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Galaxies, Volume 5, Issue 1 (March 2017)

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Editorial

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Open AccessEditorial Acknowledgement to Reviewers of Galaxies in 2016
Galaxies 2017, 5(1), 5; doi:10.3390/galaxies5010005
Received: 10 January 2017 / Revised: 10 January 2017 / Accepted: 10 January 2017 / Published: 10 January 2017
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Abstract The editors of Galaxies would like to express their sincere gratitude to the following reviewers for assessing manuscripts in 2016.[...] Full article

Research

Jump to: Editorial, Review

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 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 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 Exploring the Behaviour of Long Gamma-Ray Bursts with Intrinsic Afterglow Correlations: log L200s−α>200s
Galaxies 2017, 5(1), 4; doi:10.3390/galaxies5010004
Received: 31 August 2016 / Revised: 10 December 2016 / Accepted: 22 December 2016 / Published: 10 January 2017
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Abstract
In these proceedings, we summarise the exploration so far of the relationship between the afterglow luminosity (measured at rest frame 200s; logL200s) and average afterglow decay rate (measured from rest frame 200s onwards, α>200s) of
[...] Read more.
In these proceedings, we summarise the exploration so far of the relationship between the afterglow luminosity (measured at rest frame 200s; log L 200 s ) and average afterglow decay rate (measured from rest frame 200s onwards, α > 200 s ) of long duration Gamma-ray Bursts (GRBs), first reported in the optical/UV light curves of GRB afterglows. We show that this correlation is also present in the X-ray afterglows of GRBs as observed by Swift-XRT. We explore how the parameters of the correlation observed in both the X-ray and optical/UV light curves relate to each other and the prompt emission phase and whether these correlations are consistent with predictions of the standard afterglow model. We find that the observed correlations are consistent with a common underlying physical mechanism producing GRBs and their afterglows regardless of the detailed temporal behaviour. However, a basic afterglow model has difficulty explaining all the observed correlations. This leads us to briefly discuss alternative more complex models. Full article
(This article belongs to the Special Issue Gamma-Ray Bursts: Recent Theoretical Models and Observations)
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Open AccessArticle Challenging the Forward Shock Model with the 80 Ms Follow up of the X-ray Afterglow of Gamma-Ray Burst 130427A
Galaxies 2017, 5(1), 6; doi:10.3390/galaxies5010006
Received: 31 August 2016 / Revised: 13 December 2016 / Accepted: 26 December 2016 / Published: 16 January 2017
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Abstract
GRB 130427A was the most luminous gamma-ray burst detected in the last 30 years. With an isotropic energy output of 8.5×1053 erg and redshift of 0.34, it combined very high energetics with a relative proximity to Earth in an unprecedented
[...] Read more.
GRB 130427A was the most luminous gamma-ray burst detected in the last 30 years. With an isotropic energy output of 8.5 × 10 53 erg and redshift of 0.34, it combined very high energetics with a relative proximity to Earth in an unprecedented way. Sensitive X-ray observatories such as XMM-Newton and Chandra have detected the afterglow of this event for a record-breaking baseline longer than 80 million seconds. The light curve displays a simple power-law over more than three decades in time. In this presentation, we explore the consequences of this result for a few models put forward so far to interpret GRB 130427A, and more in general the implication of this outcome in the context of the standard forward shock model. Full article
(This article belongs to the Special Issue Gamma-Ray Bursts: Recent Theoretical Models and Observations)
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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 Area Products for H± in AdS Space
Galaxies 2017, 5(1), 10; doi:10.3390/galaxies5010010
Received: 12 November 2016 / Revised: 15 January 2017 / Accepted: 16 January 2017 / Published: 3 February 2017
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Abstract
We derive the thermodynamic products, in particular the area (or entropy) products of H± for a wide variety of black holes (BHs) in anti-de Sitter (AdS) space. We show by explicit and exact calculations that, for this class of BHs, more complicated
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We derive the thermodynamic products, in particular the area (or entropy) products of H ± for a wide variety of black holes (BHs) in anti-de Sitter (AdS) space. We show by explicit and exact calculations that, for this class of BHs, more complicated functions of the event horizon area and Cauchy horizon area are indeed mass-independent. This mass-independent results indicate that they could turn out to be a “universal” quantity provided that they depend only on the quantized angular momentum, quantized charges, and cosmological constant, etc. Furthermore, these area (or entropy) product relations for several classes of BHs in AdS space gives us strong indication to understanding the nature of non-extremal BH entropy (both inner and outer) at the microscopic level. Moreover, we compute the famous Cosmic Censorship Inequality (which requires Cosmic-Censorship hypothesis) for these classes of BHs in AdS space. Local thermodynamic stability has been discussed for these BHs and under certain conditions, these classes of BHs displayed second order phase transition. The super-entropic BH does not provide any kind of second order phase transition. Full article
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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 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 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 Black Hole Accretion in Gamma Ray Bursts
Galaxies 2017, 5(1), 15; doi:10.3390/galaxies5010015
Received: 31 August 2016 / Revised: 6 December 2016 / Accepted: 23 January 2017 / Published: 13 February 2017
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Abstract
We study the structure and evolution of the hyperaccreting disks and outflows in the gamma ray bursts central engines. The torus around a stellar mass black hole is composed of free nucleons, Helium, electron-positron pairs, and is cooled by neutrino emission. Accretion of
[...] Read more.
We study the structure and evolution of the hyperaccreting disks and outflows in the gamma ray bursts central engines. The torus around a stellar mass black hole is composed of free nucleons, Helium, electron-positron pairs, and is cooled by neutrino emission. Accretion of matter powers the relativistic jets, responsible for the gamma ray prompt emission. The significant number density of neutrons in the disk and outflowing material will cause subsequent formation of heavier nuclei. We study the process of nucleosynthesis and its possible observational consequences. We also apply our scenario to the recent observation of the gravitational wave signal, detected on 14 September 2015 by the two Advanced LIGO detectors, and related to an inspiral and merger of a binary black hole system. A gamma ray burst that could possibly be related with the GW150914 event was observed by the Fermi satellite. It had a duration of about 1 s and appeared about 0.4 s after the gravitational-wave signal. We propose that a collapsing massive star and a black hole in a close binary could lead to the event. The gamma ray burst was powered by a weak neutrino flux produced in the star remnant’s matter. Low spin and kick velocity of the merged black hole are reproduced in our simulations. Coincident gravitational-wave emission originates from the merger of the collapsed core and the companion black hole. Full article
(This article belongs to the Special Issue Gamma-Ray Bursts: Recent Theoretical Models and Observations)
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Open AccessArticle Extended Radio Emission in the Perhipheral Regions of the Shapley Concentration Core
Galaxies 2017, 5(1), 16; doi:10.3390/galaxies5010016
Received: 3 November 2016 / Revised: 3 February 2017 / Accepted: 3 February 2017 / Published: 14 February 2017
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Abstract
The Shapley Concentration (SC) is a galaxy supercluster (few tens of degrees) in the Local Universe (<z>∼0.048) which is currently undergoing cluster mergers and group accretion. It is a diversified environment, with cluster complexes in advanced evolutionary stage, groups of clusters in the
[...] Read more.
The Shapley Concentration (SC) is a galaxy supercluster (few tens of degrees) in the Local Universe (<z>∼0.048) which is currently undergoing cluster mergers and group accretion. It is a diversified environment, with cluster complexes in advanced evolutionary stage, groups of clusters in the very early stages of merger, fairly massive clusters with ongoing accretion activity, and smaller groups located in filaments. These features make the SC an ideal place to observe the signatures of the formation of large-scale structures in the Universe. As a matter of fact, the SC has been observed over a broad range of frequencies with the most important observatories, allowing for a unique multiband study. In this paper, we will present new results from an ongoing study of the Shapley Concentration Core, which is being carried out with the Giant Metrewave Radio Telescope (GMRT). Our work confirms the role played by radio observations in disentangling the details of the accretion and merging processes, and delivers a wealth of information in regions well outside the cluster cores. In particular, we will report on the discovery of a relic in the region between the two clusters A 3558 and A 3562, and of the radio properties of the brightest galaxy in the peripheral cluster A 3556. Full article
(This article belongs to the Special Issue Exploring the Outskirts of Galaxy Clusters)
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Open AccessArticle Small Scale Problems of the ΛCDM Model: A Short Review
Galaxies 2017, 5(1), 17; doi:10.3390/galaxies5010017
Received: 30 May 2016 / Revised: 26 November 2016 / Accepted: 9 December 2016 / Published: 20 February 2017
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Abstract
The ΛCDM model, or concordance cosmology, as it is often called, is a paradigm at its maturity. It is clearly able to describe the universe at large scale, even if some issues remain open, such as the cosmological constant problem, the small-scale problems
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The ΛCDM model, or concordance cosmology, as it is often called, is a paradigm at its maturity. It is clearly able to describe the universe at large scale, even if some issues remain open, such as the cosmological constant problem, the small-scale problems in galaxy formation, or the unexplained anomalies in the CMB. ΛCDM clearly shows difficulty at small scales, which could be related to our scant understanding, from the nature of dark matter to that of gravity; or to the role of baryon physics, which is not well understood and implemented in simulation codes or in semi-analytic models. At this stage, it is of fundamental importance to understand whether the problems encountered by the ΛDCM model are a sign of its limits or a sign of our failures in getting the finer details right. In the present paper, we will review the small-scale problems of the ΛCDM model, and we will discuss the proposed solutions and to what extent they are able to give us a theory accurately describing the phenomena in the complete range of scale of the observed universe. Full article
(This article belongs to the Special Issue Dark Matter: Large versus Small Scale Structures)
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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 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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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
[...] Read more.
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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