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Galaxies, Volume 4, Issue 4 (December 2016)

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Open AccessArticle
Conical Stream of the Two-Sided Jets in NGC 4261 over the Range of 103–109 Schwarzschild Radii
Received: 6 September 2016 / Revised: 23 November 2016 / Accepted: 14 December 2016 / Published: 20 December 2016
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Abstract
We report the jet width profile of of the nearby (30Mpc) AGN NGC 4261 for both the approaching jet and the counter jet at radial distances ranging from 103109 Schwarzschild radius (RS [...] Read more.
We report the jet width profile of of the nearby ( 30 Mpc ) AGN NGC 4261 for both the approaching jet and the counter jet at radial distances ranging from 10 3 10 9 Schwarzschild radius ( R S ) from the central engine. Our Very Large Array (VLA) and Very Long Baseline Array (VLBA) observations reveal that the jets maintain a conical structure on both sides over the range 10 3 10 9 R S without any structural transition (i.e., parabolic to conical) like in the approaching jet in M87. Thus, NGC 4261 will provide a unique opportunity to examine the conical jet hypothesis in blazars, while it may require some additional consideration on the acceleration and collimation process in AGN jets. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessFeature PaperArticle
Using the Outskirts of Galaxy Clusters to Determine Their Mass Accretion Rate
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 [...] Read more.
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
On Effective Degrees of Freedom in the Early Universe
Received: 20 October 2016 / Revised: 8 December 2016 / Accepted: 8 December 2016 / Published: 17 December 2016
Cited by 21 | Viewed by 2216 | 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 [...] Read more.
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
Asymmetric Star Formation Efficiency Due to Ram Pressure Stripping
Received: 6 September 2016 / Revised: 25 November 2016 / Accepted: 5 December 2016 / Published: 13 December 2016
Cited by 2 | Viewed by 1167 | PDF Full-text (428 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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
Nonparametric Reconstruction of the Om Diagnostic to Test ΛCDM
Received: 13 October 2016 / Revised: 2 December 2016 / Accepted: 3 December 2016 / Published: 10 December 2016
Cited by 5 | Viewed by 1109 | PDF Full-text (1075 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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
Emission Knots and Polarization Swings of Swinging Jets
Received: 26 September 2016 / Revised: 21 November 2016 / Accepted: 23 November 2016 / Published: 7 December 2016
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Abstract
Knots (emission features in jets of active galactic nuclei) often show non-ballistic dynamics and variable emission/polarization properties. We model these features as emission pattern propagating in a jet that carries a helical magnetic field and is launched along a changing direction. The model [...] Read more.
Knots (emission features in jets of active galactic nuclei) often show non-ballistic dynamics and variable emission/polarization properties. We model these features as emission pattern propagating in a jet that carries a helical magnetic field and is launched along a changing direction. The model can reproduce a wide range of phenomena observed in the motion of knots: non-ballistic motion (both smooth and occasional sudden change of direction, and/or oscillatory behavior), variable brightness, and confinement of knots’ motion within an overlaying envelope. The model also reproduces smooth large polarization angle swings, and at the same time allows for the seemingly random behavior of synchrotron fluxes, polarization fraction, and occasional π / 2 polarization jumps. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessFeature PaperArticle
Tidal Disruption of Milky Way Satellites with Shallow Dark Matter Density Profiles
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 [...] Read more.
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
Galaxy Cluster Outskirts from the Thermal SZ and Non-Thermal Synchrotron Link
Received: 11 October 2016 / Revised: 21 November 2016 / Accepted: 23 November 2016 / Published: 29 November 2016
Cited by 1 | Viewed by 1578 | PDF Full-text (4598 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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 AccessArticle
Multi-Frequency Monitoring of the Flat Spectrum Radio Quasar PKS 1222+216 in 2008–2015
Received: 15 July 2016 / Revised: 20 November 2016 / Accepted: 21 November 2016 / Published: 28 November 2016
Cited by 1 | Viewed by 1274 | PDF Full-text (1446 KB) | HTML Full-text | XML Full-text
Abstract
We analyze the broadband activity of the flat spectrum radio quasar PKS 1222+216 from 2008 to 2015 using multi-frequency monitoring which involves γ-ray data from the Fermi Large Area Telescope, total intensity and linear polarization observations from different optical telescopes in R [...] Read more.
We analyze the broadband activity of the flat spectrum radio quasar PKS 1222+216 from 2008 to 2015 using multi-frequency monitoring which involves γ-ray data from the Fermi Large Area Telescope, total intensity and linear polarization observations from different optical telescopes in R band, and imaging of the inner jet structure with the Very Long Baseline Array (VLBA) at 43 GHz. During the observations, the source showed several dramatic flares at γ rays and optical bands, with the rising branch of a γ-ray flare accompanied by a rapid rotation of the polarization position angle (EVPA), a fast increase of the degree of polarization in the optical band, brightening of the VLBI core, and appearance of a new superluminal component in the parsec-scale jet. The rapid variability of the optical linear polarization may be explained by a strong turbulence in the jet plasma. We find a correlation between the γ rays, optical R band, and 43 GHz variability on a long-term scale (months and years), and a good general alignment between EVPAs in R band and at 43 GHz, while the correlation between short-term variations (days and weeks) is weaker. Synchronous activity across the bands supports the idea that the emission regions responsible for the γ-ray and optical flares are co-spatial and located in the vicinity of the mm-wave core of the parsec-scale jet. However, these connections do not completely explain the challenging behaviour of PKS 1222+216, since there are some γ-ray flares which are not accompanied by jet events, and vice versa. We need a continuation of multi-frequency monitoring along with high resolution imaging of the parsec-scale jet to understand in detail the origin of high energy emission in blazars. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle
Studying the Effect of Shock Obliquity on the γ-ray and Diffuse Radio Emission in Galaxy Clusters
Received: 30 August 2016 / Revised: 16 November 2016 / Accepted: 19 November 2016 / Published: 25 November 2016
Cited by 6 | Viewed by 1249 | PDF Full-text (7013 KB) | HTML Full-text | XML Full-text
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, [...] Read more.
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
Direct Imaging of a Toroidal Magnetic Field in the Inner Jet of NRAO 150
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, [...] Read more.
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
Multiwavelength Picture of the Blazar S5 0716+714 during Its Brightest Outburst
Received: 15 July 2016 / Revised: 11 November 2016 / Accepted: 17 November 2016 / Published: 24 November 2016
Cited by 1 | Viewed by 1341 | PDF Full-text (8740 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
S5 0716+714 is a well known BL Lac object, and one of the brightest and most active blazars. The discovery in the Very High Energy band (VHE, E > 100 GeV) by MAGIC happened in 2008. In January 2015, the source went through the brightest optical state ever observed, triggering MAGIC follow-up and a VHE detection with ∼ 13 σ significance (ATel 6999 ). Rich multiwavelength coverage of the flare allowed us to construct the broad-band spectral energy distribution of S5 0716+714 during its brightest outburst. In this work, we will present the preliminary analysis of MAGIC and Fermi-LAT data of the flaring activity in January and February 2015 for the HE (0.1 < HE < 300 GeV) and VHE band, together with radio (Metsähovi, OVRO, VLBA, Effelsberg), sub-millimeter (SMA), optical (Tuorla, Perkins, Steward, AZT-8+ST7, LX-200, Kanata), X-ray and UV (Swift-XRT and UVOT), in the same time-window and discuss the time variability of the multiwavelength light curves during this impressive outburst. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle
JVLA Wideband Polarimetry Observations on a Sample of High Rotation Measure Sources
Received: 17 July 2016 / Revised: 9 November 2016 / Accepted: 12 November 2016 / Published: 24 November 2016
Cited by 1 | Viewed by 1401 | PDF Full-text (980 KB) | HTML Full-text | XML Full-text
Abstract
We present preliminary results of JVLA wideband full polarization observations of a sample of Active Galactic Nuclei (AGN) with very high Rotation Measure (RM) values, a sign of extreme environment. Polarization properties show a complex behaviour such that the polarization angle (PA) and [...] Read more.
We present preliminary results of JVLA wideband full polarization observations of a sample of Active Galactic Nuclei (AGN) with very high Rotation Measure (RM) values, a sign of extreme environment. Polarization properties show a complex behaviour such that the polarization angle (PA) and fractional polarization (fp) change dramatically within the wide band. The measured RM is not constant within the wide band. Its complex behaviour reflects the complexity of the medium with the presence of several Faraday components. The depolarization has been studied by modelling the variations of the Stokes parameters Q and U together with the polarization parameters (PA and fp) with wavelength using combinations of the simplest existing depolarization models. With this JVLA study we could spectrally resolve multiple polarized components of unresolved AGN. These preliminary results reveal the complexity of these objects, but improvements to the depolarization modelling are needed to better understand the polarization structure of these sources. Full article
(This article belongs to the Special Issue Blazars through Sharp Multi-wavelength Eyes)
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Open AccessArticle
Relic—Shock Connection in Abell 115
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 [...] Read more.
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
Multiwavelength Monitoring of the Gamma-Bright Blazar Mkn 421
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 [...] Read more.
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
Recent Progress in Understanding the Large Scale Jets of Powerful Quasars
Received: 7 September 2016 / Revised: 8 November 2016 / Accepted: 10 November 2016 / Published: 23 November 2016
Cited by 4 | Viewed by 1073 | PDF Full-text (1887 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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
Correlation Analysis of Delays between Variations of Gamma-Ray and Optical Light Curves of Blazars
Received: 14 July 2016 / Revised: 11 November 2016 / Accepted: 15 November 2016 / Published: 23 November 2016
Cited by 1 | Viewed by 897 | PDF Full-text (1265 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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
AP Librae: The Extended Jet as the Source of VHE Emission?
Received: 8 July 2016 / Revised: 25 October 2016 / Accepted: 9 November 2016 / Published: 15 November 2016
Cited by 1 | Viewed by 879 | PDF Full-text (334 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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 AccessReview
Interstellar Scintillation and Scattering of Micro-arc-second AGN
Received: 5 September 2016 / Revised: 30 October 2016 / Accepted: 3 November 2016 / Published: 14 November 2016
Cited by 5 | Viewed by 1047 | PDF Full-text (3194 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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 AccessArticle
Investigating the Puzzling Synchrotron Behaviour of Mrk 421
Received: 9 September 2016 / Revised: 6 October 2016 / Accepted: 15 October 2016 / Published: 4 November 2016
Cited by 2 | Viewed by 1223 | PDF Full-text (2059 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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
On the Non-Thermal Energy Content of Cosmic Structures
Received: 29 August 2016 / Revised: 25 October 2016 / Accepted: 26 October 2016 / Published: 2 November 2016
Cited by 2 | Viewed by 1467 | PDF Full-text (1382 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
(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 AccessReview
Consequences of Proton Acceleration in Blazar Jets
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, [...] Read more.
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 AccessArticle
Physical Conditions and Variability Processes in AGN Jets through Multi-Frequency Linear and Circular Radio Polarization Monitoring
Received: 5 September 2016 / Revised: 21 October 2016 / Accepted: 25 October 2016 / Published: 2 November 2016
Cited by 5 | Viewed by 1539 | PDF Full-text (465 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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
The Truncated Lognormal Distribution as a Luminosity Function for SWIFT-BAT Gamma-Ray Bursts
Received: 20 May 2016 / Revised: 12 September 2016 / Accepted: 18 October 2016 / Published: 1 November 2016
Cited by 3 | Viewed by 1519 | PDF Full-text (993 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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
Investigating the Innermost Jet Structures of Blazar S5 0716+714 Using Uniquely Dense Intra-day Photo-polarimetric Observations
Received: 7 September 2016 / Revised: 11 October 2016 / Accepted: 17 October 2016 / Published: 27 October 2016
Cited by 1 | Viewed by 1068 | PDF Full-text (976 KB) | HTML Full-text | XML Full-text
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; [...] Read more.
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
The RadioAstron Dedicated DiFX Distribution
Received: 18 July 2016 / Revised: 16 October 2016 / Accepted: 17 October 2016 / Published: 27 October 2016
Cited by 3 | Viewed by 1290 | PDF Full-text (262 KB) | HTML Full-text | XML Full-text
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, [...] Read more.
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
Observing—and Imaging—Active Galactic Nuclei with the Event Horizon Telescope
Received: 11 July 2016 / Revised: 13 October 2016 / Accepted: 15 October 2016 / Published: 27 October 2016
Cited by 33 | Viewed by 5054 | PDF Full-text (3054 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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 AccessReview
Approximate Methods for the Generation of Dark Matter Halo Catalogs in the Age of Precision Cosmology
Received: 24 May 2016 / Revised: 6 October 2016 / Accepted: 13 October 2016 / Published: 27 October 2016
Cited by 25 | Viewed by 1431 | PDF Full-text (20060 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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 AccessArticle
Ringo2 Optical Polarimetry of Blazars
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 [...] Read more.
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
Probing the Internal Structure of Magnetized, Relativistic Jets with Numerical Simulations
Received: 19 July 2016 / Revised: 20 October 2016 / Accepted: 21 October 2016 / Published: 26 October 2016
Cited by 2 | Viewed by 1005 | PDF Full-text (2245 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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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