Galaxies

8 pages, 384 KiB

Open AccessArticle

Effects of 2HDM in Electroweak Phase Transition

by Arnab Chaudhuri, Maxim Yu. Khlopov and Shiladitya Porey

Galaxies 2021, 9(2), 45; https://doi.org/10.3390/galaxies9020045 - 20 Jun 2021

Cited by 7 | Viewed by 2181

The entropy production scenarios due to the electroweak phase transition (EWPT) in the framework of the minimal extension of standard model, namely the two Higgs doublet model (2HDM), are revisited. The possibility of first order phase transition is discussed. Intense parameter scanning was [...] Read more.

The entropy production scenarios due to the electroweak phase transition (EWPT) in the framework of the minimal extension of standard model, namely the two Higgs doublet model (2HDM), are revisited. The possibility of first order phase transition is discussed. Intense parameter scanning was done with the help of BSMPT, a C++ package. We perform numerical calculations in order to calculate the entropy production with numerous benchmark points. Full article

(This article belongs to the Collection A Trip across the Universe: Our Present Knowledge and Future Perspectives)

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10 pages, 345 KiB

Open AccessFeature PaperArticle

Constraints on Lorentz Invariance Violation with Multiwavelength Polarized Astrophysical Sources

by Qi-Qi Zhou, Shuang-Xi Yi, Jun-Jie Wei and Xue-Feng Wu

Galaxies 2021, 9(2), 44; https://doi.org/10.3390/galaxies9020044 - 20 Jun 2021

Cited by 10 | Viewed by 2100

Abstract

Possible violations of Lorentz invariance (LIV) can produce vacuum birefringence, which results in a frequency-dependent rotation of the polarization plane of linearly polarized light from distant sources. In this paper, we try to search for a frequency-dependent change of the linear polarization angle [...] Read more.

Possible violations of Lorentz invariance (LIV) can produce vacuum birefringence, which results in a frequency-dependent rotation of the polarization plane of linearly polarized light from distant sources. In this paper, we try to search for a frequency-dependent change of the linear polarization angle arising from vacuum birefringence in the spectropolarimetric data of astrophysical sources. We collect five blazars with multiwavelength polarization measurements in different optical bands

(U B V R I)

. Taking into account the observed polarization angle contributions from both the intrinsic polarization angle and the rotation angle induced by LIV, and assuming that the intrinsic polarization angle is an unknown constant, we obtain new constraints on LIV by directly fitting the multiwavelength polarimetric data of the five blazars. Here, we show that the birefringence parameter

η

quantifying the broken degree of Lorentz invariance is limited to be in the range of $- 9.63 \times 10^{- 8} < η < 6.55 \times 10^{- 6}$ at the

2 σ

confidence level, which is as good as or represents one order of magnitude improvement over the results previously obtained from ultraviolet/optical polarization observations. Much stronger limits can be obtained by future multiwavelength observations in the gamma-ray energy band. Full article

(This article belongs to the Special Issue Lorentz Violation in Astroparticles and Gravitational Waves)

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26 pages, 3058 KiB

Open AccessArticle

Ergosphere, Photon Region Structure, and the Shadow of a Rotating Charged Weyl Black Hole

by Mohsen Fathi, Marco Olivares and José R. Villanueva

Galaxies 2021, 9(2), 43; https://doi.org/10.3390/galaxies9020043 - 18 Jun 2021

Cited by 13 | Viewed by 4178

Abstract

In this paper, we explore the photon region and the shadow of the rotating counterpart of a static charged Weyl black hole, which has been previously discussed according to null and time-like geodesics. The rotating black hole shows strong sensitivity to the electric [...] Read more.

In this paper, we explore the photon region and the shadow of the rotating counterpart of a static charged Weyl black hole, which has been previously discussed according to null and time-like geodesics. The rotating black hole shows strong sensitivity to the electric charge and the spin parameter, and its shadow changes from being oblate to being sharp by increasing in the spin parameter. Comparing the calculated vertical angular diameter of the shadow with that of M87*, we found that the latter may possess about

10^{36}

protons as its source of electric charge, if it is a rotating charged Weyl black hole. A complete derivation of the ergosphere and the static limit is also presented. Full article

(This article belongs to the Collection A Trip across the Universe: Our Present Knowledge and Future Perspectives)

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9 pages, 404 KiB

Open AccessFeature PaperReview

Polarimetric Properties of Blazars Caught by the WEBT

by Claudia M. Raiteri and Massimo Villata

Galaxies 2021, 9(2), 42; https://doi.org/10.3390/galaxies9020042 - 11 Jun 2021

Cited by 9 | Viewed by 2537

Abstract

Active galactic nuclei come in many varieties. A minority of them are radio-loud, and exhibit two opposite prominent plasma jets extending from the proximity of the supermassive black hole up to megaparsec distances. When one of the relativistic jets is oriented closely to [...] Read more.

Active galactic nuclei come in many varieties. A minority of them are radio-loud, and exhibit two opposite prominent plasma jets extending from the proximity of the supermassive black hole up to megaparsec distances. When one of the relativistic jets is oriented closely to the line of sight, its emission is Doppler beamed and these objects show extreme variability properties at all wavelengths. These are called “blazars”. The unpredictable blazar variability, occurring on a continuous range of time-scales, from minutes to years, is most effectively investigated in a multi-wavelength context. Ground-based and space observations together contribute to give us a comprehensive picture of the blazar emission properties from the radio to the

γ

-ray band. Moreover, in recent years, a lot of effort has been devoted to the observation and analysis of the blazar polarimetric radio and optical behaviour, showing strong variability of both the polarisation degree and angle. The Whole Earth Blazar Telescope (WEBT) Collaboration, involving many tens of astronomers all around the globe, has been monitoring several blazars since 1997. The results of the corresponding data analysis have contributed to the understanding of the blazar phenomenon, particularly stressing the viability of a geometrical interpretation of the blazar variability. We review here the most significant polarimetric results achieved in the WEBT studies. Full article

(This article belongs to the Special Issue Polarimetry as a Probe of Magnetic Fields in AGN Jets)

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21 pages, 7017 KiB

Open AccessFeature PaperArticle

Magnetic Fields in Molecular Clouds—Observation and Interpretation

by Hua-Bai Li

Galaxies 2021, 9(2), 41; https://doi.org/10.3390/galaxies9020041 - 8 Jun 2021

Cited by 20 | Viewed by 10831

Abstract

The Zeeman effect and dust grain alignment are two major methods for probing magnetic fields (B-fields) in molecular clouds, largely motivated by the study of star formation, as the B-field may regulate gravitational contraction and channel turbulence velocity. This review summarizes our observations [...] Read more.

The Zeeman effect and dust grain alignment are two major methods for probing magnetic fields (B-fields) in molecular clouds, largely motivated by the study of star formation, as the B-field may regulate gravitational contraction and channel turbulence velocity. This review summarizes our observations of B-fields over the past decade, along with our interpretation. Galactic B-fields anchor molecular clouds down to cloud cores with scales around 0.1 pc and densities of 10^4–5 H₂/cc. Within the cores, turbulence can be slightly super-Alfvénic, while the bulk volumes of parental clouds are sub-Alfvénic. The consequences of these largely ordered cloud B-fields on fragmentation and star formation are observed. The above paradigm is very different from the generally accepted theory during the first decade of the century, when cloud turbulence was assumed to be highly super-Alfvénic. Thus, turbulence anisotropy and turbulence-induced ambipolar diffusion are also revisited. Full article

(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)

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8 pages, 255 KiB

Open AccessArticle

Geodesic Circular Orbits Sharing the Same Orbital Frequencies in the Black String Spacetime

by Sanjar Shaymatov and Farruh Atamurotov

Galaxies 2021, 9(2), 40; https://doi.org/10.3390/galaxies9020040 - 28 May 2021

Cited by 17 | Viewed by 2533

Abstract

We consider isofrequency pairing of geodesic orbits that share the same three orbital frequencies associated with

Ω^{\hat{r}}

,

Ω^{\hat{φ}}

, and

Ω^{\hat{ω}}

in a particular region of parameter space around black string spacetime geometry. We study the [...] Read more.

We consider isofrequency pairing of geodesic orbits that share the same three orbital frequencies associated with

Ω^{\hat{r}}

,

Ω^{\hat{φ}}

, and

Ω^{\hat{ω}}

in a particular region of parameter space around black string spacetime geometry. We study the effect of a compact extra spatial dimension on the isofrequency pairing of geodesic orbits and show that such orbits would occur in the allowed region when particles move along the black string. We find that the presence of the compact extra dimension leads to an increase in the number of the isofrequency pairing of geodesic orbits. Interestingly we also find that isofrequency pairing of geodesic orbits in the region of parameter space cannot be realized beyond the critical value

J_{c r} \approx 0.096

of the conserved quantity of the motion arising from the compact extra spatial dimension. Full article

(This article belongs to the Special Issue Global Understanding of Accretion and Ejection around Black Holes)

21 pages, 657 KiB

Open AccessArticle

Study of Accretion Flow Dynamics of V404 Cygni during Its 2015 Outburst

by Arghajit Jana, Jie-Rou Shang, Dipak Debnath, Sandip K. Chakrabarti, Debjit Chatterjee and Hsiang-Kuang Chang

Galaxies 2021, 9(2), 39; https://doi.org/10.3390/galaxies9020039 - 25 May 2021

Cited by 3 | Viewed by 2903

Abstract

The 2015 Outburst of V404 Cygni is an unusual one with several X-ray and radio flares and rapid variation in the spectral and timing properties. The outburst occurred after 26 years of inactivity of the black hole. We study the accretion flow properties [...] Read more.

The 2015 Outburst of V404 Cygni is an unusual one with several X-ray and radio flares and rapid variation in the spectral and timing properties. The outburst occurred after 26 years of inactivity of the black hole. We study the accretion flow properties of the source during its initial phase of the outburst using Swift/XRT and Swift/BAT data in the energy range of 0.5–150 keV. We have done spectral analysis with the two component advective flow (TCAF) model fits file. Several flow parameters such as two types of accretion rates (Keplerian disk and sub-Keplerian halo), shock parameters (location and compression ratio) are extracted to understand the accretion flow dynamics. We calculated equipartition magnetic field

B_{e q}

for the outburst and found that the highest

B_{e q} \sim 900

Gauss. Power density spectra (PDS) showed no break, which indicates no or very less contribution of the Keplerian disk component, which is also seen from the result of the spectral analysis. No signature of prominent quasi-periodic oscillations (QPOs) is observed in the PDS. This is due to the non-satisfaction of the condition for the resonance shock oscillation as we observed mismatch between the cooling timescale and infall timescale of the post-shock matter. Full article

(This article belongs to the Special Issue Global Understanding of Accretion and Ejection around Black Holes)

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7 pages, 777 KiB

Open AccessArticle

Astronomical Seeing at Maidanak Observatory during the Year 2018

by Yusufjon Tillayev, Azimjon Azimov and Aktam Hafizov

Galaxies 2021, 9(2), 38; https://doi.org/10.3390/galaxies9020038 - 21 May 2021

Cited by 6 | Viewed by 2885

Abstract

Results of a four-month campaign of astronomical seeing measurements at Maidanak astronomical observatory (MAO) are presented. A differential image motion monitor (DIMM) was used for seeing estimations during the period from August to November 2018. The observation was organized within the framework of [...] Read more.

Results of a four-month campaign of astronomical seeing measurements at Maidanak astronomical observatory (MAO) are presented. A differential image motion monitor (DIMM) was used for seeing estimations during the period from August to November 2018. The observation was organized within the framework of a site testing for a new telescope which is going to be installed at the observatory. The median value of seeing for the entire period was determined as 0.70 arcseconds, which agrees well with the results of the period 1996–2003. The comparison of monthly values showed that some monthly median values differ from the seasonal trend of the previous period. Full article

(This article belongs to the Collection A Trip across the Universe: Our Present Knowledge and Future Perspectives)

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17 pages, 3474 KiB

Open AccessFeature PaperReview

Probing Magnetic Fields and Acceleration Mechanisms in Blazar Jets with X-ray Polarimetry

by Fabrizio Tavecchio

Galaxies 2021, 9(2), 37; https://doi.org/10.3390/galaxies9020037 - 18 May 2021

Cited by 20 | Viewed by 2960

Abstract

X-ray polarimetry promises us an unprecedented look at the structure of magnetic fields and on the processes at the base of acceleration of particles up to ultrarelativistic energies in relativistic jets. Crucial pieces of information are expected from observations of blazars (that are [...] Read more.

X-ray polarimetry promises us an unprecedented look at the structure of magnetic fields and on the processes at the base of acceleration of particles up to ultrarelativistic energies in relativistic jets. Crucial pieces of information are expected from observations of blazars (that are characterized by the presence of a jet pointing close to the Earth), in particular of the subclass defined by a synchrotron emission extending to the X-ray band (so-called high synchrotron peak blazars, HSP). In this review, I give an account of some of the models and numerical simulations developed to predict the polarimetric properties of HSP at high energy, contrasting the predictions of scenarios assuming particle acceleration at shock fronts with those that are based on magnetic reconnection, and I discuss the prospects for the observations of the upcoming Imaging X-ray Polarimetry Explorer (IXPE) satellite. Full article

(This article belongs to the Special Issue Polarimetry as a Probe of Magnetic Fields in AGN Jets)

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24 pages, 1342 KiB

Open AccessReview

Gamma-ray and Neutrino Signals from Accretion Disk Coronae of Active Galactic Nuclei

by Yoshiyuki Inoue, Dmitry Khangulyan and Akihiro Doi

Galaxies 2021, 9(2), 36; https://doi.org/10.3390/galaxies9020036 - 15 May 2021

Cited by 11 | Viewed by 3877

Abstract

To explain the X-ray spectra of active galactic nuclei (AGN), non-thermal activity in AGN coronae such as pair cascade models has been extensively discussed in the past literature. Although X-ray and gamma-ray observations in the 1990s disfavored such pair cascade models, recent millimeter-wave [...] Read more.

To explain the X-ray spectra of active galactic nuclei (AGN), non-thermal activity in AGN coronae such as pair cascade models has been extensively discussed in the past literature. Although X-ray and gamma-ray observations in the 1990s disfavored such pair cascade models, recent millimeter-wave observations of nearby Seyferts have established the existence of weak non-thermal coronal activity. In addition, the IceCube collaboration reported NGC 1068, a nearby Seyfert, as the hottest spot in their 10 year survey. These pieces of evidence are enough to investigate the non-thermal perspective of AGN coronae in depth again. This article summarizes our current observational understanding of AGN coronae and describes how AGN coronae generate high-energy particles. We also provide ways to test the AGN corona model with radio, X-ray, MeV gamma ray, and high-energy neutrino observations. Full article

(This article belongs to the Special Issue Searching for Astrophysical Sources of Cosmic Rays, Gamma-Rays and Neutrinos: Real-Time Multimessenger Programs and Theoretical Models)

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10 pages, 278 KiB

Open AccessArticle

Exploiting the IRT-THESEUS Capability to Observe Lensed Quasars

by Lindita Hamolli, Mimoza Hafizi, Francesco De Paolis and Achille A. Nucita

Galaxies 2021, 9(2), 35; https://doi.org/10.3390/galaxies9020035 - 14 May 2021

Cited by 1 | Viewed by 2210

Abstract

THESEUS is an ESA space based project, which aims to explore the early universe by unveiling a complete census of Gamma-ray Burst (GRB) population in the first billion years. This goal is expected to be realized by the combined observations of its three [...] Read more.

THESEUS is an ESA space based project, which aims to explore the early universe by unveiling a complete census of Gamma-ray Burst (GRB) population in the first billion years. This goal is expected to be realized by the combined observations of its three instruments on board: the Soft X-ray Imager (SXI), the X and Gamma Imaging Spectrometer (XGIS), and the InfraRed Telescope (IRT). This last one will identify, localise, and study the afterglow of the GRBs detected by SXI and XGIS, and about 40% of its time will be devoted to an all-sky photometric survey, which will certainly detect a relevant number of extragalactic sources, including Quasars. In this paper, we focus on the capability of IRT-THESEUS Telescope to observe Quasars and, in particular, Quasars lensed by foreground galaxies. In our analysis we consider the recent results for the Quasar Luminosity Function (QLF) in the infrared band based on the Spitzer Space Telescope imaging survey. In order to estimate the number of lensed Quasars, we develop Monte Carlo simulations using the mass-luminosity distribution function of galaxies and the galaxy and Quasar redshift distributions. We predict about 2.14 × 10⁵ Quasars to be observed during IRT-Theseus sky survey, and approximately 140 of them lensed by foreground galaxies. Detailed studies of these events would provide a powerful probe of the physical properties of Quasars and the mass distribution models of the galaxies. Full article

(This article belongs to the Special Issue Astrophysical Applications of Gravitational Microlensing)

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14 pages, 4984 KiB

Open AccessArticle

The Effects of Inertial Forces on the Dynamics of Disk Galaxies

by Roy Gomel and Tomer Zimmerman

Galaxies 2021, 9(2), 34; https://doi.org/10.3390/galaxies9020034 - 13 May 2021

Cited by 5 | Viewed by 4239

Abstract

When dealing with galactic dynamics, or more specifically, with galactic rotation curves, one basic assumption is always taken: the frame of reference relative to which the rotational velocities are given is assumed to be inertial. In other words, fictitious forces are assumed to [...] Read more.

When dealing with galactic dynamics, or more specifically, with galactic rotation curves, one basic assumption is always taken: the frame of reference relative to which the rotational velocities are given is assumed to be inertial. In other words, fictitious forces are assumed to vanish relative to the observational frame of a given galaxy. It might be interesting, however, to explore the outcomes of dropping that assumption; that is, to search for signatures of non-inertial behavior in the observed data. In this work, we show that the very discrepancy in galaxy rotation curves could be attributed to non-inertial effects. We derive a model for spiral galaxies that takes into account the possible influence of fictitious forces and find that the additional terms in the new model, due to fictitious forces, closely resemble dark halo profiles. Following this result, we apply the new model to a wide sample of galaxies, spanning a large range of luminosities and radii. It turns out that the new model accurately reproduces the structures of the rotation curves and provides very good fittings to the data. Full article

(This article belongs to the Special Issue Relativistic Cosmology, Numerical Analysis, General Relativity and Modified Gravity Theories)

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18 pages, 483 KiB

Open AccessReview

Naturally-Coupled Dark Sectors

by Durmuş Demir

Galaxies 2021, 9(2), 33; https://doi.org/10.3390/galaxies9020033 - 12 May 2021

Cited by 7 | Viewed by 2617

Abstract

The dark sector, composed of fields that are neutral under the standard model (SM) gauge group, can couple to the SM through the Higgs, hypercharge and neutrino portals, and pull the SM towards its scale by loop corrections. This instability, which is not [...] Read more.

The dark sector, composed of fields that are neutral under the standard model (SM) gauge group, can couple to the SM through the Higgs, hypercharge and neutrino portals, and pull the SM towards its scale by loop corrections. This instability, which is not possible to prevent in the known SM completions, such as supersymmetry, due to the sizable couplings to the SM, calls for alternative mechanisms that can neutralize sensitivities of the SM to the dark sector scale and to the ultraviolet cutoff above it. Here we review such a mechanism in which incorporation of gravity into the SM predicts the existence of a dark sector and allows it to be naturally coupled to the SM. We discuss and illustrate salient processes that can probe the naturally coupled dark sectors. Full article

(This article belongs to the Collection A Trip across the Universe: Our Present Knowledge and Future Perspectives)

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10 pages, 277 KiB

Open AccessFeature PaperArticle

On Plane Wave Solutions in Lorentz-Violating Extensions of Gravity

by J. R. Nascimento, A. Yu. Petrov and A. R. Vieira

Galaxies 2021, 9(2), 32; https://doi.org/10.3390/galaxies9020032 - 8 May 2021

Cited by 8 | Viewed by 2276

Abstract

In this paper, we obtain dispersion relations corresponding to plane wave solutions in Lorentz-breaking extensions of gravity with dimension 3, 4, 5 and 6 operators. We demonstrate that these dispersion relations display a usual Lorentz-invariant mode when the corresponding additive term involves higher [...] Read more.

In this paper, we obtain dispersion relations corresponding to plane wave solutions in Lorentz-breaking extensions of gravity with dimension 3, 4, 5 and 6 operators. We demonstrate that these dispersion relations display a usual Lorentz-invariant mode when the corresponding additive term involves higher derivatives. Full article

(This article belongs to the Special Issue Lorentz Violation in Astroparticles and Gravitational Waves)

8 pages, 364 KiB

Open AccessArticle

Dust-Acoustic Rogue Waves in an Electron-Positron-Ion-Dust Plasma Medium

by Md. Habibur Rahman, Nure Alam Chowdhury, Abdul Mannan and A. A. Mamun

Galaxies 2021, 9(2), 31; https://doi.org/10.3390/galaxies9020031 - 6 May 2021

Cited by 9 | Viewed by 2799

Abstract

In this work, the modulational instability of dust-acoustic (DA) waves (DAWs) is theoretically studied in a four-component plasma medium with electrons, positrons, ions, and negative dust grains. The nonlinear and dispersive coefficients of the nonlinear Schrödinger equation (NLSE) are used to recognize the [...] Read more.

In this work, the modulational instability of dust-acoustic (DA) waves (DAWs) is theoretically studied in a four-component plasma medium with electrons, positrons, ions, and negative dust grains. The nonlinear and dispersive coefficients of the nonlinear Schrödinger equation (NLSE) are used to recognize the stable and unstable parametric regimes of the DAWs. It can be seen from the numerical analysis that the amplitude of the DA rogue waves decreases with increasing populations of positrons and ions. It is also observed that the direction of the variation of the critical wave number is independent (dependent) of the sign (magnitude) of q. The applications of the outcomes from the present investigation are briefly addressed. Full article

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30 pages, 5875 KiB

Open AccessEditor’s ChoiceArticle

Features of Secular Changes in the Flux Density of the Cas a Supernova Remnant, from Centimeter to Decameter Wavelengths

by Artem Gorbynov, Artem Sukharev, Michail Ryabov, Vladislavs Bezrukovs and Arturs Orbidans

Galaxies 2021, 9(2), 30; https://doi.org/10.3390/galaxies9020030 - 4 May 2021

Cited by 2 | Viewed by 2769

Abstract

The purpose of this work is to summarize a large amount of observational data about the secular decrease of Cas A flux in the radio range, as an indicator of physical processes both in the source itself and as a consequence of the [...] Read more.

The purpose of this work is to summarize a large amount of observational data about the secular decrease of Cas A flux in the radio range, as an indicator of physical processes both in the source itself and as a consequence of the influence of the propagation medium. This paper presents results of observations Cas A and Cyg A on radio telescopes of International Radio Astronomy Center (Latvia), and URAN-4 phased array antenna (Institute of Radio Astronomy NAS of Ukraine). According to URAN-4 observations, there were seasonal–diurnal changes in Cas A/Cyg A flux ratios due to the effects of solar activity on the ionosphere, and there may be no secular decrease in Cas A flux density, or a weak tendency to decrease it. The significant influence of the ionosphere makes it difficult to use Cyg A as a reference source in the decameter radio range. In the centimeter radio range, there were episodic intra-day variations at the level 8–10% of Cas A averaged flux. Additionally, in the period January–February 2021, Cas A flux was about 1.7 times that of Cyg A. Taking into account the observed complex type of secular decrease in Cas A flux against the background of changes in space weather variations, further observations of Cas A were planned at radio observatories in Latvia and Ukraine. Full article

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28 pages, 19436 KiB

Open AccessEditor’s ChoiceArticle

Modeling of Spiral Structure in a Multi-Component Milky Way-Like Galaxy

by Sergey Khrapov, Alexander Khoperskov and Vladimir Korchagin

Galaxies 2021, 9(2), 29; https://doi.org/10.3390/galaxies9020029 - 28 Apr 2021

Cited by 7 | Viewed by 4264

Abstract

Using recent observational data, we construct a set of multi-component equilibrium models of the disk of a Milky Way-like galaxy. The disk dynamics are studied using collisionless-gaseous numerical simulations, based on the joined integration of the equations of motion for the collision-less particles [...] Read more.

Using recent observational data, we construct a set of multi-component equilibrium models of the disk of a Milky Way-like galaxy. The disk dynamics are studied using collisionless-gaseous numerical simulations, based on the joined integration of the equations of motion for the collision-less particles using direct integration of gravitational interaction and the gaseous SPH-particles. We find that after approximately one Gyr, a prominent central bar is formed having a semi-axis length of about three kpc, together with a multi-armed spiral pattern represented by a superposition of

m =

2-, 3-, and 4-armed spirals. The spiral structure and the bar exist for at least 3 Gyr in our simulations. The existence of the Milky Way bar imposes limitations on the density distributions in the subsystems of the Milky Way galaxy. We find that a bar does not form if the radial scale length of the density distribution in the disk exceeds 2.6 kpc. As expected, the bar formation is also suppressed by a compact massive stellar bulge. We also demonstrate that the maximum value in the rotation curve of the disk of the Milky Way galaxy, as found in its central regions, is explained by non-circular motion due to the presence of a bar and its orientation relative to an observer. Full article

(This article belongs to the Collection A Trip across the Universe: Our Present Knowledge and Future Perspectives)

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15 pages, 299 KiB

Open AccessEditor’s ChoiceReview

Eclipsing Systems with Pulsating Components (Types β Cep, δ Sct, γ Dor or Red Giant) in the Era of High-Accuracy Space Data

by Patricia Lampens

Galaxies 2021, 9(2), 28; https://doi.org/10.3390/galaxies9020028 - 28 Apr 2021

Cited by 23 | Viewed by 2643

Abstract

Eclipsing systems are essential objects for understanding the properties of stars and stellar systems. Eclipsing systems with pulsating components are furthermore advantageous because they provide accurate constraints on the component properties, as well as a complementary method for pulsation mode determination, crucial for [...] Read more.

Eclipsing systems are essential objects for understanding the properties of stars and stellar systems. Eclipsing systems with pulsating components are furthermore advantageous because they provide accurate constraints on the component properties, as well as a complementary method for pulsation mode determination, crucial for precise asteroseismology. The outcome of space missions aiming at delivering high-accuracy light curves for many thousands of stars in search of planetary systems has also generated new insights in the field of variable stars and revived the interest of binary systems in general. The detection of eclipsing systems with pulsating components has particularly benefitted from this, and progress in this field is growing fast. In this review, we showcase some of the recent results obtained from studies of eclipsing systems with pulsating components based on data acquired by the space missions Kepler or TESS. We consider different system configurations including semi-detached eclipsing binaries in (near-)circular orbits, a (near-)circular and non-synchronized eclipsing binary with a chemically peculiar component, eclipsing binaries showing the heartbeat phenomenon, as well as detached, eccentric double-lined systems. All display one or more pulsating component(s). Among the great variety of known classes of pulsating stars, we discuss unevolved or slightly evolved pulsators of spectral type B, A or F and red giants with solar-like oscillations. Some systems exhibit additional phenomena such as tidal effects, angular momentum transfer, (occasional) mass transfer between the components and/or magnetic activity. How these phenomena and the orbital changes affect the different types of pulsations excited in one or more components, offers a new window of opportunity to better understand the physics of pulsations. Full article

(This article belongs to the Special Issue Astrophysics of Eclipsing Binaries in the Era of Space-Borne Telescopes)

17 pages, 9296 KiB

Open AccessFeature PaperArticle

Frequency and Time Dependence of Linear Polarization in Turbulent Jets of Blazars

by Alan P. Marscher and Svetlana G. Jorstad

Galaxies 2021, 9(2), 27; https://doi.org/10.3390/galaxies9020027 - 27 Apr 2021

Cited by 37 | Viewed by 2875

Abstract

Time-variable polarization is an extremely valuable observational tool to probe the dynamical physical conditions of blazar jets. Since 2008, we have been monitoring the flux and linear polarization of a sample of gamma-ray bright blazars at optical frequencies. Some of the observations were [...] Read more.

Time-variable polarization is an extremely valuable observational tool to probe the dynamical physical conditions of blazar jets. Since 2008, we have been monitoring the flux and linear polarization of a sample of gamma-ray bright blazars at optical frequencies. Some of the observations were performed on nightly or intra-night time-scales in four optical bands, providing information on the frequency and time dependence of the polarization. The observed behavior is similar to that found in simulations of turbulent plasma in a relativistic jet that contains a standing shock and/or a helical background magnetic field. Similar simulations predict the characteristics of X-ray synchrotron polarization of blazars that will be measured in the future by the Imaging X-ray Polarimetry Explorer (IXPE). Full article

(This article belongs to the Special Issue Polarimetry as a Probe of Magnetic Fields in AGN Jets)

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9 pages, 289 KiB

Open AccessFeature PaperArticle

Constraints on Space-Time-Matter Theory in the Framework of the Standard-Model Extension

by James Overduin, Hamna Ali and Francis Walz

Galaxies 2021, 9(2), 26; https://doi.org/10.3390/galaxies9020026 - 26 Apr 2021

Cited by 3 | Viewed by 2732

Abstract

We use experimental limits on Lorentz violation within the framework of the Standard-Model Extension to derive quantitative constraints on Space-Time-Matter theory, a version of Kaluza–Klein theory in which the cylinder condition is relaxed so that four-dimensional physics can in principle depend on the [...] Read more.

We use experimental limits on Lorentz violation within the framework of the Standard-Model Extension to derive quantitative constraints on Space-Time-Matter theory, a version of Kaluza–Klein theory in which the cylinder condition is relaxed so that four-dimensional physics can in principle depend on the extra coordinates. The extra dimensions are not necessarily compact or length-like. We find that the associated variation in fundamental quantities such as rest mass must occur slowly, on cosmological scales. Full article

(This article belongs to the Special Issue Lorentz Violation in Astroparticles and Gravitational Waves)

10 pages, 812 KiB

Open AccessArticle

Properties of Faint X-ray Activity of XTE J1908+094 in 2019

by Debjit Chatterjee, Arghajit Jana, Kaushik Chatterjee, Riya Bhowmick, Sujoy Kumar Nath, Sandip K. Chakrabarti, A. Mangalam and Dipak Debnath

Galaxies 2021, 9(2), 25; https://doi.org/10.3390/galaxies9020025 - 16 Apr 2021

Cited by 6 | Viewed by 2165

Abstract

We study the properties of the faint X-ray activity of Galactic transient black hole candidate XTE J1908+094 during its 2019 outburst. Here, we report the results of detailed spectral and temporal analysis during this outburst using observations from Nuclear Spectroscopic Telescope Array (NuSTAR) [...] Read more.

We study the properties of the faint X-ray activity of Galactic transient black hole candidate XTE J1908+094 during its 2019 outburst. Here, we report the results of detailed spectral and temporal analysis during this outburst using observations from Nuclear Spectroscopic Telescope Array (NuSTAR). We have not observed any quasi-periodic-oscillations (QPOs) in the power density spectrum (PDS). The spectral study suggests that the source remained in the softer (more precisely, in the soft–intermediate) spectral state during this short period of X-ray activity. We notice a faint but broad Fe K

α

emission line at around 6.5 keV. We also estimate the probable mass of the black hole to be

6 . 5_{- 0.7}^{+ 0.5} M_{⊙}

, with 90% confidence. Full article

(This article belongs to the Special Issue Global Understanding of Accretion and Ejection around Black Holes)

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53 pages, 947 KiB

Open AccessArticle

History of Astronomy in Australia: Big-Impact Astronomy from World War II until the Lunar Landing (1945–1969)

by Alister W. Graham, Katherine H. Kenyon, Lochlan J. Bull, Visura C. Lokuge Don and Kazuki Kuhlmann

Galaxies 2021, 9(2), 24; https://doi.org/10.3390/galaxies9020024 - 31 Mar 2021

Viewed by 5706

Abstract

Radio astronomy commenced in earnest after World War II, with Australia keenly engaged through the Council for Scientific and Industrial Research. At this juncture, Australia’s Commonwealth Solar Observatory expanded its portfolio from primarily studying solar phenomena to conducting stellar and extragalactic research. Subsequently, [...] Read more.

Radio astronomy commenced in earnest after World War II, with Australia keenly engaged through the Council for Scientific and Industrial Research. At this juncture, Australia’s Commonwealth Solar Observatory expanded its portfolio from primarily studying solar phenomena to conducting stellar and extragalactic research. Subsequently, in the 1950s and 1960s, astronomy gradually became taught and researched in Australian universities. However, most scientific publications from this era of growth and discovery have no country of affiliation in their header information, making it hard to find the Australian astronomy articles from this period. In 2014, we used the then-new Astrophysics Data System (ADS) tool Bumblebee to overcome this challenge and track down the Australian-led astronomy papers published during the quarter of a century after World War II, from 1945 until the lunar landing in 1969. This required knowledge of the research centres and facilities operating at the time, which are briefly summarised herein. Based on citation counts—an objective, universally-used measure of scientific impact—we report on the Australian astronomy articles which had the biggest impact. We have identified the top-ten most-cited papers, and thus also their area of research, from five consecutive time-intervals across that blossoming quarter-century of astronomy. Moreover, we have invested a substantial amount of time researching and providing a small tribute to each of the 62 scientists involved, including several trail-blazing women. Furthermore, we provide an extensive list of references and point out many interesting historical connections and anecdotes. Full article

(This article belongs to the Collection A Trip across the Universe: Our Present Knowledge and Future Perspectives)

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10 pages, 534 KiB

Open AccessReview

Big and Young Supermassive Black Holes in the Early Universe

by Tullia Sbarrato

Galaxies 2021, 9(2), 23; https://doi.org/10.3390/galaxies9020023 - 30 Mar 2021

Cited by 12 | Viewed by 2589

Abstract

Blazars are active galactic nuclei characterized by relativistic jets launched in the vicinity of the central engine (i.e., a supermassive black hole), which are oriented close to our line of sight. Their peculiar orientation makes them very efficient tracers of the overall jetted [...] Read more.

Blazars are active galactic nuclei characterized by relativistic jets launched in the vicinity of the central engine (i.e., a supermassive black hole), which are oriented close to our line of sight. Their peculiar orientation makes them very efficient tracers of the overall jetted population, and due to their brightness they can be visible up to very high redshifts. A deep knowledge of these objects can provide fundamental clues to the models of formation and growth of the first supermassive black holes, but the search for them in the early Universe must be careful and follow a systematic approach. The discovery in the last ∼15 years of extremely massive blazars at very high redshifts (

M_{BH} > 10^{9} M_{⊙}, z > 4

) revolutionized our perception of their earliest evolution: there seem to be different formation epochs for extremely massive black holes hosted in jetted (

z \sim 4

) and non-jetted (

z \sim 2.5

) systems. This is not easy to explain, since one would expect jetted sources to accrete less efficiently. Small differences in the population are also derived from the search for such high-z sources. We will go through the open questions in order to understand where the common knowledge stands and which steps must be taken to better understand the formation and common evolution of supermassive black holes and jets in the early Universe. Full article

(This article belongs to the Special Issue Jet Physics of Accreting Super Massive Black Holes)

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24 pages, 391 KiB

Open AccessArticle

Neutrino Charge in a Magnetized Media

by Avijit K. Ganguly, Venktesh Singh, Damini Singh and Ankur Chaubey

Galaxies 2021, 9(2), 22; https://doi.org/10.3390/galaxies9020022 - 30 Mar 2021

Cited by 1 | Viewed by 2736

Abstract

In the presence of a thermal medium or an external electro-magnetic field, neutrinos can interact with photon, mediated by the corresponding charged leptons (real or virtual). The effect of a medium or an electromagnetic field is two-fold—to induce an effective

ν γ

vertex [...] Read more.

In the presence of a thermal medium or an external electro-magnetic field, neutrinos can interact with photon, mediated by the corresponding charged leptons (real or virtual). The effect of a medium or an electromagnetic field is two-fold—to induce an effective

ν γ

vertex and to modify the dispersion relations of all the particles involved to render the processes kinematically viable. It has already been noted that, in a medium, neutrinos acquire an effective charge, which in the standard model of electroweak interaction comes from the vector type vertex of weak interaction. On the other hand, in a magnetized plasma, the axial vector part also starts contributing to the effective charge of a neutrino. This contribution corresponding to the axial vector part in the interaction Lagrangian is denoted as the axial polarisation tensor. In this note, we outline the calculation of the axial polarization tensor to odd and even powers in

e B

. We further show its gauge invariance properties. Finally, we infer upon the zero external momentum limit of this axial polarisation tensor. Full article

(This article belongs to the Collection A Trip across the Universe: Our Present Knowledge and Future Perspectives)

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14 pages, 1386 KiB

Open AccessArticle

Changing Accretion Geometry of Seyfert 1 Mrk 335 with NuSTAR: A Comparative Study

by Santanu Mondal and C. S. Stalin

Galaxies 2021, 9(2), 21; https://doi.org/10.3390/galaxies9020021 - 25 Mar 2021

Cited by 7 | Viewed by 2570

Abstract

We present a detailed spectral study of the narrow-line Seyfert 1 galaxy, Markarian 335, using eight epoch observations made between 2013 and 2020 with the Nuclear Spectroscopic Telescope Array. The source was variable during this period both in spectral flux and flow [...] Read more.

We present a detailed spectral study of the narrow-line Seyfert 1 galaxy, Markarian 335, using eight epoch observations made between 2013 and 2020 with the Nuclear Spectroscopic Telescope Array. The source was variable during this period both in spectral flux and flow geometry. We estimated the height of the Compton cloud from the model fitted parameters for the whole observation period. This allowed us to investigate the underlying physical processes that drive the variability in X-rays. Our model fitted mass varies in a narrow range, between

(2.44 \pm 0.45 - 3.04 \pm 0.56) \times 10^{7} M_{⊙}

, however, given the large error bars, it is consistent with being constant and is in agreement with that known from optical reverberation mapping observations. The disk mass accretion rate reached a maximum of 10% of the Eddington rate during June 2013. Our study sheds light on mass outflows from the system and also compares different aspects of accretion with X-ray binaries. Full article

(This article belongs to the Special Issue Global Understanding of Accretion and Ejection around Black Holes)

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12 pages, 471 KiB

Open AccessArticle

A Possible Quasi-Periodic Oscillation in the X-ray Emission of 3C 120

by Aditi Agarwal, Priyanka Rani, Raj Prince, C. S. Stalin, G. C. Anupama and Vipul Agrawal

Galaxies 2021, 9(2), 20; https://doi.org/10.3390/galaxies9020020 - 25 Mar 2021

Cited by 6 | Viewed by 3314

Abstract

We present here the detection of a possible quasi-periodic oscillation (QPO) signal in the X-ray light curve of the active galactic nucleus 3C 120, a broad line radio galaxy at z = 0.033. The hint of a QPO at the 3

σ

level [...] Read more.

We present here the detection of a possible quasi-periodic oscillation (QPO) signal in the X-ray light curve of the active galactic nucleus 3C 120, a broad line radio galaxy at z = 0.033. The hint of a QPO at the 3

σ

level at 7.1 × 10

^{- 6}

Hz (∼1.65 days) was detected based on the analysis of X-ray data acquired in the 3–79 keV band by the Nuclear Spectroscopic Telescope Array (NuSTAR). The data, when processed separately in the soft (3–10 keV), hard (10–79 keV) and the total (3–79 keV) bands using four different techniques, namely discrete correlation function, Lomb Scargle periodogram, structure–function, and power spectral density indicated the presence of a QPO. 3C 120 very well fits in the negative correlation in the frequency of the QPO versus the black hole mass (F

_{Q P O}

versus M

_{B H}

) diagram known for stellar-mass and supermassive black hole sources. Considering the observed signs of QPO to represent the innermost stable orbit of the accretion disk, we found a black hole mass of

1.9 \times 10^{9}

M

_{⊙}

for a Kerr black hole and

3.04 \times 10^{8}

M

_{⊙}

for a Schwarzschild black hole. This deduced black hole mass from QPO measurement is a few times larger than the black hole mass obtained from reverberation mapping observations. Full article

(This article belongs to the Collection A Trip across the Universe: Our Present Knowledge and Future Perspectives)

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