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Galaxies, Volume 4, Issue 4 (December 2016)

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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 On the Non-Thermal Energy Content of Cosmic Structures
Galaxies 2016, 4(4), 60; doi:10.3390/galaxies4040060
Received: 29 August 2016 / Revised: 25 October 2016 / Accepted: 26 October 2016 / Published: 2 November 2016
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Abstract
(1) Background: the budget of non-thermal energy in galaxy clusters is not well constrained, owing to the observational and theoretical difficulties in studying these diluted plasmas on large scales; (2) Method: we use recent cosmological simulations with complex physics in order to connect
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(1) Background: the budget of non-thermal energy in galaxy clusters is not well constrained, owing to the observational and theoretical difficulties in studying these diluted plasmas on large scales; (2) Method: we use recent cosmological simulations with complex physics in order to connect the emergence of non-thermal energy to the underlying evolution of gas and dark matter; (3) Results: the impact of non-thermal energy (e.g., cosmic rays, magnetic fields and turbulent motions) is found to increase in the outer region of galaxy clusters. Within numerical and theoretical uncertainties, turbulent motions dominate the budget of non-thermal energy in most of the cosmic volume; (4) Conclusion: assessing the distribution non-thermal energy in galaxy clusters is crucial to perform high-precision cosmology in the future. Constraining the level of non-thermal energy in cluster outskirts will improve our understanding of the acceleration of relativistic particles and of the origin of extragalactic magnetic fields. Full article
(This article belongs to the Special Issue Exploring the Outskirts of Galaxy Clusters)
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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 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 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
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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 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 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 Studying the Effect of Shock Obliquity on the γ-ray and Diffuse Radio Emission in Galaxy Clusters
Galaxies 2016, 4(4), 71; doi:10.3390/galaxies4040071
Received: 30 August 2016 / Revised: 16 November 2016 / Accepted: 19 November 2016 / Published: 25 November 2016
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Abstract
Observations of diffuse radio emission in galaxy clusters indicate that cosmic-ray electrons are accelerated on Mpc scales. However, protons appear to be accelerated less efficiently since their associated hadronic γ-ray emission has not yet been detected. Inspired by recent particle-in-cell simulations,
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Observations of diffuse radio emission in galaxy clusters indicate that cosmic-ray electrons are accelerated on Mpc scales. However, protons appear to be accelerated less efficiently since their associated hadronic γ-ray emission has not yet been detected. Inspired by recent particle-in-cell simulations, we study the cosmic-ray production and its signatures under the hypothesis that the efficiency of shock acceleration depends on the Mach number and on the shock obliquity. For this purpose, we combine ENZO cosmological magneto-hydrodynamical simulations with a Lagrangian tracer code to follow the properties of the cosmic rays. Our simulations suggest that the distribution of obliquities in galaxy clusters is random to first order. Quasi-perpendicular shocks are able to accelerate cosmic-ray electrons to the energies needed to produce observable radio emission. However, the γ-ray emission is lowered by a factor of a few, ∼3 , if cosmic-ray protons are only accelerated by quasi-parallel shocks, reducing (yet not entirely solving) the tension with the non-detection of hadronic γ-ray emission by the Fermi-satellite. Full article
(This article belongs to the Special Issue Exploring the Outskirts of Galaxy Clusters)
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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 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 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 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
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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 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 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
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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 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 Galaxy Cluster Outskirts from the Thermal SZ and Non-Thermal Synchrotron Link
Galaxies 2016, 4(4), 73; doi:10.3390/galaxies4040073
Received: 11 October 2016 / Revised: 21 November 2016 / Accepted: 23 November 2016 / Published: 29 November 2016
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Abstract
Galaxy cluster merger shocks are the main agent for the thermalization of the intracluster medium and the energization of cosmic ray particles in it. Shock propagation changes the state of the tenuous intracluster plasma, and the corresponding signal variations are measurable with the
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Galaxy cluster merger shocks are the main agent for the thermalization of the intracluster medium and the energization of cosmic ray particles in it. Shock propagation changes the state of the tenuous intracluster plasma, and the corresponding signal variations are measurable with the current generation of X-ray and Sunyaev–Zel’dovich (SZ) effect instruments. Additionally, non-thermal electrons (re-)energized by the shocks sometimes give rise to extended and luminous synchrotron sources known as radio relics, which are prominent indicators of shocks propagating roughly in the plane of the sky. In this short review, we discuss how the joint modeling of the non-thermal and thermal signal variations across radio relic shock fronts is helping to advance our knowledge of the gas thermodynamical properties and magnetic field strengths in the cluster outskirts. We describe the first use of the SZ effect to measure the Mach numbers of relic shocks, for both the nearest (Coma) and the farthest (El Gordo) clusters with known radio relics. Full article
(This article belongs to the Special Issue Exploring the Outskirts of Galaxy Clusters)
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Open AccessFeature PaperArticle Tidal Disruption of Milky Way Satellites with Shallow Dark Matter Density Profiles
Galaxies 2016, 4(4), 74; doi:10.3390/galaxies4040074
Received: 28 September 2016 / Revised: 17 November 2016 / Accepted: 22 November 2016 / Published: 30 November 2016
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Abstract
Dwarf galaxies of the Local Group provide unique possibilities to test current theories of structure formation. Their number and properties have put the broadly accepted cold dark matter model into question, posing a few problems. These problems now seem close to resolution due
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Dwarf galaxies of the Local Group provide unique possibilities to test current theories of structure formation. Their number and properties have put the broadly accepted cold dark matter model into question, posing a few problems. These problems now seem close to resolution due to the improved treatment of baryonic processes in dwarf galaxy simulations which now predict cored rather than cuspy dark matter profiles in isolated dwarfs with important consequences for their subsequent environmental evolution. Using N-body simulations, we study the evolution of a disky dwarf galaxy with such a shallow dark matter profile on a typical orbit around the Milky Way. The dwarf survives the first pericenter passage but is disrupted after the second due to tidal forces from the host. We discuss the evolution of the dwarf’s properties in time prior to and at the time of disruption. We demonstrate that the dissolution occurs on a rather short timescale as the dwarf expands from a spheroid into a stream with non-zero mean radial velocity. We point out that the properties of the dwarf at the time of disruption may be difficult to distinguish from bound configurations, such as tidally induced bars, both in terms of surface density and line-of-sight kinematics. Full article
(This article belongs to the Special Issue Dark Matter: Large versus Small Scale Structures)
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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 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 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 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 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 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 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 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
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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 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 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 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
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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 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 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 Nonparametric Reconstruction of the Om Diagnostic to Test ΛCDM
Galaxies 2016, 4(4), 76; doi:10.3390/galaxies4040076
Received: 13 October 2016 / Revised: 2 December 2016 / Accepted: 3 December 2016 / Published: 10 December 2016
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Abstract
In this work, we consider an Om diagnostic using a non-parametric reconstruction by employing the Loess–Simex factory. This procedure allows us to perform a model-independent comparison for w(z) with the astrophysical data. The concordance model can be tested with
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In this work, we consider an Om diagnostic using a non-parametric reconstruction by employing the Loess–Simex factory. This procedure allows us to perform a model-independent comparison for w(z) with the astrophysical data. The concordance model can be tested with the advantage that our approach represents an alternative and efficient way to relax the use of priors and find a possible w that reliably describes the data with no previous knowledge of a cosmological model. Full article
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Open AccessArticle Asymmetric Star Formation Efficiency Due to Ram Pressure Stripping
Galaxies 2016, 4(4), 77; doi:10.3390/galaxies4040077
Received: 6 September 2016 / Revised: 25 November 2016 / Accepted: 5 December 2016 / Published: 13 December 2016
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Abstract
Previous works have shown that a dense cluster environment affects satellite galaxy properties and accelerates or truncates their evolutionary processes. In this work, we use the EAGLE simulation to study this effect, dissecting the galaxies in two halves: the one that is falling
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Previous works have shown that a dense cluster environment affects satellite galaxy properties and accelerates or truncates their evolutionary processes. In this work, we use the EAGLE simulation to study this effect, dissecting the galaxies in two halves: the one that is falling directly to the cluster (leading half) and the one behind (trailing half). Considering all galaxies within the virial radius of the most massive groups and clusters of the simulation ( M h a l o > 10 13 . 8 [ M ] ), we find that on average the leading half presents an enhancement of the star formation rate with respect to the trailing half. We conclude that galaxies falling into the intra-cluster medium experience a boost in star-formation in their leading half due to ram pressure. Sparse observations of jellyfish galaxies have revealed visually the enhancement of the star formation in the leading half. In order to confirm this effect statistically using observations, different cases must be investigated using the simulation as a test dataset. Full article
(This article belongs to the Special Issue Exploring the Outskirts of Galaxy Clusters)
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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 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
Cited by 2 | PDF Full-text (1037 KB) | HTML Full-text | XML Full-text
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 The Truncated Lognormal Distribution as a Luminosity Function for SWIFT-BAT Gamma-Ray Bursts
Galaxies 2016, 4(4), 57; doi:10.3390/galaxies4040057
Received: 20 May 2016 / Revised: 12 September 2016 / Accepted: 18 October 2016 / Published: 1 November 2016
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Abstract
The determination of the luminosity function (LF) in Gamma ray bursts (GRBs) depends on the adopted cosmology, each one characterized by its corresponding luminosity distance. Here, we analyze three cosmologies: the standard cosmology, the plasma cosmology and the pseudo-Euclidean universe. The LF of
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The determination of the luminosity function (LF) in Gamma ray bursts (GRBs) depends on the adopted cosmology, each one characterized by its corresponding luminosity distance. Here, we analyze three cosmologies: the standard cosmology, the plasma cosmology and the pseudo-Euclidean universe. The LF of the GRBs is firstly modeled by the lognormal distribution and the four broken power law and, secondly, by a truncated lognormal distribution. The truncated lognormal distribution fits acceptably the range in luminosity of GRBs as a function of the redshift. Full article
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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 Relic—Shock Connection in Abell 115
Galaxies 2016, 4(4), 68; doi:10.3390/galaxies4040068
Received: 9 September 2016 / Revised: 12 November 2016 / Accepted: 15 November 2016 / Published: 23 November 2016
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Abstract
Giant radio relics are arc-shaped diffuse sources with Mpc-scale found in the peripheries of some dynamically disturbed galaxy clusters. According to the leading scenario of relic formation, shock waves occurring in merger events amplify the local magnetic field and (re)accelerate particles. However, Mach
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Giant radio relics are arc-shaped diffuse sources with Mpc-scale found in the peripheries of some dynamically disturbed galaxy clusters. According to the leading scenario of relic formation, shock waves occurring in merger events amplify the local magnetic field and (re)accelerate particles. However, Mach numbers associated with merger shocks are typically low, and hence inefficient at accelerating particles from the thermal pool. We analyzed a deep Chandra observation (334 ks) to study the relic region in the cluster Abell 115. Temperature and surface brightness profiles taken across the relic both show a clear discontinuity, which is consistent with a shock. This result supports the relic–shock connection and represents a test case to study the origin of radio relics. In this particular case, we suggest that a re-acceleration scenario is more suitable. The relic morphology and position are consistent with a shock produced in an off-axis merger between clusters with different masses. Full article
(This article belongs to the Special Issue Exploring the Outskirts of Galaxy Clusters)
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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 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 On Effective Degrees of Freedom in the Early Universe
Galaxies 2016, 4(4), 78; doi:10.3390/galaxies4040078
Received: 20 October 2016 / Revised: 8 December 2016 / Accepted: 8 December 2016 / Published: 17 December 2016
Cited by 1 | PDF Full-text (1845 KB) | HTML Full-text | XML Full-text
Abstract
We explore the effective degrees of freedom in the early Universe, from before the electroweak scale at a few femtoseconds after the Big Bang until the last positrons disappeared a few minutes later. We look at the established concepts of effective degrees of
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We explore the effective degrees of freedom in the early Universe, from before the electroweak scale at a few femtoseconds after the Big Bang until the last positrons disappeared a few minutes later. We look at the established concepts of effective degrees of freedom for energy density, pressure, and entropy density, and introduce effective degrees of freedom for number density as well. We discuss what happens with particle species as their temperature cools down from relativistic to semi- and non-relativistic temperatures, and then annihilates completely. This will affect the pressure and the entropy per particle. We also look at the transition from a quark-gluon plasma to a hadron gas. Using a list a known hadrons, we use a “cross-over” temperature of 214 MeV, where the effective degrees of freedom for a quark-gluon plasma equals that of a hadron gas. Full article
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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 AccessFeature PaperArticle Using the Outskirts of Galaxy Clusters to Determine Their Mass Accretion Rate
Galaxies 2016, 4(4), 79; doi:10.3390/galaxies4040079
Received: 27 September 2016 / Revised: 8 December 2016 / Accepted: 9 December 2016 / Published: 20 December 2016
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Abstract
We explore the possibility of using the external regions of galaxy clusters to measure their mass accretion rate (MAR). The main goal is to provide a method to observationally investigate the growth of structures on the nonlinear scales of galaxy clusters. We derive
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We explore the possibility of using the external regions of galaxy clusters to measure their mass accretion rate (MAR). The main goal is to provide a method to observationally investigate the growth of structures on the nonlinear scales of galaxy clusters. We derive the MAR by using the mass profile beyond the splashback radius, evaluating the mass of a spherical shell and the time it takes to fall in. The infall velocity of the shell is extracted from N-body simulations. The average MAR returned by our prescription in the redshift range z = [ 0 , 2 ] is within 20%–40% of the average MAR derived from the merger trees of dark matter haloes in the reference N-body simulations. Our result suggests that the external regions of galaxy clusters can be used to measure the mean MAR of a sample of clusters. Full article
(This article belongs to the Special Issue Exploring the Outskirts of Galaxy Clusters)
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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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Review

Jump to: Research

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 Approximate Methods for the Generation of Dark Matter Halo Catalogs in the Age of Precision Cosmology
Galaxies 2016, 4(4), 53; doi:10.3390/galaxies4040053
Received: 24 May 2016 / Revised: 6 October 2016 / Accepted: 13 October 2016 / Published: 27 October 2016
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Abstract
Precision cosmology has recently triggered new attention on the topic of approximate methods for the clustering of matter on large scales, whose foundations date back to the period from the late 1960s to early 1990s. Indeed, although the prospect of reaching sub-percent accuracy
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Precision cosmology has recently triggered new attention on the topic of approximate methods for the clustering of matter on large scales, whose foundations date back to the period from the late 1960s to early 1990s. Indeed, although the prospect of reaching sub-percent accuracy in the measurement of clustering poses a challenge even to full N-body simulations, an accurate estimation of the covariance matrix of clustering statistics, not to mention the sampling of parameter space, requires usage of a large number (hundreds in the most favourable cases) of simulated (mock) galaxy catalogs. Combination of few N-body simulations with a large number of realizations performed with approximate methods gives the most promising approach to solve these problems with a reasonable amount of resources. In this paper I review this topic, starting from the foundations of the methods, then going through the pioneering efforts of the 1990s, and finally presenting the latest extensions and a few codes that are now being used in present-generation surveys and thoroughly tested to assess their performance in the context of future surveys. Full article
(This article belongs to the Special Issue Dark Matter: Large versus Small Scale Structures)
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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
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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,
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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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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)

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