these (previously thought TeV-quiet) quasar jets likely makes them the dominant class among active galactic nuclei (AGN), exceeding the TeV production of so-called TeV blazars. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Multiwavelength Monitoring of the Gamma-Bright Blazar Mkn 421
Galaxies 2016, 4(4), 67; doi:10.3390/galaxies4040067
Received: 13 July 2016 / Revised: 15 November 2016 / Accepted: 16 November 2016 / Published: 23 November 2016
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Abstract
We present the results of photo-polarimetric monitoring observations of the blazar Markarian 421 carried out with different telescopes (the 0.4 m telescopes of St. Petersburg State University and the Pulkovo Observatory, the 0.7 m telescope of the Crimean Astrophysical Observatory) during 2008–2015. We
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We present the results of photo-polarimetric monitoring observations of the blazar Markarian 421 carried out with different telescopes (the 0.4 m telescopes of St. Petersburg State University and the Pulkovo Observatory, the 0.7 m telescope of the Crimean Astrophysical Observatory) during 2008–2015. We analyse the optical data as well as gamma-ray ligh t curves obtained with the Fermi Large Area Telescope. The multiwavelength flux variations are discussed. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle AP Librae: The Extended Jet as the Source of VHE Emission?
Galaxies 2016, 4(4), 63; doi:10.3390/galaxies4040063
Received: 8 July 2016 / Revised: 25 October 2016 / Accepted: 9 November 2016 / Published: 15 November 2016
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Abstract
The LBL AP Librae has been detected in the TeV gamma-ray range, which is unexpected for this kind of blazar. The broad-band SED cannot be explained by a one-zone model, and especially fails in the TeV domain. An extended jet has been detected
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The LBL AP Librae has been detected in the TeV gamma-ray range, which is unexpected for this kind of blazar. The broad-band SED cannot be explained by a one-zone model, and especially fails in the TeV domain. An extended jet has been detected on arcsec scales both in radio and X-ray frequencies. The spectral index of the X-ray band indicates an inverse Compton origin. Using an IC/CMB model, the TeV emission can be successfully modeled as originating in the extended jet. Several arguments in favor of this model, as well as observational tests, will be presented. The observations suggest that acceleration of particles to very high energies is still efficient at large distances from the core. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Investigating the Puzzling Synchrotron Behaviour of Mrk 421
Galaxies 2016, 4(4), 61; doi:10.3390/galaxies4040061
Received: 9 September 2016 / Revised: 6 October 2016 / Accepted: 15 October 2016 / Published: 4 November 2016
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Abstract
We investigate the multiwavelength behaviour of the high-energy peaked BL Lac object (HBL) Mrk 421 at redshift z=0.031 in the period 2007–2015. We use optical photometric, spectroscopic, and polarimetric data and near-infrared data obtained by 35 observatories participating in
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We investigate the multiwavelength behaviour of the high-energy peaked BL Lac object (HBL) Mrk 421 at redshift z = 0 . 031 in the period 2007–2015. We use optical photometric, spectroscopic, and polarimetric data and near-infrared data obtained by 35 observatories participating in the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT), as well as by the Steward Observatory Support of the Fermi Mission. We also employ high-energy data from the Swift (UV and X-rays) satellite to study correlations among emission in different bands. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Physical Conditions and Variability Processes in AGN Jets through Multi-Frequency Linear and Circular Radio Polarization Monitoring
Galaxies 2016, 4(4), 58; doi:10.3390/galaxies4040058
Received: 5 September 2016 / Revised: 21 October 2016 / Accepted: 25 October 2016 / Published: 2 November 2016
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Abstract
Radio polarimetry is an invaluable tool to investigate the physical conditions and variability processes in active galactic nuclei (AGN) jets. However, detecting their linear and circular polarization properties is a challenging endeavor due to their low levels and possible depolarization effects. We have
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Radio polarimetry is an invaluable tool to investigate the physical conditions and variability processes in active galactic nuclei (AGN) jets. However, detecting their linear and circular polarization properties is a challenging endeavor due to their low levels and possible depolarization effects. We have developed an end-to-end data analysis methodology to recover the polarization properties of unresolved sources with high accuracy. It has been applied to recover the linear and circular polarization of 87 AGNs measured by the F-GAMMA program from July 2010 to January 2015 with a mean cadence of 1.3 months. Their linear polarization was recovered at four frequencies between 2.64 and 10.45 GHz and the circular polarization at 4.85 and 8.35 GHz. The physical conditions required to reproduce the observed polarization properties and the processes which induce their variability were investigated with a full-Stokes radiative transfer code which emulates the synchrotron emission of modeled jets. The model was used to investigate the conditions needed to reproduce the observed polarization behavior for the blazar 3C 454.3, assuming that the observed variability is attributed to evolving internal shocks propagating downstream. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Observing—and Imaging—Active Galactic Nuclei with the Event Horizon Telescope
Galaxies 2016, 4(4), 54; doi:10.3390/galaxies4040054
Received: 11 July 2016 / Revised: 13 October 2016 / Accepted: 15 October 2016 / Published: 27 October 2016
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Abstract
Originally developed to image the shadow region of the central black hole in Sagittarius A* and in the nearby galaxy M87, the Event Horizon Telescope (EHT) provides deep, very high angular resolution data on other active galactic nucleus (AGN) sources too. The challenges
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Originally developed to image the shadow region of the central black hole in Sagittarius A* and in the nearby galaxy M87, the Event Horizon Telescope (EHT) provides deep, very high angular resolution data on other active galactic nucleus (AGN) sources too. The challenges of working with EHT data have spurred the development of new image reconstruction algorithms. This work briefly reviews the status of the EHT and its utility for observing AGN sources, with emphasis on novel imaging techniques that offer the promise of better reconstructions at 1.3 mm and other wavelengths. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle The RadioAstron Dedicated DiFX Distribution
Galaxies 2016, 4(4), 55; doi:10.3390/galaxies4040055
Received: 18 July 2016 / Revised: 16 October 2016 / Accepted: 17 October 2016 / Published: 27 October 2016
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Abstract
Distributed FX-architecture (DiFX) is a software Very Long Baseline Interferometry (VLBI) correlator currently adopted by several main correlation sites around the globe. After the launch of the RadioAstron Space-VLBI mission in 2011, an extension was necessary to handle processing of an orbiting antenna,
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Distributed FX-architecture (DiFX) is a software Very Long Baseline Interferometry (VLBI) correlator currently adopted by several main correlation sites around the globe. After the launch of the RadioAstron Space-VLBI mission in 2011, an extension was necessary to handle processing of an orbiting antenna, to be correlated with supporting ground arrays. Here, we present a branch of the main DiFX distribution (2.4), uploaded on the publicly available repository during July 2016, that the Max Planck Institute for Radio Astronomy (MPIfR) developed to process data of the three key active galactic nuclei (AGN)-imaging RadioAstron science projects, as well as part of the AGN survey project, and General Observing Time (GOT) projects proposed since Announcement of Opportunity 2 (AO-2, July 2014–July 2015). It can account for general relativistic correction of an orbiting antenna with variable position/velocity, providing a routine to convert the native RadioAstron Data Format (RDF) format to the more common Mark5 B (M5B). The possibility of introducing a polynomial clock allows one to mitigate the effects of spacecraft acceleration terms in near-perigee observations. Additionally, since for the first time polarimetry on space-baselines is available thanks to RadioAstron, this DiFX branch allows one to include the spacecraft orientation information at the correlation stage, in order to perform proper polarization calibration during data reduction. Finally, a fringe-finding algorithm able to manage an arbitrarily large fringe-search window is included, allowing one to increase the search space normally adopted by common software packages like HOPS. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Investigating the Innermost Jet Structures of Blazar S5 0716+714 Using Uniquely Dense Intra-day Photo-polarimetric Observations
Galaxies 2016, 4(4), 56; doi:10.3390/galaxies4040056
Received: 7 September 2016 / Revised: 11 October 2016 / Accepted: 17 October 2016 / Published: 27 October 2016
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Abstract
The sub-hour timescale variability commonly observed in blazars—widely known as intra-day or microvariability—has been extensively studied in optical photo-polarimetric bands over the past 25–30 years. In addition, there have been comprehensive theoretical discussions on the topic, with various models and scenarios proposed;
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The sub-hour timescale variability commonly observed in blazars—widely known as intra-day or microvariability—has been extensively studied in optical photo-polarimetric bands over the past 25–30 years. In addition, there have been comprehensive theoretical discussions on the topic, with various models and scenarios proposed; however, the phenomenon still remains relatively poorly understood. Here we present the summary of our optical microvariability studies over the past few years based on multi-frequency photo-polarimetric Whole Earth Blazar Telescope (WEBT) observation campaigns. The primary objective of the study was to explore the characteristics of the source microvariability on timescales of a few minutes to a few days using exceptionally dense photo-polarimetric observations. The results show that the source often displays fast variability with an amplitude as large as 0.3 mag within a few hours, as well as color variability on similar time scales often characterized by “bluer-when-brighter” trend. Similarly, the correlation between variability in flux and polarization appears to depend upon the configuration of the optical polarization angle relative to the positional angle of the innermost radio core of the jet. Other fascinating observations include a sudden and temporary disappearance in the observed variability lasting for ∼6 h. In addition, the modeling of individual microflares strongly suggests that the phenomenon of microvariability can be best explained by convolved emission from compact emission sites distributed stochastically in the turbulent jet. Besides, analysis of some of the well resolved micro-flares exhibiting high degrees of polarization points towards a complex magnetic geometry pervading the jet with the possible presence of small-scale regions of highly ordered and enhanced magnetic field similar to so-called “magnetic islands”. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Probing the Internal Structure of Magnetized, Relativistic Jets with Numerical Simulations
Galaxies 2016, 4(4), 51; doi:10.3390/galaxies4040051
Received: 19 July 2016 / Revised: 20 October 2016 / Accepted: 21 October 2016 / Published: 26 October 2016
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Abstract
From an observational point of view, unveiling the physical processes behind the nature of the jets emanating from radio-loud AGN demands the resolution of the structure across the jet with the highest angular resolutions. Relying on a magneto-fluid dynamical description, numerical simulations can
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From an observational point of view, unveiling the physical processes behind the nature of the jets emanating from radio-loud AGN demands the resolution of the structure across the jet with the highest angular resolutions. Relying on a magneto-fluid dynamical description, numerical simulations can help to characterize the internal structure of jets (transversal structure, magnetic field structure, internal shocks, etc.). In the first part of the paper, we shall discuss equilibrium models of magnetized, relativistic, infinite, axisymmetric jets with rotation propagating through a homogeneous, static, unmagnetized ambient medium. Then, these transversal equilibrium profiles will be used to build steady models of overpressured, superfast-magnetosonic, relativistic jets, with the aim of characterizing their internal structure in connection with their dominant type of energy (internal energy: hot jets; rest-mass energy: kinetically-dominated jets; magnetic energy: Poynting-flux-dominated jets). Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Ringo2 Optical Polarimetry of Blazars
Galaxies 2016, 4(4), 52; doi:10.3390/galaxies4040052
Received: 15 July 2016 / Revised: 19 September 2016 / Accepted: 23 September 2016 / Published: 26 October 2016
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Abstract
We present polarimetric and photometric observations from a sample of 15 γ-ray bright blazars with data from the Tuorla blazar monitoring program (KVA DIPOL) and Liverpool Telescope (LT) Ringo2 polarimeters (supplemented with γ-ray data from Fermi-LAT). We find that (1) The
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We present polarimetric and photometric observations from a sample of 15 γ-ray bright blazars with data from the Tuorla blazar monitoring program (KVA DIPOL) and Liverpool Telescope (LT) Ringo2 polarimeters (supplemented with γ-ray data from Fermi-LAT). We find that (1) The optical magnitude and γ-ray flux are positively correlated; (2) electric vector position angle rotations can occur in any blazar subclass; (3) there is no difference in the γ-ray flaring rates in the sample between subclasses; flares can occur during and outside of rotations with no preference for this behaviour; (4) the average degree of polarisation (P), optical magnitude and γ-ray flux are lower during a rotation compared with during non-rotation; (5) the number of observed flaring events and optical polarisation rotations are correlated and (6) the maximum observed P increases from ∼10% to ∼30% to ∼40% for subclasses with synchrotron peaks at high, intermediate and low frequencies respectively. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Through the Looking Glass: Faraday Conversion in Turbulent Blazar Jets
Galaxies 2016, 4(4), 50; doi:10.3390/galaxies4040050
Received: 15 July 2016 / Revised: 30 September 2016 / Accepted: 15 October 2016 / Published: 25 October 2016
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Abstract
Faint levels of circular polarization (Stokes V) have been detected in several relativistic jets. While typically less than a few percent, circular polarization can give us critical insight into the underlying nature of the jet plasma. Circular polarization can be produced through
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Faint levels of circular polarization (Stokes V) have been detected in several relativistic jets. While typically less than a few percent, circular polarization can give us critical insight into the underlying nature of the jet plasma. Circular polarization can be produced through a process known as linear birefringence, in which initially linearly polarized emission produced in one region of the jet is altered by Faraday rotation as it propagates through other regions of the jet with distinct magnetic field orientations. Recently, Marscher has developed the Turbulent Extreme Multi-Zone (TEMZ) model for blazar emission, in which turbulent plasma crossing a standing shock in the jet is represented by a collection of thousands of individual plasma cells, each with distinct magnetic field orientation. In order to test whether the TEMZ model can reproduce circularly polarized radiation at levels comparable to those observed in blazars, I have developed a numerical algorithm to solve the full Stokes equations of polarized radiative transfer. I have embedded this algorithm into the ray-tracing code RADMC3D (http://ascl.net/1202.015). RADMC3D was originally developed to model continuum radiative transfer in dusty media. This code, however, has been written in a modularized fashion that allows the user to specify the physics that is incorporated into the radiative transfer. I have replaced RADMC3D’s thermal emission and absorption coefficients with non-thermal coefficients pertaining to polarized synchrotron emission. This code is applied to ray-tracing through the 3-D TEMZ computational grid. Here I present a suite of synthetic polarized emission maps that highlight the effect that thousands of distinct cells of plasma within a jet can have on the observed linear and circular polarization. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle The VLBA-BU-BLAZAR Multi-Wavelength Monitoring Program
Galaxies 2016, 4(4), 47; doi:10.3390/galaxies4040047
Received: 18 July 2016 / Revised: 13 October 2016 / Accepted: 13 October 2016 / Published: 20 October 2016
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Abstract
We describe a multiwavelength program of monitoring of a sample of bright γ-ray blazars, which the Boston University (BU) group has being carrying out since June 2007. The program includes monthly monitoring with the Very Long Baseline Array at 43 GHz, optical
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We describe a multiwavelength program of monitoring of a sample of bright γ-ray blazars, which the Boston University (BU) group has being carrying out since June 2007. The program includes monthly monitoring with the Very Long Baseline Array at 43 GHz, optical photometric and polarimetric observations, construction and analysis of UV and X-ray light curves obtained with the Rossi X-ray Timing Explorer (RXTE) and Swift satellites, and construction and analysis of γ-ray light curves based on data provided by the Large Area Telescope of the Fermi Gamma-ray Space Telescope. We present general results about the kinematics of parsec-scale radio jets, as well as the connection between γ-ray outbursts and jet events. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Millimeter VLBI of NGC 1052: Dynamics
Galaxies 2016, 4(4), 48; doi:10.3390/galaxies4040048
Received: 15 July 2016 / Revised: 30 September 2016 / Accepted: 9 October 2016 / Published: 20 October 2016
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Abstract
The LINER galaxy NGC 1052 is an ideal target to study the innermost regions of active galactic nuclei (AGN), given its close distance of about 20 Mpc. The source was observed at 29 epochs from 2005 to 2009 with the Very Long Baseline
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The LINER galaxy NGC 1052 is an ideal target to study the innermost regions of active galactic nuclei (AGN), given its close distance of about 20 Mpc. The source was observed at 29 epochs from 2005 to 2009 with the Very Long Baseline Array (VLBA) at 43 GHz. Here, we present a kinematic study of its twin-jet system from a subset of 9 epochs at 43 GHz carried out in 2005 and 2006, finding a bright central feature as the dynamic center. The resulting mean velocities of β = v / c = 0 . 46 ± 0 . 08 and β = 0 . 69 ± 0 . 02 for the western and eastern jet, respectively, give hints towards higher velocities in the eastern jet. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Observations of the Structure and Dynamics of the Inner M87 Jet
Galaxies 2016, 4(4), 46; doi:10.3390/galaxies4040046
Received: 1 September 2016 / Revised: 5 October 2016 / Accepted: 8 October 2016 / Published: 18 October 2016
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Abstract
M87 is the best source in which to study a jet at high resolution in gravitational units because it has a very high mass black hole and is nearby. The angular size of the black hole is second only to Sgr A*, which
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M87 is the best source in which to study a jet at high resolution in gravitational units because it has a very high mass black hole and is nearby. The angular size of the black hole is second only to Sgr A*, which does not have a strong jet. The jet structure is edge brightened with a wide opening angle base and a weak counterjet. We have roughly annual observations for 17 years plus intensive monitoring at three week intervals for a year and five day intervals for 2.5 months made with the Very Long Baseline Array (VLBA) at 43 GHz. The inner jet shows very complex dynamics, with apparent motions both along and across the jet. Speeds from zero to over 2c are seen, with acceleration observed over the first 3 milli-arcseconds. The counterjet decreases in brightness much more rapidly than the main jet, as is expected from relativistic beaming in an accelerating jet oriented near the line-of-sight. Details of the structure and dynamics are discussed. The roughly annual observations show side-to-side motion of the whole jet with a characteristic time scale of about 9 years. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Theoretical Study of the Effects of Magnetic Field Geometry on the High-Energy Emission of Blazars
Galaxies 2016, 4(4), 45; doi:10.3390/galaxies4040045
Received: 6 September 2016 / Revised: 6 October 2016 / Accepted: 8 October 2016 / Published: 14 October 2016
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Abstract
The knowledge of the structure of the magnetic field inside a blazar jet, as deduced from polarization observations at radio to optical wavelengths, is closely related to the formation and propagation of relativistic jets that result from accretion onto supermassive black holes. However,
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The knowledge of the structure of the magnetic field inside a blazar jet, as deduced from polarization observations at radio to optical wavelengths, is closely related to the formation and propagation of relativistic jets that result from accretion onto supermassive black holes. However, a largely unexplored aspect of the theoretical understanding of radiation transfer physics in blazar jets has been the magnetic field geometry as revealed by the polarized emission and the connection between the variability in polarization and flux across the spectrum. Here, we explore the effects of various magnetic geometries that can exist inside a blazar jet: parallel, oblique, toroidal, and tangled. We investigate the effects of changing the orientation of the magnetic field, according to the above-mentioned geometries, on the resulting high-energy spectral energy distributions (SEDs) and spectral variability patterns (SVPs) of a typical blazar. We use the MUlti-ZOne Radiation Feedback (MUZORF) model of Joshi et al. (2014) to carry out this study and to relate the geometry of the field to the observed SEDs at X-ray and γ-ray energies. One of the goals of the study is to understand the relationship between synchrotron and inverse Compton peaks in blazar SEDs and the reason for the appearance of γ-ray “orphan flares” observed in some blazars. This can be associated with the directionality of the magnetic field, which creates a difference in the radiation field as seen by an observer versus that seen by the electrons in the emission region. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Polarization Vector Rotations: Real, Spurious, Hidden and Imaginary
Galaxies 2016, 4(4), 43; doi:10.3390/galaxies4040043
Received: 14 July 2016 / Revised: 26 September 2016 / Accepted: 27 September 2016 / Published: 11 October 2016
Cited by 2 | PDF Full-text (3663 KB) | HTML Full-text | XML Full-text
Abstract
Large and variable polarization is an inherent property of a majority of blazars. Systematic rotations of the polarization vector have been claimed for several blazars. In some cases, however, the reality of these rotations may be questionable. We suggest an additional method for
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Large and variable polarization is an inherent property of a majority of blazars. Systematic rotations of the polarization vector have been claimed for several blazars. In some cases, however, the reality of these rotations may be questionable. We suggest an additional method for the verification of the reality of polarization position angle rotations based on the correlation of the normalized Stokes parameters. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Parsec-Scale Structure and Kinematics of Faint TeV HBLs
Galaxies 2016, 4(4), 44; doi:10.3390/galaxies4040044
Received: 6 September 2016 / Revised: 29 September 2016 / Accepted: 30 September 2016 / Published: 11 October 2016
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Abstract
We present new multi-epoch Very Long Baseline Array (VLBA) observations of a set of TeV blazars drawn from our VLBA program to monitor all TeV-detected high-frequency peaked BL Lac objects (HBLs) at parsec scales. Most of these sources are faint in the radio,
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We present new multi-epoch Very Long Baseline Array (VLBA) observations of a set of TeV blazars drawn from our VLBA program to monitor all TeV-detected high-frequency peaked BL Lac objects (HBLs) at parsec scales. Most of these sources are faint in the radio, so they have not been well observed with VLBI by other surveys. Our previous measurements of apparent jet speeds in TeV HBLs showed apparent jet speeds that were subluminal or barely superluminal, suggesting jets with velocity structures at the parsec-scale. Here we present apparent jet speed measurements for eight new TeV HBLs, which for the first time show a superluminal tail to the apparent speed distribution for the TeV HBLs. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Radio-to-γ-Ray, Broadband Variability Study of the Classical BL Lac Object PKS 0735+178
Galaxies 2016, 4(4), 42; doi:10.3390/galaxies4040042
Received: 18 July 2016 / Revised: 16 September 2016 / Accepted: 23 September 2016 / Published: 10 October 2016
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Abstract
The power-law shape of the power spectral density (PSD) of blazar light curves—P(νk)νkβ, where νk is the temporal frequency—indicates that blazar variability is, in general, of the colored-type noise (β
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The power-law shape of the power spectral density (PSD) of blazar light curves— P ( ν k ) ν k β , where ν k is the temporal frequency—indicates that blazar variability is, in general, of the colored-type noise ( β 1 3 ). A precise characterisation of PSD slopes, normalizations, or characteristic timescales (if any) manifesting as distinct features in the power spectra of blazars is important for constraining the physics of the emission and energy dissipation processes in relativistic jets. Here we present the results of the PSD analysis for the BL Lac object PKS 0735+178 at GeV (Fermi-LAT), optical (R-band), and radio (GHz band from UMRAO and OVRO programmes) frequencies, covering a broad range in variability timescales. The novelty of our approach is that in the optical regime, by combining the long-term and densely sampled R-band intra-night light curves, we constructed the PSD for time periods ranging from 23 years down to minutes. Our analysis reveals that: (1) the nature of processes generating flux variability at optical and radio frequencies is different from those operating at GeV photon energies ( β 2 and 1, respectively); (2) the main driver behind the optical variability is the same on timescales of years, months, days, and hours (a single power-law with β 2). We discuss our findings in the framework of a model where the overall blazar variability is generated by an underlying single stochastic process (radio and optical frequencies), or a linear superposition of such processes (γ-ray regime). Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle A Search for QPOs in the Blazar OJ287: Preliminary Results from the 2015/2016 Observing Campaign
Galaxies 2016, 4(4), 41; doi:10.3390/galaxies4040041
Received: 16 July 2016 / Revised: 7 September 2016 / Accepted: 9 September 2016 / Published: 9 October 2016
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Abstract
We analyse the light curve in the R band of the blazar OJ287, gathered during the 2015/2016 observing season. We did a search for quasi-periodic oscillations (QPOs) using several methods over a wide range of timescales. No statistically significant periods were found in
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We analyse the light curve in the R band of the blazar OJ287, gathered during the 2015/2016 observing season. We did a search for quasi-periodic oscillations (QPOs) using several methods over a wide range of timescales. No statistically significant periods were found in the high-frequency domain both in the ground-based data and in Kepler observations. In the longer-period domain, the Lomb–Scargle periodogram revealed several peaks above the 99% significance level. The longest one—about 95 days—corresponds to the innermost stable circular orbit (ISCO) period of the more massive black hole. The 43-day period could be an alias, or it can be attributed to accretion in the form of a two-armed spiral wave. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Magnetic Dissipation in Relativistic Jets
Galaxies 2016, 4(4), 40; doi:10.3390/galaxies4040040
Received: 27 July 2016 / Revised: 22 September 2016 / Accepted: 26 September 2016 / Published: 7 October 2016
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Abstract
The most promising mechanisms for producing and accelerating relativistic jets, and maintaining collimated structure of relativistic jets involve magnetohydrodynamical (MHD) processes. We have investigated the magnetic dissipation mechanism in relativistic jets via relativistic MHD simulations. We found that the relativistic jets involving a
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The most promising mechanisms for producing and accelerating relativistic jets, and maintaining collimated structure of relativistic jets involve magnetohydrodynamical (MHD) processes. We have investigated the magnetic dissipation mechanism in relativistic jets via relativistic MHD simulations. We found that the relativistic jets involving a helical magnetic field are unstable for the current-driven kink instability, which leads to helically distorted structure in relativistic jets. We identified the regions of high current density in filamentary current sheets, indicative of magnetic reconnection, which are associated to the kink unstable regions and correlated to the converted regions of magnetic to kinetic energies of the jets. We also found that an over-pressured relativistic jet leads to the generation of a series of stationary recollimation shocks and rarefaction structures by the nonlinear interaction of shocks and rarefaction waves. The differences in the recollimation shock structure due to the difference of the magnetic field topologies and strengths may be observable through mm-VLBI observations and space-VLBI mission. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Resolving the Base of the Relativistic Jet in M87 at 6Rsch Resolution with Global mm-VLBI
Galaxies 2016, 4(4), 39; doi:10.3390/galaxies4040039
Received: 15 July 2016 / Revised: 22 September 2016 / Accepted: 23 September 2016 / Published: 30 September 2016
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Abstract
M87 is one of the nearest radio galaxies with a central Super-Massive Black Hole (SMBH) and a prominent relativistic jet. Due to its close distance to the observer and the large SMBH mass, the source is one of the best laboratories to obtain
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M87 is one of the nearest radio galaxies with a central Super-Massive Black Hole (SMBH) and a prominent relativistic jet. Due to its close distance to the observer and the large SMBH mass, the source is one of the best laboratories to obtain strong observational constraints on the theoretical models for the formation and evolution of the AGN jets. In this article, we present preliminary results from our ongoing observational study about the innermost jet of M87 at an ultra-high resolution of ∼50 μ as achieved by the Global Millimeter-Very Long Baseline Interferometry Array (GMVA). The data obtained between 2004 and 2015 clearly show limb-brightened jets at extreme resolution and sensitivity. Our preliminary analysis reveals that the innermost jet expands in an edge-brightened cone structure (parabolic shape) but with the jet expansion profile slightly different from the outer regions of the jet. Brightness temperatures of the Very Long Baseline Interferometry (VLBI) core obtained from cm- to mm-wavelengths show a systematic evolution, which can be interpreted as the evolution as a function of distance from the BH. We also adopt an alternative imaging algorithm, Bi-Spectrum Maximum Entropy Method (BSMEM), to test reliable imaging at higher angular resolution than provided by the standard CLEAN method (i.e., super-resolution). A demonstration with a VLBA 7 mm example data set shows consistent results with a near-in-time 3 mm VLBI image. Application of the method to the 2009 GMVA data yields an image with remarkable fine-scale structures that have been never imaged before. We present a brief interpretation of the complexity in the structure. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Microscopic Processes in Global Relativistic Jets Containing Helical Magnetic Fields
Galaxies 2016, 4(4), 38; doi:10.3390/galaxies4040038
Received: 13 July 2016 / Revised: 14 September 2016 / Accepted: 22 September 2016 / Published: 29 September 2016
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Abstract
In the study of relativistic jets one of the key open questions is their interaction with the environment on the microscopic level. Here, we study the initial evolution of both electron–proton (ep+) and electron–positron (e±
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In the study of relativistic jets one of the key open questions is their interaction with the environment on the microscopic level. Here, we study the initial evolution of both electron–proton ( e p + ) and electron–positron ( e ± ) relativistic jets containing helical magnetic fields, focusing on their interaction with an ambient plasma. We have performed simulations of “global” jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities such as the Weibel instability, the kinetic Kelvin-Helmholtz instability (kKHI) and the Mushroom instability (MI). In our initial simulation study these kinetic instabilities are suppressed and new types of instabilities can grow. In the e p + jet simulation a recollimation-like instability occurs and jet electrons are strongly perturbed. In the e ± jet simulation a recollimation-like instability occurs at early times followed by a kinetic instability and the general structure is similar to a simulation without helical magnetic field. Simulations using much larger systems are required in order to thoroughly follow the evolution of global jets containing helical magnetic fields. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle The Blazar Sequence 2.0
Galaxies 2016, 4(4), 36; doi:10.3390/galaxies4040036
Received: 28 July 2016 / Revised: 20 September 2016 / Accepted: 21 September 2016 / Published: 27 September 2016
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Abstract
This paper discusses the spectral energy distribution (SED) of all blazars with redshift detected by the Fermi satellite and listed in the 3LAC catalog. The so called “blazar sequence” from the phenomenological point of view will be updated, with no theory or modelling.
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This paper discusses the spectral energy distribution (SED) of all blazars with redshift detected by the Fermi satellite and listed in the 3LAC catalog. The so called “blazar sequence” from the phenomenological point of view will be updated, with no theory or modelling. It will be shown that: (i) pure data show that jet and accretion power are related; (ii) the updated blazar sequence maintains the properties of the old version, albeit with a less pronounced dominance of the γ-ray emission; (iii) at low bolometric luminosities, two different types of objects have the same high energy power: low black hole mass flat spectrum radio quasars and high mass BL Lacs. Therefore, at low luminosities, there is a very large dispersion of SED shapes; (iv) in low power BL Lacs, the contribution of the host galaxy is important. Remarkably, the luminosity distribution of the host galaxies of BL Lacs are spread in a very narrow range; (v) a simple sum of two smoothly joining power laws can describe the blazar SEDs very well. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Broad Band Observations of Gravitationally Lensed Blazar during a Gamma-Ray Outburst
Galaxies 2016, 4(4), 31; doi:10.3390/galaxies4040031
Received: 15 July 2016 / Accepted: 2 September 2016 / Published: 27 September 2016
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Abstract
QSO B0218+357 is a gravitationally lensed blazar located at a cosmological redshift of 0.944. In July 2014 a GeV flare was observed by Fermi-LAT, triggering follow-up observations with the MAGIC telescopes at energies above 100 GeV. The MAGIC observations at the expected
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QSO B0218+357 is a gravitationally lensed blazar located at a cosmological redshift of 0.944. In July 2014 a GeV flare was observed by Fermi-LAT, triggering follow-up observations with the MAGIC telescopes at energies above 100 GeV. The MAGIC observations at the expected time of arrival of the trailing component resulted in the first detection of QSO B0218+357 in Very-High-Energy (VHE, >100 GeV) gamma rays. We report here the observed multiwavelength emission during the 2014 flare. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle The Connection between the Radio Jet and the γ-ray Emission in the Radio Galaxy 3C 120 and the Blazar CTA 102
Galaxies 2016, 4(4), 34; doi:10.3390/galaxies4040034
Received: 16 July 2016 / Revised: 14 September 2016 / Accepted: 15 September 2016 / Published: 27 September 2016
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Abstract
We present multi-wavelength studies of the radio galaxy 3C 120 and the blazar CTA 102 during unprecedented γ-ray flares for both sources. In both studies the analysis of γ-ray data has been compared with a series of 43 GHz VLBA images
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We present multi-wavelength studies of the radio galaxy 3C 120 and the blazar CTA 102 during unprecedented γ-ray flares for both sources. In both studies the analysis of γ-ray data has been compared with a series of 43 GHz VLBA images from the VLBA-BU-BLAZAR program, providing the necessary spatial resolution to probe the parsec scale jet evolution during the high energy events. To extend the radio dataset for 3C 120 we also used 15 GHz VLBA data from the MOJAVE sample. These two objects which represent very different classes of AGN, have similar properties during the γ-ray events. The γ-ray flares are associated with the passage of a new superluminal component through the mm VLBI core, but not all ejections of new components lead to γ-ray events. In both sources γ-ray events occurred only when the new components are moving in a direction closer to our line of sight. We locate the γ-ray dissipation zone a short distance from the radio core but outside of the broad line region, suggesting synchrotron self-Compton scattering as the probable mechanism for the γ-ray production. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Radiative Transfer Modeling of Radio-Band Linear Polarization Observations as a Probe of the Physical Conditions in the Jets of γ-Ray Flaring Blazars
Galaxies 2016, 4(4), 35; doi:10.3390/galaxies4040035
Received: 29 August 2016 / Accepted: 13 September 2016 / Published: 26 September 2016
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Abstract
Since the mid-1980s, the shock-in-jet model has been the preferred paradigm to explain radio-band flaring in blazar jets. We describe our radiative transfer model incorporating relativistically-propagating shocks, and illustrate how the 4.8, 8, and 14.5 GHz linear polarization and total flux density data
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Since the mid-1980s, the shock-in-jet model has been the preferred paradigm to explain radio-band flaring in blazar jets. We describe our radiative transfer model incorporating relativistically-propagating shocks, and illustrate how the 4.8, 8, and 14.5 GHz linear polarization and total flux density data from the University of Michigan monitoring program, in combination with the model, constrain jet flow conditions and shock attributes. Results from strong Fermi-era flares in 4 blazars with widely-ranging properties are presented. Additionally, to investigate jet evolution on decadal time scales we analyze 3 outbursts in OT 081 spanning nearly 3 decades and find intrinsic changes attributable to flow changes at a common spatial location, or, alternatively, to a change in the jet segment viewed. The model’s success in reproducing these data supports a scenario in which relativistic shocks compress a plasma with an embedded passive, initially-turbulent magnetic field, with additional ordered magnetic field components, one of which may be helical. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Exploring the Magnetic Field Configuration in BL Lac Using GMVA
Galaxies 2016, 4(3), 32; doi:10.3390/galaxies4030032
Received: 15 July 2016 / Revised: 9 September 2016 / Accepted: 13 September 2016 / Published: 20 September 2016
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Abstract
The high radio frequency polarization imaging of non-thermal emission from active galactic nuclei (AGN) is a direct way to probe the magnetic field strength and structure in the immediate vicinity of supermassive black holes (SMBHs) and is crucial in testing the jet-launching scenario.
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The high radio frequency polarization imaging of non-thermal emission from active galactic nuclei (AGN) is a direct way to probe the magnetic field strength and structure in the immediate vicinity of supermassive black holes (SMBHs) and is crucial in testing the jet-launching scenario. To explore the the magnetic field configuration at the base of jets in blazars, we took advantage of the full polarization capabilities of the Global Millimeter VLBI Array (GMVA). With an angular resolution of ∼50 micro-arcseconds (μas) at 86 GHz, one could resolve scales up to ∼450 gravitational radii (for a 10 9 solar mass black hole at a redshift of 0.1). We present here the preliminary results of our study on the blazar BL Lac. Our results suggest that on sub-mas scales the core and the central jet of BL Lac are significantly polarized with two distinct regions of polarized intensity. We also noted a great morphological similarity between the 7 mm/3 mm VLBI images at very similar angular resolution. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle The Radio/Gamma-Ray Connection from 120 MHz to 230 GHz
Galaxies 2016, 4(3), 30; doi:10.3390/galaxies4030030
Received: 14 July 2016 / Revised: 3 September 2016 / Accepted: 5 September 2016 / Published: 13 September 2016
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Abstract
Radio loud active galactic nuclei are composed of different spatial features, each one characterized by different spectral properties in the radio band. Among them, blazars are the most common class of sources detected at gamma-rays by Fermi, and their radio emission is
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Radio loud active galactic nuclei are composed of different spatial features, each one characterized by different spectral properties in the radio band. Among them, blazars are the most common class of sources detected at gamma-rays by Fermi, and their radio emission is dominated by the flat spectrum compact core. In this contribution, we explore the connection between emission at high energy revealed by Fermi and at radio frequencies. Taking as a reference the strong and very highly significant correlation found between gamma rays and cm-λ radio emission, we explore the different behaviours found as we change the energy range in gamma rays and in radio, therefore changing the physical parameters of the zones involved in the emitted radiation. We find that the correlation weakens when we consider (1) gamma rays of energy above 10 GeV (except for high synchrotron peaked blazars) or (2) low frequency radio data taken by the Murchison Widefield Array; on the other hand, the correlation strengthens when we consider mm-λ data taken by Atacama Large Millimeter Array (ALMA). Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle AGN Jet Kinematics on Parsec-Scales: The MOJAVE Program
Galaxies 2016, 4(3), 29; doi:10.3390/galaxies4030029
Received: 20 July 2016 / Accepted: 2 September 2016 / Published: 9 September 2016
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Abstract
Very long baseline interferometry offers the best means of investigating the complex dynamics of relativistic jets powered by active galactic nuclei, via multi-epoch, sub-milliarcsecond, full-polarization imaging at radio wavelengths. Although targeted studies have yielded important information on the structures of individual AGN jets,
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Very long baseline interferometry offers the best means of investigating the complex dynamics of relativistic jets powered by active galactic nuclei, via multi-epoch, sub-milliarcsecond, full-polarization imaging at radio wavelengths. Although targeted studies have yielded important information on the structures of individual AGN jets, the strong selection effects associated with relativistically beaming imply that general aspects of the flows can only be determined via large statistical studies. In this review I discuss major results from the Monitoring of Jets in Active Galactic Nuclei With VLBA Experiments (MOJAVE) program, which has gathered multi-epoch Very Long Baseline Array (VLBA) data at 15 GHz on over 400 AGN jets over the course of two decades. The sample is large enough to encompass a range of AGN optical class, radio luminosity and synchrotron peak frequency, and has been used to show that within a particular jet, individual bright features have a spread of apparent speed and velocity vector position angle about a characteristic value. We have found that in some cases there is a secular evolution of launch angle direction over time, indicative of evolving narrow energized channels within a wider outflow. The majority of the jet features are superluminal and accelerating, with changes in speed more common than changes in direction. Within approximately 100 pc of the AGN, the flows are generally accelerating, while beyond this distance the flows begin to decelerate or remain nearly constant in speed. We also find evidence for a maximum bulk flow Lorentz factor of 50 in the pc-scale radio regime, and a trend of higher jet speeds in lower-synchrotron peaked and gamma-ray-loud blazars. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Applying Relativistic Reconnection to Blazar Jets
Galaxies 2016, 4(3), 28; doi:10.3390/galaxies4030028
Received: 15 July 2016 / Revised: 2 September 2016 / Accepted: 3 September 2016 / Published: 8 September 2016
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Abstract
Rapid and luminous flares of non-thermal radiation observed in blazars require an efficient mechanism of energy dissipation and particle acceleration in relativistic active galactic nuclei (AGN) jets. Particle acceleration in relativistic magnetic reconnection is being actively studied by kinetic numerical simulations. Relativistic reconnection
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Rapid and luminous flares of non-thermal radiation observed in blazars require an efficient mechanism of energy dissipation and particle acceleration in relativistic active galactic nuclei (AGN) jets. Particle acceleration in relativistic magnetic reconnection is being actively studied by kinetic numerical simulations. Relativistic reconnection produces hard power-law electron energy distributions N ( γ ) γ p exp ( γ / γ max ) with index p 1 and exponential cut-off Lorentz factor γ max σ in the limit of magnetization σ = B 2 / ( 4 π w ) 1 (where w is the relativistic enthalpy density). Reconnection in electron-proton plasma can additionally boost γ max by the mass ratio m p / m e . Hence, in order to accelerate particles to γ max 10 6 in the case of BL Lacs, reconnection should proceed in plasma of very high magnetization σ max 10 3 . On the other hand, moderate mean jet magnetization values are required for magnetic bulk acceleration of relativistic jets, σ mean Γ j 20 (where Γ j is the jet bulk Lorentz factor). I propose that the systematic dependence of γ max on blazar luminosity class—the blazar sequence—may result from a systematic trend in σ max due to homogeneous loading of leptons by pair creation regulated by the energy density of high-energy external radiation fields. At the same time, relativistic AGN jets should be highly inhomogeneous due to filamentary loading of protons, which should determine the value of σ mean roughly independently of the blazar class. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
Open AccessArticle Sharp Polarimetric Eyes: More Trees than Forest?
Galaxies 2016, 4(3), 27; doi:10.3390/galaxies4030027
Received: 14 July 2016 / Revised: 29 August 2016 / Accepted: 30 August 2016 / Published: 7 September 2016
Cited by 2 | PDF Full-text (4828 KB) | HTML Full-text | XML Full-text
Abstract
The Fermi Gamma-ray Space Telescope (Fermi) has focused the intensive multi-wavelength and international observational effort on blazars since it was launched in 2008. Part of this effort involves systematic monitoring of the highly variable polarization of the continuum emission from these
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The Fermi Gamma-ray Space Telescope (Fermi) has focused the intensive multi-wavelength and international observational effort on blazars since it was launched in 2008. Part of this effort involves systematic monitoring of the highly variable polarization of the continuum emission from these objects. These observations are valuable in that they provide direct information on the degree of ordering and orientation on the sky of the magnetic field within the non-thermal emission region(s). Unfortunately, it is not yet possible to measure the polarization of the inverse-Compton continuum, only that of the lower-energy synchrotron emission. The inability to directly compare the polarization of the two dominant continuum sources in blazars is a drawback and leads to more ambiguities in determining their relative locations. There are many compelling examples of strong connections between γ-ray, X-ray, UV/optical/IR, and radio behavior in blazars that suggest the same region produces much of the observed emission at all wavelengths at least some of the time. However, the wealth of polarization behavior seen relative to flux changes invariably results in a complex situation that is difficult to interpret and model. The long-term blazar monitoring program undertaken at Steward Observatory is designed to primarily obtain accurate optical polarimetry of γ-ray-bright blazars during the Fermi mission with the goal of gaining important insights into the jet structure and physics of these objects. Data from this program are available to all researchers as soon as reductions are completed. I briefly detail the current status and progress of the program and the data products available. Although the wide variety of polarization behavior in blazars adds another layer of complexity to an already difficult problem, I summarize several important conclusions that can be drawn from the polarization information gathered during the Fermi era. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Intra-Day Simultaneous Optical Monitoring of S5 0716+714
Galaxies 2016, 4(3), 25; doi:10.3390/galaxies4030025
Received: 14 July 2016 / Revised: 31 August 2016 / Accepted: 31 August 2016 / Published: 6 September 2016
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Abstract
We present the results of simultaneous optical multi-color observations of BL Lac object 0716+714 in November 2014 and February 2016. The intra-day variability (IDV) varies from 0.04 to 0.3 mags. Both achromatic and bluer-when-brighter (BWB) color behaviors were detected. A probable quasi-periodic oscillation
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We present the results of simultaneous optical multi-color observations of BL Lac object 0716+714 in November 2014 and February 2016. The intra-day variability (IDV) varies from 0.04 to 0.3 mags. Both achromatic and bluer-when-brighter (BWB) color behaviors were detected. A probable quasi-periodic oscillation overlapping on a significant flare was also observed. We used the interpolated cross-correlation function to calculate time lags between light curves in different bands. Variations in the B and R lagging behind that in the I band were found, which corresponds with anti-clockwise loops on the color-magnitude diagrams. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Flaring γ-Ray Emission from High Redshift Blazars
Galaxies 2016, 4(3), 26; doi:10.3390/galaxies4030026
Received: 14 July 2016 / Revised: 16 August 2016 / Accepted: 16 August 2016 / Published: 6 September 2016
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Abstract
High redshift blazars are among the most powerful objects in the Universe. Although they represent a significant fraction of the extragalactic hard X-ray sky, they are not commonly detected in γ-rays. High redshift (z>2) objects represent <10
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High redshift blazars are among the most powerful objects in the Universe. Although they represent a significant fraction of the extragalactic hard X-ray sky, they are not commonly detected in γ-rays. High redshift ( z > 2 ) objects represent < 10 per cent of the active galactic nuclei (AGN) population observed by Fermi so far, and γ-ray flaring activity from these sources is even more uncommon. The characterization of the radio-to-γ-ray properties of high redshift blazars represents a powerful tool for the study of the energetics of such extreme objects and the Extragalactic Background Light. This contribution will present results of multi-band campaigns, from radio to γ-rays, on PKS 0836+710, PKS 2149−306, and TXS 0536+145. The latter is the highest redshift detection of a flaring γ-ray blazar so far. At the peaks of their respective flares these sources reached an apparent isotropic gamma-ray luminosity of about 10 50 erg·s 1 , which is comparable with the luminosity observed from the most powerful blazars. The physical properties derived from the multi-wavelength observations of these sources are then compared with those shown by the high redshift population. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle TimeTubes: Visualization of Polarization Variations in Blazars
Galaxies 2016, 4(3), 23; doi:10.3390/galaxies4030023
Received: 15 July 2016 / Revised: 26 August 2016 / Accepted: 30 August 2016 / Published: 3 September 2016
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Abstract
Optical polarization provides important clues to the magnetic field in blazar jets. It is easy to find noteworthy patterns in the time-series data of the polarization degree (PD) and position angle (PA). On the other hand, we need to see the trajectory of
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Optical polarization provides important clues to the magnetic field in blazar jets. It is easy to find noteworthy patterns in the time-series data of the polarization degree (PD) and position angle (PA). On the other hand, we need to see the trajectory of the object in the Stokes Q U plane when the object has multiple polarized components. In this case, ironically, the more data we have, the more difficult it is to gain any knowledge from it. Here, we introduce TimeTubes, a new visualization scheme to explore the time-series data of polarization observed in blazars. In TimeTubes, the data is represented by tubes in 3D (Q, U, and time) space. The measurement errors of Q and U, color, and total flux of objects are expressed as the size, color, and brightness of the tubes. As a result, TimeTubes allows us to see the behavior of six variables in one view. We used TimeTubes for our data taken by the Kanata telescope between 2008 and 2014. We found that this tool facilitates the recognition of the patterns in blazar variations; for example, favored PA of flares and PA rotations associated with a series of flares. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Optical Outburst of the Blazar S4 0954+658 in Early 2015
Galaxies 2016, 4(3), 24; doi:10.3390/galaxies4030024
Received: 12 July 2016 / Revised: 29 August 2016 / Accepted: 30 August 2016 / Published: 3 September 2016
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Abstract
We analyze the behavior of the BL Lac object S4 0954+658 during an unprecedented bright optical flare in early 2015. The optical flare was accompanied by a powerful γ-ray flare and the detection of very-high-energy γ-ray emission. We analyze total and
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We analyze the behavior of the BL Lac object S4 0954+658 during an unprecedented bright optical flare in early 2015. The optical flare was accompanied by a powerful γ -ray flare and the detection of very-high-energy γ -ray emission. We analyze total and polarized intensity images obtained with the VLBA at 43 GHz and discover a new bright polarized superluminal knot, which was ejected from the VLBI-core during the peak of the flare. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Spectral and Polarization Signatures of Relativistic Shocks in Blazars
Galaxies 2016, 4(3), 22; doi:10.3390/galaxies4030022
Received: 7 July 2016 / Revised: 29 August 2016 / Accepted: 30 August 2016 / Published: 2 September 2016
PDF Full-text (2709 KB) | HTML Full-text | XML Full-text
Abstract
Relativistic shocks are one of the most plausible sites of the emission of strongly variable, polarized multi-wavelength emission from relativistic jet sources such as blazars, via the diffusive shock acceleration (DSA) of relativistic particles. This paper summarizes recent results on a self-consistent coupling
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Relativistic shocks are one of the most plausible sites of the emission of strongly variable, polarized multi-wavelength emission from relativistic jet sources such as blazars, via the diffusive shock acceleration (DSA) of relativistic particles. This paper summarizes recent results on a self-consistent coupling of diffusive shock acceleration and radiation transfer in blazar jets. We demonstrate that the observed spectral energy distributions (SEDs) of blazars strongly constrain the nature of hydromagnetic turbulence responsible for pitch-angle scattering by requiring a strongly energy-dependent pitch-angle mean free path. The prominent soft X-ray excess (“Big Blue Bump”) in the SED of the BL Lac object AO 0235+164 can be modelled as the signature of bulk Compton scattering of external radiation fields by the thermal electron population, which places additional constraints on the level of hydromagnetic turbulence. It has further been demonstrated that internal shocks propagating in a jet pervaded by a helical magnetic field naturally produce polarization-angle swings by 180 , in tandem with multi-wavelength flaring activity, without requiring any helical motion paths or other asymmetric jet structures. The specific application of this model to 3C279 presents the first consistent simultaneous modeling of snap-shot SEDs, multi-wavelength light curves, and time-dependent polarization signatures of a blazar during a polarization-angle (PA) rotation. This model has recently been generalized to a lepto-hadronic model, in which the high-energy emission is dominated by proton synchrotron radiation. It is shown that in this case, the high-energy (X-ray and γ-ray) polarization signatures are expected to be significantly more stable (not showing PA rotations) than the low-energy (electron-synchrotron) signatures. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Gamma-Ray Variability Induced by Microlensing on Intermediate Size Structures in Lensed Blazars
Galaxies 2016, 4(3), 20; doi:10.3390/galaxies4030020
Received: 12 July 2016 / Revised: 24 August 2016 / Accepted: 25 August 2016 / Published: 31 August 2016
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Abstract
Changes of the magnification ratio of images in a lensed blazar, caused by microlensing on individual stars, have been proposed as a probe of the size and velocity of the emission region in the lensed source. We study whether similar changes in the
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Changes of the magnification ratio of images in a lensed blazar, caused by microlensing on individual stars, have been proposed as a probe of the size and velocity of the emission region in the lensed source. We study whether similar changes in the magnification ratio can be caused by the microlensing on the intermediate size structures in the lensing galaxy, namely stellar clusters and giant molecular clouds. Our numerical simulations show that changes in the magnification ratio of two images with similar time scales (as seen in QSO B0218+357) can be obtained for relativistically-moving emission regions with sizes up to 0.01 pc in the case of microlensing on clumps in giant molecular clouds. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Time-Dependent Injection as a Model for Rapid Blazar Flares
Galaxies 2016, 4(3), 19; doi:10.3390/galaxies4030019
Received: 8 July 2016 / Revised: 22 August 2016 / Accepted: 26 August 2016 / Published: 31 August 2016
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Abstract
Time-dependent injection can cause non-linear cooling effects, which lead to a faster energy loss of the electrons in jets. The most obvious result is the appearance of unique breaks in the SED, which would normally be attributed to a complicated electron distribution. Knowledge
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Time-dependent injection can cause non-linear cooling effects, which lead to a faster energy loss of the electrons in jets. The most obvious result is the appearance of unique breaks in the SED, which would normally be attributed to a complicated electron distribution. Knowledge of the observation time and duration is important for the interpretation of the observed spectra, because of the non-trivial evolution of the spectral energy distributions (SED). Intrinsic gamma–gamma absorption processes in the emission region are only of minor importance. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle 18–22 cm VLBA Observational Evidence for Toroidal B-Field Components in Six AGN Jets
Galaxies 2016, 4(3), 18; doi:10.3390/galaxies4030018
Received: 5 July 2016 / Revised: 20 August 2016 / Accepted: 26 August 2016 / Published: 30 August 2016
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Abstract
The formation of relativistic jets in Active Galactic Nuclei (AGN) is related to accretion onto their central supermassive black holes, and magnetic (B) fields are believed to play a central role in launching, collimating, and accelerating the jet streams from very
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The formation of relativistic jets in Active Galactic Nuclei (AGN) is related to accretion onto their central supermassive black holes, and magnetic (B) fields are believed to play a central role in launching, collimating, and accelerating the jet streams from very compact regions out to kiloparsec scales. We present results of Faraday rotation studies based on Very Long Baseline Array (VLBA) data obtained at 18–22 cm for six well known AGN (OJ 287, 3C 279, PKS 1510-089, 3C 345, BL Lac, and 3C 454.3), which probe projected distances out to tens of parsecs from the observed cores. We have identified statistically significant, monotonic, transverse Faraday rotation gradients across the jets of all but one of these sources, indicating the presence of toroidal B fields, which may be one component of helical B fields associated with these AGN jets. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle The Optical Variability of the BL Lac AO 0235+164
Galaxies 2016, 4(3), 17; doi:10.3390/galaxies4030017
Received: 14 July 2016 / Revised: 17 August 2016 / Accepted: 18 August 2016 / Published: 30 August 2016
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Abstract
In this work, we present optical R band observations of AO 0235+164 carried out during the period of November 2006 to December 2012 using the Ap6E CCD camera attached to the primary focus of the 70 cm meniscus telescope at Abastumani Observatory, Georgia.
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In this work, we present optical R band observations of AO 0235+164 carried out during the period of November 2006 to December 2012 using the Ap6E CCD camera attached to the primary focus of the 70 cm meniscus telescope at Abastumani Observatory, Georgia. It shows a large variation of Δ R = 4.88 mag (14.19–19.07 mag) and a short time scale of Δ T v = 73.5 min during our monitoring period. When periodicity analysis methods are applied to the R-band data from both historic and our observations, periods P 1 = 8.26 yr and P 2 = 0.54 yr are found. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Multi-Frequency Blazar Micro-Variability as a Tool to Investigate Relativistic Jets
Galaxies 2016, 4(3), 15; doi:10.3390/galaxies4030015
Received: 29 June 2016 / Revised: 15 August 2016 / Accepted: 17 August 2016 / Published: 29 August 2016
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Abstract
For the past 12 years we have been studying optical micro-variability of a sample of 15 Blazars. We summarize the results of this study and draw some basic conclusions about the characteristics of micro-variability. The intermittency, the stochastic nature, and the similar profile
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For the past 12 years we have been studying optical micro-variability of a sample of 15 Blazars. We summarize the results of this study and draw some basic conclusions about the characteristics of micro-variability. The intermittency, the stochastic nature, and the similar profile shapes seen in micro-variations at different times and in different objects have led us to a possible model to explain the observed micro-variations. The model is based on a strong shock propagating down a relativistic jet and encountering turbulence which causes density or magnetic field enhancements. We use the theory of Kirk, Reiger, and Mastichiadis (1998) to describe the pulse of synchrotron emission emanating from individual density enhancements energized by the shock. By fitting these “pulses” to micro-variability observations, we obtain excellent fits to actual micro-variations. The model predicts that the spectral index changes as a function of pulse duration. This effect should be observable in multi-frequency micro-variability data. We present the theoretical model, model fits of our micro-variability light curves, and preliminary multi-frequency micro-variability observations that support this model. A further test that has yet to be carried out involves observing polarization changes in different pulses. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle The Classifications and Some Correlations for Fermi Blazars
Galaxies 2016, 4(3), 16; doi:10.3390/galaxies4030016
Received: 14 July 2016 / Revised: 17 August 2016 / Accepted: 17 August 2016 / Published: 29 August 2016
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Abstract
In a recent paper, we constructed the spectral energy distributions (SEDs) for 1425 Fermi blazars. We classify them as low synchrotron peak sources (LSPs) if log νp(Hz) 14.0, intermediate synchrotron peak sources (ISPs) if 14.0<logν
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In a recent paper, we constructed the spectral energy distributions (SEDs) for 1425 Fermi blazars. We classify them as low synchrotron peak sources (LSPs) if log ν p ( Hz ) 14.0, intermediate synchrotron peak sources (ISPs) if 14.0 < log ν p ( Hz ) 15.3 , and high synchrotron peak sources (HSPs) if log ν p ( Hz ) > 15.3 . We obtain an empirical relation to estimate the synchrotron peak frequency, ν p Eq . from effective spectral indexes α o x and α r o as log ν p Eq . = 16 + 4.238 X if X < 0 , and log ν p Eq . = 16 + 4.005 Y if X > 0 , where X = 1.0 1.262 α r o 0.623 α o x and Y = 1.0 + 0.034 α r o 0.978 α o x . In the present work, we investigate the correlation between the peak frequency and the radio-to-X-ray spectral index, between peak luminosity (bolometric luminosity) and γ-ray/optical luminosity, and between peak luminosity and bolometric luminosity. Some discussion is presented. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Search for High-Confidence Blazar Candidates and Their MWL Counterparts in the Fermi-LAT Catalog Using Machine Learning
Galaxies 2016, 4(3), 14; doi:10.3390/galaxies4030014
Received: 15 July 2016 / Revised: 18 August 2016 / Accepted: 23 August 2016 / Published: 26 August 2016
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Abstract
A large fraction of the gamma-ray sources presented in the Third Fermi-LAT source catalog (3FGL) is affiliated with counterparts and source types, but 1010 sources remain unassociated and 573 sources are associated with active galaxies of uncertain type. The purpose of this
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A large fraction of the gamma-ray sources presented in the Third Fermi-LAT source catalog (3FGL) is affiliated with counterparts and source types, but 1010 sources remain unassociated and 573 sources are associated with active galaxies of uncertain type. The purpose of this study is to assign blazar classes to these unassociated and uncertain sources, and to link counterparts to the unassociated. A machine learning algorithm is used for the classification, based on properties extracted from the 3FGL, an infrared and an X-ray catalog. To estimate the reliability of the classification, performance measures are considered through validation techniques. The classification yielded purity values around 90% with efficiency values of roughly 50%. The prediction of high-confidence blazar candidates has been conducted successfully, and the possibility to link counterparts in the same procedure has been proven. These findings confirm the relevance of this novel multiwavelength approach. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle The Extremes in Intra-Night Blazar Variability: The S4 0954+65 Case
Galaxies 2016, 4(3), 13; doi:10.3390/galaxies4030013
Received: 7 July 2016 / Revised: 18 August 2016 / Accepted: 19 August 2016 / Published: 26 August 2016
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Abstract
We present results of optical observations of the extremely violently variable blazar S4 0954+65 on intra-night time scales. The object showed flux changes of up to 100% within a few hours. Time delays between optical bands, color changes and “rms-flux” relations are investigated
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We present results of optical observations of the extremely violently variable blazar S4 0954+65 on intra-night time scales. The object showed flux changes of up to 100% within a few hours. Time delays between optical bands, color changes and “rms-flux” relations are investigated and the results are discussed in terms of existing models of blazar variability. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Powers and Magnetization of Blazar Jets
Galaxies 2016, 4(3), 12; doi:10.3390/galaxies4030012
Received: 15 July 2016 / Revised: 21 August 2016 / Accepted: 23 August 2016 / Published: 26 August 2016
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Abstract
In this work I review the observational constraints imposed on the energetics and magnetisation of quasar jets, in the context of theoretical expectations. The discussion is focused on issues regarding the jet production efficiency, matter content, and particle acceleration. I show that if
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In this work I review the observational constraints imposed on the energetics and magnetisation of quasar jets, in the context of theoretical expectations. The discussion is focused on issues regarding the jet production efficiency, matter content, and particle acceleration. I show that if the ratio of electron-positron-pairs to protons is of order 15, as is required to achieve agreement between jet powers computed using blazar spectral fits and those computed using radio-lobe calorimetry, the magnetization of blazar jets in flat-spectrum-radio-quasars (FSRQ) must be significant. This result favors the reconnection mechanism for particle acceleration and explains the large Compton-dominance of blazar spectra that is often observed, without the need to postulate very low jet magnetization. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle A Panchromatic View of Relativistic Jets in Narrow-Line Seyfert 1 Galaxies
Galaxies 2016, 4(3), 11; doi:10.3390/galaxies4030011
Received: 15 July 2016 / Revised: 16 August 2016 / Accepted: 18 August 2016 / Published: 25 August 2016
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Abstract
The discovery by the Large Area Telescope on board Fermi of variable γ-ray emission from radio-loud narrow-line Seyfert 1 (NLSy1) galaxies revealed the presence of a possible third class of Active Galactic Nuclei (AGN) with relativistic jets in addition to blazars and
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The discovery by the Large Area Telescope on board Fermi of variable γ-ray emission from radio-loud narrow-line Seyfert 1 (NLSy1) galaxies revealed the presence of a possible third class of Active Galactic Nuclei (AGN) with relativistic jets in addition to blazars and radio galaxies. Considering that NLSy1 are usually hosted in spiral galaxies, this finding poses intriguing questions about the nature of these objects and the formation of relativistic jets. We report on a systematic investigation of the γ-ray properties of a sample of radio-loud NLSy1, including the detection of new objects, using 7 years of Fermi-LAT data with the new Pass 8 event-level analysis. In addition we discuss the radio-to-very-high-energy properties of the γ-ray emitting NLSy1, their host galaxy, and black hole mass in the context of the blazar scenario and the unification of relativistic jets at different scales. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle Precessing Jet in the High-Redshift Blazar J0017+8135
Galaxies 2016, 4(3), 10; doi:10.3390/galaxies4030010
Received: 12 July 2016 / Revised: 17 August 2016 / Accepted: 18 August 2016 / Published: 24 August 2016
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Abstract
The prominent flat-spectrum radio quasar J0017+8135 (S5 0014+81) at z = 3.366 is one of the most luminous active galactic nuclei (AGN) known. Its milliarcsecond-scale radio jet structure has been studied with very long baseline interferometry (VLBI) since the 1980s. The quasar was
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The prominent flat-spectrum radio quasar J0017+8135 (S5 0014+81) at z = 3.366 is one of the most luminous active galactic nuclei (AGN) known. Its milliarcsecond-scale radio jet structure has been studied with very long baseline interferometry (VLBI) since the 1980s. The quasar was selected as one of the original defining objects of the International Celestial Reference Frame, but left out from its current second realization (ICRF2) because of systematic long-term positional variations. Here we analyse archival 8.6- and 2.3-GHz VLBI imaging data collected at nearly 100 different epochs during more than 20 years, to obtain information about the kinematics of jet components. Because of the cosmological time dilation, extensive VLBI monitoring data are essential to reveal changes in the jet structure of high-redshift AGN. In the case of J0017+8135, the data can be described with a simple kinematic model of jet precession with a 12-year periodicity in the observer’s frame. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Review

Jump to: Editorial, Research

Open AccessReview A Search for Blazar-Like Radio-Loud Narrow-Line Seyfert 1 Galaxies
Galaxies 2017, 5(1), 20; doi:10.3390/galaxies5010020
Received: 2 September 2016 / Revised: 8 February 2017 / Accepted: 23 February 2017 / Published: 21 March 2017
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Abstract
We report the results of an observational program to investigate the gamma-ray and optical variability properties of the vRL NLSY1 galaxies listed in the Yuan et al. sample. We have identified 17 members of the Yuan et al. sample possibly associated with gamma-ray
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We report the results of an observational program to investigate the gamma-ray and optical variability properties of the vRL NLSY1 galaxies listed in the Yuan et al. sample. We have identified 17 members of the Yuan et al. sample possibly associated with gamma-ray sources based on a combination of their optical polarization and optical variability and their gamma-ray properties. Eight have previously been associated with gamma-ray sources. We find nine additional members that we predict are excellent candidates to be associated with gamma-ray sources in the future. All 17 sources have many properties in common with flat spectrum radio quasars (FSRQs), suggesting that they may, in fact, constitute a new subclass of FSRQs. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessReview Interstellar Scintillation and Scattering of Micro-arc-second AGN
Galaxies 2016, 4(4), 62; doi:10.3390/galaxies4040062
Received: 5 September 2016 / Revised: 30 October 2016 / Accepted: 3 November 2016 / Published: 14 November 2016
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Abstract
The discovery of the first quasar 3C 273 led directly to the discovery of their variability at optical and radio wavelengths. We review the radio variability observations, in particular the variability found at frequencies below 1 GHz, as well as those exhibiting intra-day
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The discovery of the first quasar 3C 273 led directly to the discovery of their variability at optical and radio wavelengths. We review the radio variability observations, in particular the variability found at frequencies below 1 GHz, as well as those exhibiting intra-day variability (IDV) at cm wavelengths. Observations have shown that IDV arises principally from scintillation caused by scattering in the ionized interstellar medium of our Galaxy. The sensitivity of interstellar scintillation towards source angular sizes has provided a powerful tool for studying the most compact components of radio-loud AGN at microarcsecond and milliarcsecond scale resolution. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessReview Consequences of Proton Acceleration in Blazar Jets
Galaxies 2016, 4(4), 59; doi:10.3390/galaxies4040059
Received: 5 September 2016 / Revised: 15 October 2016 / Accepted: 18 October 2016 / Published: 2 November 2016
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Abstract
Hadronic models of blazar emission constitute an interesting alternative to the more popular leptonic ones. Using the BL Lac object Mrk 421 as a characteristic example, we present two distinct ways of modeling the spectral energy distribution of blazars in the hadronic context,
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Hadronic models of blazar emission constitute an interesting alternative to the more popular leptonic ones. Using the BL Lac object Mrk 421 as a characteristic example, we present two distinct ways of modeling the spectral energy distribution of blazars in the hadronic context, and we discuss the predictions of each variant on the spectral shape, the multi-wavelength variability, the cosmic-ray flux, and the high-energy neutrino emission. Focusing on the latter, we then present an application of the hadronic model to individual BL Lacs that were recently suggested to be the counterparts of some of the IceCube neutrinos. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessReview Multifrequency Studies of Active Galactic Nuclei in the Planck Satellite Era
Galaxies 2016, 4(4), 49; doi:10.3390/galaxies4040049
Received: 5 July 2016 / Revised: 18 October 2016 / Accepted: 19 October 2016 / Published: 21 October 2016
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Abstract
The multi-epoch single-survey Planck satellite data have given a rare glimpse into how the radio spectra of active galactic nuclei (AGN) evolve in time. Using Planck and simultaneous auxiliary radio data ranging from 1 GHz to 857 GHz, spectra for 104 bright northern
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The multi-epoch single-survey Planck satellite data have given a rare glimpse into how the radio spectra of active galactic nuclei (AGN) evolve in time. Using Planck and simultaneous auxiliary radio data ranging from 1 GHz to 857 GHz, spectra for 104 bright northern extragalactic radio sources (most of them AGN) have been assembled; in these, the various stages of flare development can be identified. The results are compared with theoretical models describing relativistic jets. Evidence for particularly flat high-frequency radio spectra is found, indicating a harder accelerated electron energy spectrum than usually assumed. A set of sources also shows signs of intrinsic cold dust. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
Open AccessFeature PaperReview Variability of Blazars and Blazar Models over 38 Years
Galaxies 2016, 4(4), 37; doi:10.3390/galaxies4040037
Received: 18 July 2016 / Accepted: 14 September 2016 / Published: 27 September 2016
Cited by 1 | PDF Full-text (1457 KB) | HTML Full-text | XML Full-text
Abstract
Since 1978, when BL Lac objects and violently variable quasars were married to become “blazars”, physical interpretations of the phenomenon have evolved. Remarkably, though, the general picture of relativistic jets beaming their radiation in our direction, proposed that year by Blandford and Rees,
[...] Read more.
Since 1978, when BL Lac objects and violently variable quasars were married to become “blazars”, physical interpretations of the phenomenon have evolved. Remarkably, though, the general picture of relativistic jets beaming their radiation in our direction, proposed that year by Blandford and Rees, remains intact. The main stress on theoretical models has come from observations that reveal ever more extreme variability requiring rampant particle acceleration on time-scales of minutes, often parsecs away from the central black hole. Here the author reviews many of the observations and theoretical ideas that have shaped his studies of blazars over about 40 years. This leads to his preferred scenario that blazar jets contain a helical magnetic field close to the black hole, turbulent plasma on parsecs scales, and both standing and moving shock waves. Particle acceleration can then occur in multiple stages involving the second-order Fermi process, magnetic reconnections, and modest jumps in energy at shock fronts. The most extreme variability, as well as brightness temperatures ~100 times the inverse Compton limit, probably require occasional exceptionally high bulk Lorentz factors. These can result, for example, from supersonic, relativistic turbulence, or ultra-relativistic flows propelled from sites of magnetic reconnection. Future efforts in these and other areas can determine whether these potential solutions are valid. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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