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Galaxies, Volume 6, Issue 1 (March 2018)

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Editorial

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Open AccessEditorial Acknowledgement to Reviewers of Galaxies in 2017
Galaxies 2018, 6(1), 7; doi:10.3390/galaxies6010007
Received: 12 January 2018 / Revised: 12 January 2018 / Accepted: 12 January 2018 / Published: 12 January 2018
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Abstract
Peer review is an essential part in the publication process, ensuring that Galaxies maintains high quality standards for its published papers [...]
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Research

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Open AccessFeature PaperArticle Multi-Wavelength Intra-Day Variability and Quasi-Periodic Oscillation in Blazars
Galaxies 2018, 6(1), 1; doi:10.3390/galaxies6010001
Received: 2 December 2017 / Revised: 15 December 2017 / Accepted: 19 December 2017 / Published: 25 December 2017
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Abstract
We reviewed multi-wavelength blazars variability and detection of quasi-periodic oscillations on intra-day timescales. The variability timescale from a few minutes to up to less than a days is commonly known as intra-day variability. These fast variations are extremely useful to constrain the size
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We reviewed multi-wavelength blazars variability and detection of quasi-periodic oscillations on intra-day timescales. The variability timescale from a few minutes to up to less than a days is commonly known as intra-day variability. These fast variations are extremely useful to constrain the size of the emitting region, black hole mass estimation, etc. It is noticed that in general, blazars show intra-day variability in the complete electromagnetic spectrum. However, some classes of blazars either do not show or show very little intra-day variability in a specific band of the electromagnetic spectrum. Blazars show rarely quasi-periodic oscillations in time series data in optical and X-ray bands. Other properties and emission mechanisms of blazars are also briefly discussed. Full article
(This article belongs to the Special Issue Microvariability of Blazars)
Open AccessFeature PaperArticle Microvariability in BL Lacertae: “Zooming” into the Innermost Blazar Regions
Galaxies 2018, 6(1), 2; doi:10.3390/galaxies6010002
Received: 23 November 2017 / Revised: 18 December 2017 / Accepted: 22 December 2017 / Published: 3 January 2018
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Abstract
In this work, we present the results of our multi-band microvariability study of the famous blazar BL Lac. We performed microvariablity observations of the source in the optical VRI bands for four nights in 2016. We studied the intranight flux and spectral variability
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In this work, we present the results of our multi-band microvariability study of the famous blazar BL Lac. We performed microvariablity observations of the source in the optical VRI bands for four nights in 2016. We studied the intranight flux and spectral variability of the source in detail with an objective to characterize microvariability in the blazars, a frequently observed phenomenon in blazars. The results show that the source often displays a fast flux variability with an amplitude as large as ~0.2 magnitude within a few hours, and that the color variability in the similar time scales can be characterized as “bluer-when-brighter” trend. We also observed markedly curved optical spectrum during one of the nights. Furthermore, the correlation between multi-band emission shows that in general the emission in all the bands are highly correlated; and in one of the nights V band emission was found to lead the I band emission by ~13 min. The search for characteristic timescale using z-transformed auto-correlation function and the structure function analyses reveals characteristic timescale of ~50 min in one of the R band observations. We try to explain the observed results in the context of the passage of shock waves through the relativistic outflows in blazars. Full article
(This article belongs to the Special Issue Microvariability of Blazars)
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Open AccessFeature PaperArticle Optical/Infrared Polarised Emission in X-ray Binaries
Galaxies 2018, 6(1), 3; doi:10.3390/galaxies6010003
Received: 19 September 2017 / Revised: 12 December 2017 / Accepted: 19 December 2017 / Published: 4 January 2018
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Abstract
Recently, evidence for synchrotron emission in both black-hole (BH) and neutron star X-ray binaries has been mounting, from optical/infrared spectral, polarimetric, and fast timing signatures. The synchrotron emission of jets can be highly linearly polarised, depending on the configuration of the magnetic field
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Recently, evidence for synchrotron emission in both black-hole (BH) and neutron star X-ray binaries has been mounting, from optical/infrared spectral, polarimetric, and fast timing signatures. The synchrotron emission of jets can be highly linearly polarised, depending on the configuration of the magnetic field (B-field). Optical and infrared (OIR) polarimetric observations of X-ray binaries are presented in this brief review. The OIR polarimetric signature of relativistic jets is detected at levels of ∼1–10%, similarly to for active galactic nuclei (AGN) cores. This reveals that the magnetic geometry in the compact jets may be similar for supermassive and stellar-mass BHs. The B-fields near the jet base in most of these systems appear to be turbulent, variable and on average, aligned with the jet axis, although there are some exceptions. These measurements probe the physical conditions in the accretion (out)flow and demonstrate a new way of connecting inflow and outflow, using both rapid timing and polarisation. Variations in polarisation could be due to rapid changes of the ordering of the B-field in the emitting region, or in one case, flares from individual ejections or collisions between ejecta. It is predicted that in some cases, variable levels of X-ray polarisation from synchrotron emission originating in jets will be detected from accreting galactic BHs with upcoming spaceborne X-ray polarimeters. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
Open AccessFeature PaperArticle A Proposal of a Regular Black Hole Satisfying the Weak Energy Condition
Galaxies 2018, 6(1), 4; doi:10.3390/galaxies6010004
Received: 29 November 2017 / Revised: 28 December 2017 / Accepted: 30 December 2017 / Published: 4 January 2018
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Abstract
We discuss a black hole generated by some matter fluid, whose stress-energy tensor is known. We show that it is regular and that it satisfies the Weak Energy Condition (WEC) and the null energy condition (NEC). Finally, we look for its asymptotic behavior.
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We discuss a black hole generated by some matter fluid, whose stress-energy tensor is known. We show that it is regular and that it satisfies the Weak Energy Condition (WEC) and the null energy condition (NEC). Finally, we look for its asymptotic behavior. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
Open AccessFeature PaperArticle The Variable Rotation Measure Distribution in 3C 273 on Parsec Scales
Galaxies 2018, 6(1), 5; doi:10.3390/galaxies6010005
Received: 15 September 2017 / Revised: 17 December 2017 / Accepted: 18 December 2017 / Published: 5 January 2018
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Abstract
We briefly review how opacity affects the observed polarization in synchrotron emitting jets. We show some new multi-frequency observations of 3C 273 made with the VLBA in 1999–2000, which add significantly to the available rotation measure (RM) observations of this source. Our findings
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We briefly review how opacity affects the observed polarization in synchrotron emitting jets. We show some new multi-frequency observations of 3C 273 made with the VLBA in 1999–2000, which add significantly to the available rotation measure (RM) observations of this source. Our findings can be summarized as follows: (1) The transverse gradient in RM is amply confirmed. This implies a toroidal component to the magnetic field, which in turn requires a current of 1017–1018 A flowing down the jet. (2) The net magnetic field in the jet is longitudinal; however, whether or not the longitudinal component is vector-ordered is an open question. (3) The RM distribution is variable on timescales of months to years. We attribute this to the motion of superluminal components behind a turbulent Faraday screen that surrounds the jet. (4) Finally, we suggest that Faraday rotation measurements at higher resolution and higher frequencies, with the Event Horizon Telescope, may enable useful constraints to be placed on the accretion rate onto the central black hole. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Open AccessFeature PaperArticle Wavepacket Evolution in Unimodular Quantum Cosmology
Galaxies 2018, 6(1), 8; doi:10.3390/galaxies6010008
Received: 29 November 2017 / Revised: 29 December 2017 / Accepted: 3 January 2018 / Published: 12 January 2018
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Abstract
The unimodular theory of gravity admits a canonical quantization of minisuperspace models without the problem of time. We derive instead a kind of Schrödinger equation. We have found unitarily evolving wave packet solutions for the special case of a massless scalar field and
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The unimodular theory of gravity admits a canonical quantization of minisuperspace models without the problem of time. We derive instead a kind of Schrödinger equation. We have found unitarily evolving wave packet solutions for the special case of a massless scalar field and a spatially flat Friedmann universe. We show that the longterm behaviour of the expectation values of the canonical quantities corresponds to the evolution of the classical variables. The solutions provided in an explicit example can be continued beyond the singularity at t = 0 , passing a finite minimal extension of the universe. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
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Open AccessFeature PaperArticle Determining the Jet Poloidal B Field and Black-Hole Rotation Directions in AGNs
Galaxies 2018, 6(1), 9; doi:10.3390/galaxies6010009
Received: 21 October 2017 / Revised: 16 December 2017 / Accepted: 9 January 2018 / Published: 12 January 2018
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Abstract
It is theoretically expected that active galactic nucleus (AGN) jets should carry helical magnetic (B) fields, which arise due to the rotation of the central black hole and accretion disk combined with the jet outflow. The direction of the toroidal component
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It is theoretically expected that active galactic nucleus (AGN) jets should carry helical magnetic (B) fields, which arise due to the rotation of the central black hole and accretion disk combined with the jet outflow. The direction of the toroidal component of the helical B field B φ is determined by the direction of the poloidal component B p of the initial seed field that is “wound up” and the direction of rotation of the central black hole and accretion disk. The presence of the jet’s helical B field can be manifest both through the presence of Faraday rotation gradients across the jet, and the presence of appreciable circular polarization, which comes about when linearly polarized emission from the far side of the jet is partially converted to circularly polarized emission as it passes through the magnetized plasma at the front side of the jet on its way towards the observer. When both of these properties are manifest, they can be used jointly with the jet linear polarization structure to uniquely determine both the direction of B p and the direction of the central rotation. This technique has been applied to 12 AGNs. The results indicate statistically equal numbers of outward and inward B p and of clockwise (CW) and counter-clockwise (CCW) rotations of the central black holes on the sky. However, they suggest that the directions of B p and of the central rotation are coupled: CW/CCW central rotation is preferentially associated with inward/outward poloidal B field. This leads to a preferred orientation for the toroidal B-field component corresponding to inward current along the jet. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Open AccessFeature PaperArticle Ultra Light Axionic Dark Matter: Galactic Halos and Implications for Observations with Pulsar Timing Arrays
Galaxies 2018, 6(1), 10; doi:10.3390/galaxies6010010
Received: 29 November 2017 / Revised: 15 December 2017 / Accepted: 8 January 2018 / Published: 16 January 2018
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Abstract
The cold dark matter (CDM) paradigm successfully explains the cosmic structure over an enormous span of redshifts. However, it fails when probing the innermost regions of dark matter halos and the properties of the Milky Way’s dwarf galaxy satellites. Moreover, the lack of
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The cold dark matter (CDM) paradigm successfully explains the cosmic structure over an enormous span of redshifts. However, it fails when probing the innermost regions of dark matter halos and the properties of the Milky Way’s dwarf galaxy satellites. Moreover, the lack of experimental detection of Weakly Interacting Massive Particle (WIMP) favors alternative candidates such as light axionic dark matter that naturally arise in string theory. Cosmological N-body simulations have shown that axionic dark matter forms a solitonic core of size of ≃150 pc in the innermost region of the galactic halos. The oscillating scalar field associated to the axionic dark matter halo produces an oscillating gravitational potential that induces a time dilation of the pulse arrival time of ≃400 ns/(m B /10 22 eV) for pulsar within such a solitonic core. Over the whole galaxy, the averaged predicted signal may be detectable with current and forthcoming pulsar timing array telescopes. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
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Open AccessFeature PaperArticle Supersymmetric M-brane Solution in a Dynamical Background
Galaxies 2018, 6(1), 11; doi:10.3390/galaxies6010011
Received: 22 November 2017 / Revised: 31 December 2017 / Accepted: 3 January 2018 / Published: 16 January 2018
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Abstract
Supersymmetry arises in certain theories of fermions coupled to gauge fields and gravity in a spacetime of 11 dimensions. The dynamical brane background has mainly been studied for the class of purely bosonic solutions only, but recent developments involving a time-dependent brane solution
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Supersymmetry arises in certain theories of fermions coupled to gauge fields and gravity in a spacetime of 11 dimensions. The dynamical brane background has mainly been studied for the class of purely bosonic solutions only, but recent developments involving a time-dependent brane solution have made it clear that one can get more information by asking what happens on supersymmetric systems. In this proceeding, we construct an exact supersymmetric solution of a dynamical M-brane background in the 11-dimensional supergravity and investigate supersymmetry breaking, the geometric features near the singularity and the black hole horizon. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
Open AccessFeature PaperArticle Time in Quantum Cosmology of FRW f(R) Theories
Galaxies 2018, 6(1), 12; doi:10.3390/galaxies6010012
Received: 1 December 2017 / Revised: 9 January 2018 / Accepted: 9 January 2018 / Published: 17 January 2018
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Abstract
The time problem is a problem of canonical quantum gravity that has long been known about; it is related to the relativistic invariance and the consequent absence of an explicit time variable in the quantum equations. This fact complicates the interpretation of the
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The time problem is a problem of canonical quantum gravity that has long been known about; it is related to the relativistic invariance and the consequent absence of an explicit time variable in the quantum equations. This fact complicates the interpretation of the wave function of the universe. Following proposals to assign the clock function to a scalar field, we look at the scalar degree of freedom contained in f ( R ) theories. For this purpose we consider a quadratic f ( R ) theory in an equivalent formulation with a scalar field, with a FRW metric, and consider its Wheeler-DeWitt equation. The wave function is obtained numerically and is consistent with the interpretation of the scalar field as time by means of a conditional probability, from which an effective time-dependent wave function follows. The evolution the scale factor is obtained by its mean value, and the quantum fluctuations are consistent with the Heisenberg relations and a classical universe today. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
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Open AccessFeature PaperArticle Polarization: A Method to Reveal the True Nature of the Dusty S-Cluster Object (DSO/G2)
Galaxies 2018, 6(1), 13; doi:10.3390/galaxies6010013
Received: 15 November 2017 / Revised: 6 December 2017 / Accepted: 11 January 2018 / Published: 17 January 2018
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Abstract
There have been different scenarios describing the nature of a dusty source, noted as Dusty S-cluster Object (DSO) or G2, orbiting around the Galactic centre super-massive black hole. Observing the polarized continuum emission of this source provides information on its nature and geometry.
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There have been different scenarios describing the nature of a dusty source, noted as Dusty S-cluster Object (DSO) or G2, orbiting around the Galactic centre super-massive black hole. Observing the polarized continuum emission of this source provides information on its nature and geometry. We show that this source is intrinsically polarized with polarization degree of 30%, implying that it has a non-spherical geometry, and a varying polarization angle in the ambient medium of the black hole. Its main observable properties can be well described and modeled with a pre-main-sequence star forming a bow shock as it approaches the Sgr A* position. Full article
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
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Other

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Open AccessFeature PaperConference Report Quantum-Gravitational Effects on Primordial Power Spectra in Slow-Roll Inflationary Models
Galaxies 2018, 6(1), 6; doi:10.3390/galaxies6010006
Received: 30 November 2017 / Revised: 3 January 2018 / Accepted: 3 January 2018 / Published: 8 January 2018
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Abstract
We review the computation of the power spectra of inflationary gauge-invariant perturbations in the context of canonical quantum gravity for generic slow-roll models. A semiclassical approximation, based on an expansion in inverse powers of the Planck mass, is applied to the complete Wheeler–DeWitt
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We review the computation of the power spectra of inflationary gauge-invariant perturbations in the context of canonical quantum gravity for generic slow-roll models. A semiclassical approximation, based on an expansion in inverse powers of the Planck mass, is applied to the complete Wheeler–DeWitt equation describing a perturbed inflationary universe. This expansion leads to a hierarchy of equations at consecutive orders of the approximation and allows us to write down a corrected Schrödinger equation that encodes information about quantum-gravitational effects. The analytical dependence of the correction to the power spectrum on the wavenumber is obtained. Nonetheless, some numerical work is needed in order to obtain its precise value. Finally, it is shown that the correction turns out to be positive, which leads to an enhancement of the power spectrum especially prominent for large scales. We will also discuss whether this correction leads to a measurable effect in the cosmic microwave background anisotropies. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
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