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Galaxies, Volume 6, Issue 2 (June 2018)

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Cover Story (view full-size image) The generalized theory of gravitation Scalar-Tensor-Vector-Gravity (STVG) (modified gravity (MOG)) [...] Read more.
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Open AccessFeature PaperArticle Sliding along the Eddington Limit—Heavy-Weight Central Stars of Planetary Nebulae
Received: 23 April 2018 / Revised: 14 June 2018 / Accepted: 15 June 2018 / Published: 19 June 2018
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Abstract
Due to thermal pulses, asymptotic giant branch (AGB) stars experience periods of convective mixing that provide ideal conditions for slow neutron-capture nucleosynthesis. These processes are affected by large uncertainties and are still not fully understood. By the lucky coincidence that about a quarter
[...] Read more.
Due to thermal pulses, asymptotic giant branch (AGB) stars experience periods of convective mixing that provide ideal conditions for slow neutron-capture nucleosynthesis. These processes are affected by large uncertainties and are still not fully understood. By the lucky coincidence that about a quarter of all post-AGB stars turn hydrogen-deficient in a final flash of the helium-burning shell, they display nuclear processed material at the surface providing an unique insight to nucleosynthesis and mixing. We present results of non-local thermodynamic equilibrium spectral analyses of the extremely hot, hydrogen-deficient, PG 1159-type central stars of the Skull Nebula NGC 246 and the “Galactic Soccerballs” Abell 43 and NGC 7094. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
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Open AccessFeature PaperArticle A Catalog of Photometric Redshift and the Distribution of Broad Galaxy Morphologies
Received: 4 April 2018 / Revised: 31 May 2018 / Accepted: 8 June 2018 / Published: 11 June 2018
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Abstract
We created a catalog of photometric redshift of ∼3,000,000 SDSS galaxies annotated by their broad morphology. The photometric redshift was optimized by testing and comparing several pattern recognition algorithms and variable selection strategies, and was trained and tested on a subset of the
[...] Read more.
We created a catalog of photometric redshift of ∼3,000,000 SDSS galaxies annotated by their broad morphology. The photometric redshift was optimized by testing and comparing several pattern recognition algorithms and variable selection strategies, and was trained and tested on a subset of the galaxies in the catalog that had spectra. The galaxies in the catalog have i magnitude brighter than 18 and Petrosian radius greater than 5.5. The majority of these objects are not included in previous SDSS photometric redshift catalogs such as the photoz table of SDSS DR12. Analysis of the catalog shows that the number of galaxies in the catalog that are visually spiral increases until redshift of ∼0.085, where it peaks and starts to decrease. It also shows that the number of spiral galaxies compared to elliptical galaxies drops as the redshift increases. Full article
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Open AccessArticle Recent Development of the Atomic Line List
Received: 7 May 2018 / Revised: 1 June 2018 / Accepted: 2 June 2018 / Published: 8 June 2018
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Abstract
The Atomic Line List is an online database of wavelengths and transition probabilities of atomic lines. It is primarily set up as a tool to help identify unknown spectral features. This paper briefly describes the web interface, how the line list is constructed,
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The Atomic Line List is an online database of wavelengths and transition probabilities of atomic lines. It is primarily set up as a tool to help identify unknown spectral features. This paper briefly describes the web interface, how the line list is constructed, and what development is currently being undertaken for the next release. Full article
(This article belongs to the Special Issue Atomic and Molecular Data Needs for Astronomy and Astrophysics)
Open AccessFeature PaperReview Multi-Wavelength Observations and Modeling of Loop I
Received: 24 February 2018 / Revised: 11 May 2018 / Accepted: 4 June 2018 / Published: 7 June 2018
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Abstract
The article aims to overview the origin and current dynamical state of a giant structure on the northern galactic sky—the radio Loop I extending from ≈ 45 to ≈ +45 in longitudes and up to ≈ 80 in
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The article aims to overview the origin and current dynamical state of a giant structure on the northern galactic sky—the radio Loop I extending from ≈ 45 to ≈ + 45 in longitudes and up to ≈ 80 in latitudes over the Galactic center (GC). The main issue addressed here is a description of possible sources of mass and energy able to build up the Loop I and associated structures seen in X-ray, 21 cm, far infrared (FIR), and maintain them on long timescales. This region of the sky is highly crowded, such that contaminations from many projected structures can be tangled, and not always current direct observations look sufficient to disentangle them. At such conditions indirect arguments based on analysis of underlying star formation (SF) rate, morphological features in radio, X-ray and FIR may be important for understanding the origin of Loop I. Simple estimates show that the observed rather weak SF rate is able to create and maintain Loop I, and under certain circumstances can provide the observed east-west asymmetry. However, an explanation of an apparent coexistence of morphologically similar HI and FIR filaments close to Loop I is challenging, indicating that most likely they may belong to the foreground. Recently discovered absorptions in diffuse interstellar bands seem to confirm this picture. Full article
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Open AccessArticle Relativistic Motion of Stars near Rotating Black Holes
Received: 20 April 2018 / Revised: 31 May 2018 / Accepted: 1 June 2018 / Published: 5 June 2018
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Abstract
Formulation of the Lagrangian approach is presented for studying features of motions of stellar bodies with non-zero rest mass in the vicinity of fast-spinning black holes. The structure of Lagrangian is discussed. The general method is applied to description of body motion in
[...] Read more.
Formulation of the Lagrangian approach is presented for studying features of motions of stellar bodies with non-zero rest mass in the vicinity of fast-spinning black holes. The structure of Lagrangian is discussed. The general method is applied to description of body motion in the Kerr model of space–time to transition to the problem of tidal disruption of elastic bodies by a strong gravitational field. Full article
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Open AccessArticle Cartesian Material Space with Active-Passive Densities of Complex Charges and Yin-Yang Compensation of Energy Integrals
Received: 9 April 2018 / Revised: 21 May 2018 / Accepted: 29 May 2018 / Published: 5 June 2018
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Abstract
The non-empty space approach to Maxwell’s equations reveals the analytical solution with the continuous radial density of the extended astroparticle. The principle of equivalence works universally for active/passive masses and for active/passive electric charges. The Maxwell equations for complex continuous charges become four-vector
[...] Read more.
The non-empty space approach to Maxwell’s equations reveals the analytical solution with the continuous radial density of the extended astroparticle. The principle of equivalence works universally for active/passive masses and for active/passive electric charges. The Maxwell equations for complex continuous charges become four-vector identities for locally-compensated flows of active (yang) and passive (yin) energies. The Einstein formula for relativistic mass-energy can be supplemented by the imaginary energy of electricity. Cartesian relativistic physics should have its own non-dual analog of the Einstein equation. Full article
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Open AccessAddendum Addendum: Cohen, M.H. OJ 287 as a Rotating Helix. Galaxies 2017, 5, 12
Received: 27 April 2018 / Revised: 22 May 2018 / Accepted: 22 May 2018 / Published: 28 May 2018
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Abstract
In an earlier paper it was erroneously stated that the inner jet of OJ 287 has a right-hand helical structure. In this addendum we show that the observations underlying this statement are ambiguous, and that they cannot determine the rotation sense. Full article
Open AccessArticle Planets, Planetary Nebulae, and Intermediate Luminosity Optical Transients (ILOTs)
Received: 16 April 2018 / Revised: 24 May 2018 / Accepted: 24 May 2018 / Published: 28 May 2018
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Abstract
I review some aspects related to the influence of planets on the evolution of stars before and beyond the main sequence. Some processes include the tidal destruction of a planet on to a very young main sequence star, on to a low-mass main
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I review some aspects related to the influence of planets on the evolution of stars before and beyond the main sequence. Some processes include the tidal destruction of a planet on to a very young main sequence star, on to a low-mass main sequence star, and on to a brown dwarf. This process releases gravitational energy that might be observed as a faint intermediate luminosity optical transient (ILOT) event. I then summarize the view that some elliptical planetary nebulae are shaped by planets. When the planet interacts with a low-mass, upper asymptotic giant branch (AGB) star, it both enhances the mass-loss rate and shapes the wind to form an elliptical planetary nebula, mainly by spinning up the envelope and by exciting waves in the envelope. If no interaction with a companion, stellar or substellar, takes place beyond the main sequence, the star is termed a Jsolated star, and its mass-loss rates on the giant branches are likely to be much lower than what is traditionally assumed. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
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Open AccessFeature PaperArticle Probing Black Hole Magnetic Fields with QED
Received: 18 March 2018 / Revised: 11 May 2018 / Accepted: 20 May 2018 / Published: 24 May 2018
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Abstract
The effect of vacuum birefringence is one of the first predictions of quantum electrodynamics (QED): the presence of a charged Dirac field makes the vacuum birefringent when threaded by magnetic fields. This effect, extremely weak for terrestrial magnetic fields, becomes important for highly
[...] Read more.
The effect of vacuum birefringence is one of the first predictions of quantum electrodynamics (QED): the presence of a charged Dirac field makes the vacuum birefringent when threaded by magnetic fields. This effect, extremely weak for terrestrial magnetic fields, becomes important for highly magnetized astrophysical objects, such as accreting black holes. In the X-ray regime, the polarization of photons traveling in the magnetosphere of a black hole is not frozen at emission but is changed by the local magnetic field. We show that, for photons traveling along the plane of the disk, where the field is expected to be partially organized, this results in a depolarization of the X-ray radiation. Because the amount of depolarization depends on the strength of the magnetic field, this effect can provide a way to probe the magnetic field in black-hole accretion disks and to study the role of magnetic fields in astrophysical accretion in general. Full article
(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)
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Open AccessFeature PaperReview Large-Scale Features of the Radio Sky and a Model for Loop I
Received: 6 April 2018 / Revised: 14 May 2018 / Accepted: 17 May 2018 / Published: 22 May 2018
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Abstract
The large-scale radio/microwave sky has been mapped over a range of frequencies from tens of MHz to tens of GHz, in intensity and polarization. The emission is primarily synchrotron radiation from cosmic ray electrons spiralling in the Galactic magnetic field, in addition to
[...] Read more.
The large-scale radio/microwave sky has been mapped over a range of frequencies from tens of MHz to tens of GHz, in intensity and polarization. The emission is primarily synchrotron radiation from cosmic ray electrons spiralling in the Galactic magnetic field, in addition to free–free radiation from warm ionized gas. Away from the Galactic plane, the radio sky is dominated by very large (tens of degrees) loops, arcs, spurs and filaments, including the well-known North Polar Spur (NPS), which forms part of Loop I with a diameter of ∼ 120 . In polarization data, such features are often more discernible due to their high polarization fractions suggesting ordered magnetic fields, while the polarization angles suggest fields that are parallel to the filament. The exact nature of these features are poorly understood. We give a brief review of these features, focussing on the NPS/Loop I, whose polarization directions can be explained using a simple expanding shell model, placing the centre of the shell at a distance of ∼100–200 pc. However, there is significant evidence for a larger distance in the range ∼500–1000 pc, while larger distances including the Galactic Centre are unlikely. We also briefly discuss other large-scale curiosities in the radio sky such as the microwave haze and anti-correlation of H α filaments and synchrotron polarized intensity. Full article
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Open AccessFeature PaperArticle Constraints on the Distribution of Gas and Young Stars in the Galactic Centre in the Context of Interpreting Gamma Ray Emission Features
Received: 14 April 2018 / Revised: 7 May 2018 / Accepted: 9 May 2018 / Published: 15 May 2018
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Abstract
Gamma ray observations have found evidence of an extremely energetic outflow emanating from the Galactic Centre, and an ‘excess’ of emission at GeV energies towards the Galactic Centre over that expected from current models. Determining whether the outflow is AGN- or star formation-driven,
[...] Read more.
Gamma ray observations have found evidence of an extremely energetic outflow emanating from the Galactic Centre, and an ‘excess’ of emission at GeV energies towards the Galactic Centre over that expected from current models. Determining whether the outflow is AGN- or star formation-driven, and whether the ‘excess’ is astrophysical in origin or requires new physics (e.g., self-annihilation of dark matter), requires the accurate modelling of the expected energy injection from astrophysical sources and the subsequent interaction with the surrounding environment. We briefly summarise current constraints on the distribution of gas and young stars in the inner few hundred parsecs of the Galaxy that can be included in future 2D and 3D modelling of the astrophysical gamma ray emission. The key points to highlight with respect to predominantly axisymmetric models currently in use are: (i) the distribution of dense gas, young stars and interstellar radiation field is highly asymmetric around the Galactic Centre; (ii) star formation is almost exclusively constrained to a Galactocentric radius of ∼100 pc; and (iii) the star formation rate in this region has been constant at ≲0.1 M yr 1 to within a factor of 2 over the last ∼5 Myr. Full article
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Open AccessArticle Instrumentation and Future Missions in the Upcoming Era of X-ray Polarimetry
Received: 31 January 2018 / Revised: 11 April 2018 / Accepted: 12 April 2018 / Published: 11 May 2018
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Abstract
The maturity of current detectors based on technologies that range from solid state to gases renewed the interest for X-ray polarimetry, raising the enthusiasm of a wide scientific community to improve the performance of polarimeters as well as to produce more detailed theoretical
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The maturity of current detectors based on technologies that range from solid state to gases renewed the interest for X-ray polarimetry, raising the enthusiasm of a wide scientific community to improve the performance of polarimeters as well as to produce more detailed theoretical predictions. We will introduce the basic concepts about measuring the polarization of photons, especially in the X-rays, and we will review the current state of the art of polarimeters in a wide energy range from soft to hard X-rays, from solar flares to distant astrophysical sources. We will introduce relevant examples of polarimeters developed from the recent past up to the panorama of upcoming space missions to show how the recent development of the technology is allowing reopening the observational window of X-ray polarimetry. Full article
(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)
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Open AccessArticle Hydrodynamical Simulations of Recollimation Shocks within Relativistic Astrophysical Jets
Received: 2 March 2018 / Revised: 16 April 2018 / Accepted: 26 April 2018 / Published: 3 May 2018
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Abstract
Astrophysical jets launched from active galactic nuclei can remain tightly collimated over large distances due, in part, to recollimation shocks. Formed within the jets due to their supersonic nature, recollimation shocks are predicted to leave signatures in the observed radio emission due to
[...] Read more.
Astrophysical jets launched from active galactic nuclei can remain tightly collimated over large distances due, in part, to recollimation shocks. Formed within the jets due to their supersonic nature, recollimation shocks are predicted to leave signatures in the observed radio emission due to magnetic flux freezing and the geometric relationship between magnetic fields and the polarization of synchrotron radiation. In the course of this work, we will compare how predictions of emission from recollimation shocks change when the flow is modelled using a hydrodynamical code, as opposed to semi-dynamical and magnetohydrodynamical codes. Jets generally exhibit low levels of polarization, which implies a substantially disordered magnetic field. It is difficult to model such fields using magnetohydrodynamics, hence this work uses hydrodynamical code and a statistical treatment of the magnetic field (c.f. Scheuer and Matthews, 1990). It should then be possible to assess whether certain radio jet phenomena, such as knots and radio-cores, may be modelled as singular or multiple recollimation shocks. To date, the hydrodynamical code has been successfully built and executed on UCLan’s supercomputer cluster, and parallelepiped vector triads have been included to monitor the fluid deformation within the simulation, so that the emergent flux and polarization may be calculated. The parallelepiped advection is currently being verified and some results are discussed. Code for radiative transfer throughout the jet is also being implemented, in order to simulate images for comparison with previous works and observations. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Open AccessArticle A Changing-Look AGN to Be Probed by X-ray Polarimetry
Received: 2 March 2018 / Revised: 12 April 2018 / Accepted: 13 April 2018 / Published: 27 April 2018
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Abstract
Active galactic nuclei (AGN) produce the highest intrinsic luminosities in the Universe from within a compact region. The central engine is thought to be powered by accretion onto a supermassive black hole. A fraction of this huge release of energy influences the evolution
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Active galactic nuclei (AGN) produce the highest intrinsic luminosities in the Universe from within a compact region. The central engine is thought to be powered by accretion onto a supermassive black hole. A fraction of this huge release of energy influences the evolution of the host galaxy, and in particular, star formation. Thus, AGN are key astronomical sources not only because they play an important role in the evolution of the Universe, but also because they constitute a laboratory for extreme physics. However, these objects are under the resolution limit of current telescopes. Polarimetry is a unique technique capable of providing us with information on physical AGN structures. The incoming new era of X-ray polarimetry will give us the opportunity to explore the geometry and physical processes taking place in the innermost regions of the accretion disc. Here we exploit this future powerful tool in the particular case of changing-look AGN, which are key for understanding the complexity of AGN physics. Full article
(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)
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Open AccessFeature PaperReview Exoplanets: Past, Present, and Future
Received: 18 February 2018 / Revised: 27 March 2018 / Accepted: 20 April 2018 / Published: 26 April 2018
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Abstract
Our understanding of extra-solar planet systems is highly driven by advances in observations in the past decade. Thanks to high precision spectrographs, we are able to reveal unseen companions to stars with the radial velocity method. High precision photometry from the space, especially
[...] Read more.
Our understanding of extra-solar planet systems is highly driven by advances in observations in the past decade. Thanks to high precision spectrographs, we are able to reveal unseen companions to stars with the radial velocity method. High precision photometry from the space, especially with the Kepler mission, enables us to detect planets when they transit their stars and dim the stellar light by merely one percent or smaller. Ultra wide-field, high cadence, continuous monitoring of the Galactic bulge from different sites around the southern hemisphere provides us the opportunity to observe microlensing effects caused by planetary systems from the solar neighborhood, all the way to the Milky Way center. The exquisite AO imaging from ground-based large telescopes, coupled with high-contrast coronagraph, captured the photons directly emitted by planets around other stars. In this article, I present a concise review of the extra-solar planet discoveries, discussing the strengths and weaknesses of the major planetary detection methods, providing an overview of our current understanding of planetary formation and evolution given the tremendous observations delivered by various methods, as well as on-going and planned observation endeavors to provide a clear picture of extra-solar planetary systems. Full article
(This article belongs to the Special Issue Synoptic Astronomy)
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Open AccessFeature PaperArticle A Study of Background Conditions for Sphinx—The Satellite-Borne Gamma-Ray Burst Polarimeter
Received: 16 February 2018 / Revised: 18 April 2018 / Accepted: 19 April 2018 / Published: 24 April 2018
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Abstract
SPHiNX is a proposed satellite-borne gamma-ray burst polarimeter operating in the energy range 50–500 keV. The mission aims to probe the fundamental mechanism responsible for gamma-ray burst prompt emission through polarisation measurements. Optimising the signal-to-background ratio for SPHiNX is an important task during
[...] Read more.
SPHiNX is a proposed satellite-borne gamma-ray burst polarimeter operating in the energy range 50–500 keV. The mission aims to probe the fundamental mechanism responsible for gamma-ray burst prompt emission through polarisation measurements. Optimising the signal-to-background ratio for SPHiNX is an important task during the design phase. The Geant4 Monte Carlo toolkit is used in this work. From the simulation, the total background outside the South Atlantic Anomaly (SAA) is about 323 counts/s, which is dominated by the cosmic X-ray background and albedo gamma rays, which contribute ∼60% and ∼35% of the total background, respectively. The background from albedo neutrons and primary and secondary cosmic rays is negligible. The delayed background induced by the SAA-trapped protons is about 190 counts/s when SPHiNX operates in orbit for one year. The resulting total background level of ∼513 counts/s allows the polarisation of ∼50 GRBs with minimum detectable polarisation less than 30% to be determined during the two-year mission lifetime. Full article
(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)
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Open AccessFeature PaperArticle Effelsberg Monitoring of a Sample of RadioAstron Blazars: Analysis of Intra-Day Variability
Received: 31 October 2017 / Revised: 5 April 2018 / Accepted: 14 April 2018 / Published: 18 April 2018
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Abstract
We present the first results of an ongoing intra-day variability (IDV) flux density monitoring program of 107 blazars, which were selected from a sample of RadioAstron space very long baseline interferometry (VLBI) targets. The IDV observations were performed with the Effelsberg 100-m radio
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We present the first results of an ongoing intra-day variability (IDV) flux density monitoring program of 107 blazars, which were selected from a sample of RadioAstron space very long baseline interferometry (VLBI) targets. The IDV observations were performed with the Effelsberg 100-m radio telescope at 4.8 GHz, focusing on the statistical properties of IDV in a relatively large sample of compact active galactic nuclei (AGN). We investigated the dependence of rapid (<3 day) variability on various source properties through a likelihood approach. We found that the IDV amplitude depends on flux density and that fainter sources vary by about a factor of 3 more than their brighter counterparts. We also found a significant difference in the variability amplitude between inverted- and flat-spectrum radio sources, with the former exhibiting stronger variations. γ -ray loud sources were found to vary by up to a factor 4 more than γ -ray quiet ones, with 4 σ significance. However a galactic latitude dependence was barely observed, which suggests that it is predominantly the intrinsic properties (e.g., angular size, core-dominance) of the blazars that determine how they scintillate, rather than the directional dependence in the interstellar medium (ISM). We showed that the uncertainty in the VLBI brightness temperatures obtained from the space VLBI data of the RadioAstron satellite can be as high as ~70% due to the presence of the rapid flux density variations. Our statistical results support the view that IDV at centimeter wavelengths is predominantly caused by interstellar scintillation (ISS) of the emission from the most compact, core-dominant region in an AGN. Full article
(This article belongs to the Special Issue Microvariability of Blazars)
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Open AccessFeature PaperArticle On Nearly Newtonian Potentials and Their Implications to Astrophysics
Received: 28 February 2018 / Revised: 23 March 2018 / Accepted: 27 March 2018 / Published: 17 April 2018
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Abstract
We review the concept of the slow motion problem in General relativity. We discuss how the understanding of this process may imprint influence on the explanation of astrophysical problems Full article
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Open AccessFeature PaperArticle Hadronic Models of the Fermi Bubbles: Future Perspectives
Received: 15 February 2018 / Revised: 5 April 2018 / Accepted: 8 April 2018 / Published: 12 April 2018
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Abstract
The origin of sub-TeV gamma rays detected by Fermi-LAT from the Fermi bubbles at the Galactic center is still unknown. In a hadronic model, acceleration of protons and/or nuclei and their subsequent interactions with gas in the bubble volume can produce observed
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The origin of sub-TeV gamma rays detected by Fermi-LAT from the Fermi bubbles at the Galactic center is still unknown. In a hadronic model, acceleration of protons and/or nuclei and their subsequent interactions with gas in the bubble volume can produce observed gamma rays. Such interactions naturally produce high-energy neutrinos, and a detection of those can discriminate between a hadronic and a leptonic origin of gamma rays. Additional constraints on the Fermi bubbles gamma-ray flux in the TeV range from recent HAWC observations restrict hadronic model parameters, which in turn disfavor Fermi bubbles as the origin of a large fraction of neutrino events detected by IceCube along the bubble directions. We revisit our hadronic model and discuss future constraints on parameters from observations in very high-energy gamma rays and neutrinos. Full article
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Open AccessFeature PaperArticle Practical Aspects of X-ray Imaging Polarimetry of Supernova Remnants and Other Extended Sources
Received: 23 February 2018 / Revised: 28 March 2018 / Accepted: 29 March 2018 / Published: 10 April 2018
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Abstract
The new generation of X-ray polarisation detectors, the gas pixel detectors, which will be employed by the future space missions IXPE and eXTP, allows for spatially resolved X-ray polarisation studies. This will be of particular interest for X-ray synchrotron emission from extended sources
[...] Read more.
The new generation of X-ray polarisation detectors, the gas pixel detectors, which will be employed by the future space missions IXPE and eXTP, allows for spatially resolved X-ray polarisation studies. This will be of particular interest for X-ray synchrotron emission from extended sources like young supernova remnants and pulsar wind nebulae. Here we report on employing a polarisation statistic that can be used to makes maps in the Stokes I, Q, and U parameters, a method that we expand by correcting for the energy-dependent instrumental modulation factor, using optimal weighting of the signal. In order to explore the types of Stokes maps that can be obtained, we present a Monte Carlo simulation program called xpolim, with which different polarisation weighting schemes are explored. We illustrate its use with simulations of polarisation maps of young supernova remnants, after having described the general science case for polarisation studies of supernova remnants, and its connection to magnetic-field turbulence. We use xpolim simulations to show that in general deep, ~2 Ms observations are needed to recover polarisation signals, in particular for Cas A, for which in the polarisation fraction may be as low as 5%. Full article
(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)
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Open AccessFeature PaperConference Report Searching for Axion-Like Particles with X-ray Polarimeters
Received: 31 January 2018 / Revised: 3 April 2018 / Accepted: 4 April 2018 / Published: 10 April 2018
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Abstract
X-ray telescopes are an exceptional tool for searching for new fundamental physics. In particular, X-ray observations have already placed world-leading bounds on the interaction between photons and axion-like particles (ALPs). ALPs are hypothetical new ultra-light particles motivated by string theory models. They can
[...] Read more.
X-ray telescopes are an exceptional tool for searching for new fundamental physics. In particular, X-ray observations have already placed world-leading bounds on the interaction between photons and axion-like particles (ALPs). ALPs are hypothetical new ultra-light particles motivated by string theory models. They can also act as dark matter and dark energy, and provide a solution to the strong CP problem. In a background magnetic field, ALPs and photons may interconvert. This leads to energy dependent modulations in both the flux and polarisation of the spectra of point sources shining through large magnetic fields. The next generation of polarising X-ray telescopes will offer new detection possibilities for ALPs. Here we present techniques and projected bounds for searching for ALPs with X-ray polarimetry. We demonstrate that upcoming X-ray polarimetry missions have the potential to place world-leading bounds on ALPs. Full article
(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)
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Open AccessFeature PaperArticle Hot Coronae in Local AGN: Present Status and Future Perspectives
Received: 2 February 2018 / Revised: 19 March 2018 / Accepted: 19 March 2018 / Published: 4 April 2018
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Abstract
The nuclear X-ray emission in radio-quiet Active Galactic Nuclei (AGN) is commonly believed to be due to inverse Compton scattering of soft UV photons in a hot corona. The radiation is expected to be polarized, the polarization degree depending mainly on the geometry
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The nuclear X-ray emission in radio-quiet Active Galactic Nuclei (AGN) is commonly believed to be due to inverse Compton scattering of soft UV photons in a hot corona. The radiation is expected to be polarized, the polarization degree depending mainly on the geometry and optical depth of the corona. Nuclear Spectroscopic Telescope Array (NuSTAR) observations are providing for the first time high quality measurements of the coronal physical parameters—temperature and optical depth. We hereby review the NuSTAR results on the coronal physical parameters (temperature and optical depth) and discuss their implications for future X-ray polarimetric studies. Full article
(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)
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Open AccessArticle Applying MOG to Lensing: Einstein Rings, Abell 520 and the Bullet Cluster
Received: 30 January 2018 / Revised: 16 March 2018 / Accepted: 19 March 2018 / Published: 2 April 2018
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Abstract
We investigate gravitational lensing in the context of the MOG modified theory of gravity. Using a formulation of the theory with no adjustable or fitted parameters, we present the MOG equations of motion for slow, nonrelativistic test particles and for ultrarelativistic test particles,
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We investigate gravitational lensing in the context of the MOG modified theory of gravity. Using a formulation of the theory with no adjustable or fitted parameters, we present the MOG equations of motion for slow, nonrelativistic test particles and for ultrarelativistic test particles, such as rays of light. We demonstrate how the MOG prediction for the bending of light can be applied to astronomical observations. Our investigation first focuses on a small set of strong lensing observations where the properties of the lensing objects are found to be consistent with the predictions of the theory. We also present an analysis of the colliding clusters 1E0657-558 (known also as the Bullet Cluster) and Abell 520; in both cases, the predictions of the MOG theory are in good agreement with observation. Full article
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Open AccessFeature PaperArticle Future X-ray Polarimetry of Relativistic Accelerators: Pulsar Wind Nebulae and Supernova Remnants
Received: 29 January 2018 / Revised: 16 March 2018 / Accepted: 20 March 2018 / Published: 27 March 2018
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Abstract
Supernova remnants (SNRs) and pulsar wind nebulae (PWNs) are among the most significant sources of non-thermal X-rays in the sky, and the best means by which relativistic plasma dynamics and particle acceleration can be investigated. Being strong synchrotron emitters, they are ideal candidates
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Supernova remnants (SNRs) and pulsar wind nebulae (PWNs) are among the most significant sources of non-thermal X-rays in the sky, and the best means by which relativistic plasma dynamics and particle acceleration can be investigated. Being strong synchrotron emitters, they are ideal candidates for X-ray polarimetry, and indeed the Crab nebula is up to present the only object where X-ray polarization has been detected with a high level of significance. Future polarimetric measures will likely provide us with crucial information on the level of turbulence that is expected at particle acceleration sites, together with the spatial and temporal coherence of magnetic field geometry, enabling us to set stronger constraints on our acceleration models. PWNs will also allow us to estimate the level of internal dissipation. I will briefly review the current knowledge on the polarization signatures in SNRs and PWNs, and I will illustrate what we can hope to achieve with future missions such as IXPE/XIPE. Full article
(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)
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Open AccessFeature PaperArticle The Train Wreck Cluster Abell 520 and the Bullet Cluster 1E0657-558 in a Generalized Theory of Gravitation
Received: 19 January 2018 / Revised: 14 March 2018 / Accepted: 20 March 2018 / Published: 26 March 2018
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Abstract
A major hurdle for modified gravity theories is to explain the dynamics of galaxy clusters. A case is made for a generalized gravitational theory called Scalar-Tensor-Vector-Gravity (STVG) or MOG (Modified Gravity) to explain merging cluster dynamics. The paper presents the results of a
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A major hurdle for modified gravity theories is to explain the dynamics of galaxy clusters. A case is made for a generalized gravitational theory called Scalar-Tensor-Vector-Gravity (STVG) or MOG (Modified Gravity) to explain merging cluster dynamics. The paper presents the results of a re-analysis of the Bullet Cluster, as well as an analysis of the Train Wreck Cluster in the weak gravitational field limit without dark matter. The King- β model is used to fit the X-ray data of both clusters, and the κ -maps are computed using the parameters of this fit. The amount of galaxies in the clusters is estimated by subtracting the predicted κ -map from the κ -map data. The estimate for the Bullet Cluster is that 14.1 % of the cluster is composed of galaxies. For the Train Wreck Cluster, if the Jee et al. data are used, 25.7 % of the cluster is composed of galaxies. The baryon matter in the galaxies and the enhanced strength of gravitation in MOG shift the lensing peaks, making them offset from the gas. The work demonstrates that this generalized gravitational theory can explain merging cluster dynamics without dark matter. Full article
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Open AccessFeature PaperArticle Probing the Large Faraday Rotation Measure Environment of Compact Active Galactic Nuclei
Received: 16 February 2018 / Revised: 16 March 2018 / Accepted: 19 March 2018 / Published: 26 March 2018
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Abstract
Knowing how the ambient medium in the vicinity of active galactic nuclei (AGNs) is shaped is crucial to understanding generally the evolution of such cosmic giants as well as AGN jet formation and launching. Thanks to the new broadband capability now available at
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Knowing how the ambient medium in the vicinity of active galactic nuclei (AGNs) is shaped is crucial to understanding generally the evolution of such cosmic giants as well as AGN jet formation and launching. Thanks to the new broadband capability now available at the Jansky Very Large Array (JVLA), we can study changes in polarization properties, fractional polarization, and polarization angles, together with the total intensity spectra of a sample of 14 AGNs, within a frequency range from 1 to 12 GHz. Depolarization modeling has been performed by means of so-called “qu-fitting” to the polarized data, and a synchrotron self absorption model has been used for fitting to the total intensity data. We found complex behavior both in the polarization spectra and in the total intensity spectra, and several Faraday components with a large rotation measure (RM) and several synchrotron components were needed to represent these spectra. Here, results for three targets are shown. This new method of analyzing broadband polarization data through qu-fitting successfully maps the complex surroundings of unresolved objects. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Open AccessFeature PaperCommentary Positron Transport and Annihilation in the Galactic Bulge
Received: 29 January 2018 / Revised: 14 March 2018 / Accepted: 17 March 2018 / Published: 21 March 2018
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Abstract
The annihilation of positrons in the Milky Way Galaxy has been observed for ∼50 years; however, the production sites of these positrons remains hard to identify. The observed morphology of positron annihilation gamma-rays provides information on the annihilation sites of these Galactic positrons.
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The annihilation of positrons in the Milky Way Galaxy has been observed for ∼50 years; however, the production sites of these positrons remains hard to identify. The observed morphology of positron annihilation gamma-rays provides information on the annihilation sites of these Galactic positrons. It is understood that the positrons responsible for the annihilation signal originate at MeV energies. The majority of sources of MeV positrons occupy the star-forming thin disk of the Milky Way. If positrons propagate far from their sources, we must develop accurate models of positron propagation through all interstellar medium (ISM) phases in order to reveal the currently uncertain origin of these Galactic positrons. On the other hand, if positrons annihilate close to their sources, an alternative source of MeV positrons with a distribution that matches the annihilation morphology must be identified. In this work, I discuss the various models that have been developed to understand the origin of the 511 keV line from the direction of the Galactic bulge, and the propagation of positrons in the ISM. Full article
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