Galaxies

41 pages, 12018 KB

Open AccessReview

Timing Analysis of Black Hole X-Ray Binaries with Insight-HXMT

by Haifan Zhu and Wei Wang

Galaxies 2025, 13(5), 111; https://doi.org/10.3390/galaxies13050111 - 19 Sep 2025

The Hard X-ray Modulation Telescope (HXMT), China’s first X-ray astronomy satellite, has significantly contributed to the study of fast variability in black hole X-ray binaries through its broad energy coverage (1–250 keV), high timing resolution, and sensitivity to hard X-rays. This review presents [...] Read more.

The Hard X-ray Modulation Telescope (HXMT), China’s first X-ray astronomy satellite, has significantly contributed to the study of fast variability in black hole X-ray binaries through its broad energy coverage (1–250 keV), high timing resolution, and sensitivity to hard X-rays. This review presents a comprehensive overview of timing analysis techniques applied to black hole X-ray binaries using Insight-HXMT data. We introduce the application and comparative strengths of several time-frequency analysis methods, including traditional Fourier analysis, wavelet transform, bicoherence analysis, and Hilbert-Huang transform. These methods offer complementary insights into the non-stationary and nonlinear variability patterns observed in black hole X-ray binaries, particularly during spectral state transitions and quasi-periodic oscillations. We discuss how each technique has been employed in recent Insight-HXMT studies to characterize timing features such as low-frequency QPOs, phase lags, and power spectrum evolution across different energy bands. Moreover, we present novel phenomena revealed by Insight-HXMT observations, including the detection of high-energy QPOs, spectral parameter modulation with QPO phase, and a new classification scheme for QPO types. The integration of multiple analysis methods enables a more nuanced understanding of the accretion dynamics and the geometry of the inner accretion flow, shedding light on fundamental physical processes in relativistic environments. Full article

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27 pages, 1734 KB

Open AccessArticle

Comparative Photometry of the Quiet Quasar PDS 456 and the Radio-Loud Blazar 3C 273

by Alberto Silva Betzler, Ingrid dos Santos Delfino, Agábio Brasil dos Santos, Roberto Mendes Dias and Orahcio Felicio de Sousa

Galaxies 2025, 13(5), 110; https://doi.org/10.3390/galaxies13050110 - 15 Sep 2025

Abstract

A comparative analysis of the photometric variability of the blazar 3C 273 and the quasar PDS 456 using multi-band data from ground- and space-based platforms (2015–2025) reveals contrasting behaviors. For 3C 273, a statistically significant secular dimming was detected in the ATLASc [...] Read more.

A comparative analysis of the photometric variability of the blazar 3C 273 and the quasar PDS 456 using multi-band data from ground- and space-based platforms (2015–2025) reveals contrasting behaviors. For 3C 273, a statistically significant secular dimming was detected in the ATLASc-band light curve

((5.6 \pm 0.2) \times 10^{- 4} mag {day}^{- 1})

and confirmed by Johnson–Cousins V-band photometry. Ten optical flares were identified, two coinciding with Fermi gamma-ray enhancements, suggesting a synchrotron origin linked to jet activity. A significant bluer-when-brighter trend (

ρ = - 0.54

) was found relative to the o-band, and several color extrema align with gamma-ray activity, reinforcing the nonthermal interpretation. In contrast, PDS 456 exhibits a statistically significant secular brightening in the o-band

((- 3.1 \pm 0.2) \times 10^{- 5} mag {day}^{- 1})

and 75 optical flares, four coinciding with UV flares observed by Swift/UVOT. The

c - o

color index displays a non-Gaussian distribution with asymmetric reddening and blueing episodes. An extreme reddening event aligns with a strong UV flare, suggesting transient inner-disk heating. These results indicate jet-dominated variability in 3C 273 and disk-driven variability in PDS 456, highlighting distinct physical mechanisms in radio-loud versus radio-quiet active galactic nuclei. Full article

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17 pages, 1635 KB

Open AccessReview

Optical and X-Ray Variability of Gamma Cas Analogs

by Alexander Kholtygin

Galaxies 2025, 13(5), 109; https://doi.org/10.3390/galaxies13050109 - 15 Sep 2025

Abstract

γ

Cas analogs is an enigmatic group of Be stars with unusually hard X-rays and an X-ray luminosity of

10^{31}

–

10^{33}

erg

s^{- 1}

, which is higher than a typical value for classical Be stars. The evolutionary status [...] Read more.

γ

Cas analogs is an enigmatic group of Be stars with unusually hard X-rays and an X-ray luminosity of

10^{31}

–

10^{33}

erg

s^{- 1}

, which is higher than a typical value for classical Be stars. The evolutionary status of these mysterious objects and the nature of their X-ray emission remains disputable. I suppose that our understanding of this mystery is in the detailed studies of their optical and X-ray variability on the various time scales from very short to very long. In the present paper the optical and X-ray spectral and photometric observations of these stars are reviewed. The contemporary assumptions on the mechanisms of X-ray radiation generation of

γ

Cas analogs are discussed. It is concluded that the analysis of the binarity of

γ

Cas analog helps to understand their inexplicable nature. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

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17 pages, 2665 KB

Open AccessArticle

Testing CCC+TL Cosmology with Galaxy Rotation Curves

by Rajendra P. Gupta

Galaxies 2025, 13(5), 108; https://doi.org/10.3390/galaxies13050108 - 12 Sep 2025

Abstract

This paper aims to explore whether astrophysical observations, primarily galaxy rotation curves, result from covarying coupling constants (CCC) rather than from dark matter. We have shown in earlier papers that cosmological observations, such as supernovae type 1a (Pantheon+), the small size of galaxies [...] Read more.

This paper aims to explore whether astrophysical observations, primarily galaxy rotation curves, result from covarying coupling constants (CCC) rather than from dark matter. We have shown in earlier papers that cosmological observations, such as supernovae type 1a (Pantheon+), the small size of galaxies at cosmic dawn, baryon acoustic oscillations (BAO), the sound horizon in the cosmic microwave background (CMB), and time dilation effect, can be easily accounted for without requiring dark energy and dark matter when coupling constants are permitted to evolve in an expanding Universe, as predicted by Dirac, and the redshift is considered jointly due to the Universe’s expansion and Zwicky’s tired light (TL) effect. Here, we show that the CCC parameter α is responsible for generating the illusion of dark matter and dark energy, which we call α-matter and α-energy, and is influenced by the baryonic matter density distribution. While cosmologically α is a constant determined for the homogenous and isotropic Universe, e.g., by fitting Pantheon+ data, it can vary locally due to the extreme anisotropy of the matter distribution. Thus, in high baryonic density regions, one expects α-matter and α-energy densities to be relatively low and vice versa. We present its application to a few galaxy rotation curves from the SPARC database and find the results promising. Full article

(This article belongs to the Special Issue Alternative Interpretations of Observed Galactic Behaviors)

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14 pages, 247 KB

Open AccessArticle

The Murchison Widefield Array Enters Adolescence: A Personal Review of the Early Years of Operations

by Steven J. Tingay

Galaxies 2025, 13(5), 107; https://doi.org/10.3390/galaxies13050107 - 11 Sep 2025

Abstract

The Murchison Widefield Array (MWA) is a low frequency radio interferometer designed and developed by an international consortium, operated on behalf of the consortium by Curtin University. The MWA is a Precursor for the low frequency Square Kilometre Array (SKA) and is located [...] Read more.

The Murchison Widefield Array (MWA) is a low frequency radio interferometer designed and developed by an international consortium, operated on behalf of the consortium by Curtin University. The MWA is a Precursor for the low frequency Square Kilometre Array (SKA) and is located at the SKA site in Western Australia, Inyarrimanha Ilgari Bundara, the CSIRO Murchison Radio-astronomy Observatory. Commencing science operations in 2013 after an extended development period, the MWA has performed observations over a wide set of science objectives, has been upgraded multiple times, and has played a fundamental role in the development of the low frequency SKA. As MWA Program Manager from 2008 to 2011, as Director from 2011 until 2015, and then again from 2021 to the present, I describe some personal reflections on the MWA’s activities and successes in these different dimensions, as well as my view of some of the approaches that have enabled these successes. I offer some of the lessons I’ve perceived over the last 17+ years in the project. Full article

(This article belongs to the Special Issue Recent Advances in Radio Astronomy)

28 pages, 1155 KB

Open AccessArticle

Dynamics of Compact Stellar Solutions Admitting Anisotropic Fluid: A Comparative Analysis of GR and Non-Conserved Rastall Gravity

by Tayyab Naseer, Muhammad Sharif, Fatima Chand, Baiju Dayanandan and Ali Elrashidi

Galaxies 2025, 13(5), 106; https://doi.org/10.3390/galaxies13050106 - 9 Sep 2025

Abstract

This study proposes a couple of analytical solutions that characterize the anisotropic dense celestial bodies within the Rastall-Rainbow theoretical framework. The analysis assumes a static spherically symmetric matter distribution and derives the corresponding modified field equations. By utilizing well-established radial metric functions and [...] Read more.

This study proposes a couple of analytical solutions that characterize the anisotropic dense celestial bodies within the Rastall-Rainbow theoretical framework. The analysis assumes a static spherically symmetric matter distribution and derives the corresponding modified field equations. By utilizing well-established radial metric functions and merging them with the two principal pressures, we obtain differential equations related to the time component. Subsequently, we perform the integration of these equations to determine the remaining geometric quantity that encompasses various integration constants. The proposed interior solutions are then matched with the Schwarzschild exterior metric at the boundary of the compact object, facilitating the determination of the constants. Additionally, the incorporation of the non-minimal coupling parameter into these constants is accomplished by enforcing the null radial pressure at the boundary. Afterwards, we rigorously examine the physical characteristics and critical stability conditions of the formulated models under observational data from two pulsars, say 4U 1820-30 and LMC X-4. It is concluded that our models are well-aligned with essential criteria required to ensure the physical viability of stellar structures, subject to specific parametric values. Full article

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8 pages, 349 KB

Open AccessArticle

Photometric Monitoring of the First Eclipsing Binary Be Star: V658 Car

by Tajan H. de Amorim, Alex C. Carciofi, Alexandre Zanardo, Carlos Colesanti, Cristóvão Jacques, Denis Kulh, João Antonio Mattei, Marcelo Domingues, Marco Rocca, Sérgio Silva, Tasso Napoleão and Jonathan Labadie-Bartz

Galaxies 2025, 13(5), 105; https://doi.org/10.3390/galaxies13050105 - 8 Sep 2025

Abstract

V658 Car is the first known eclipsing binary system involving a classical Be star and an sdOB companion, offering a unique opportunity to study disk physics and binary interactions in unprecedented detail. From TESS data and multi-color observations from the comissão para a [...] Read more.

V658 Car is the first known eclipsing binary system involving a classical Be star and an sdOB companion, offering a unique opportunity to study disk physics and binary interactions in unprecedented detail. From TESS data and multi-color observations from the comissão para a colaboração entre profissionais e amadores collaboration, we analyze the system’s color–magnitude diagram and compare it with radiative transfer models that include the Be star, its circumstellar disk, and the sdOB companion. While the stellar eclipses are well reproduced, two features observed in the multi-color photometry challenge the current modeling paradigm: the discrepancy between the observed reddening and the modeled blueing during the first attenuation phase and the complete lack of modeled attenuation around the second stellar eclipse. These issues highlight the need for more sophisticated modeling approaches to capture the complex interplay between disk opacity and binary dynamics. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

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7 pages, 232 KB

Open AccessArticle

One-Dimensional Analytical Solutions of the Transport Equations for Incompressible Magnetohydrodynamic (MHD) Turbulence

by Bingbing Wang, Gary P. Zank, Laxman Adhikari and Swati Sharma

Galaxies 2025, 13(5), 104; https://doi.org/10.3390/galaxies13050104 - 3 Sep 2025

Abstract

We derive one-dimensional (1D) analytical solutions for the transport equations of incompressible magnetohydrodynamic (MHD) turbulence, including the Elsässer energies and the correlation lengths. The solutions are suitable for an arbitrary given background convection speed and Alfvén speed profiles but require near equipartition of [...] Read more.

We derive one-dimensional (1D) analytical solutions for the transport equations of incompressible magnetohydrodynamic (MHD) turbulence, including the Elsässer energies and the correlation lengths. The solutions are suitable for an arbitrary given background convection speed and Alfvén speed profiles but require near equipartition of turbulent kinetic energy and magnetic field energy. These analytical solutions provide a simple tool to investigate the evolution of turbulence and resulting energetic particle diffusion coefficients in various space and astrophysical environments that possess simple geometry. Full article

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65 pages, 19950 KB

Open AccessReview

A Review of Lunar Environment and In-Situ Resource Utilization for Achieving Long-Term Lunar Habitation

by Chang Wang, Guoqing Zhang, Yaohui Wang and Lei Song

Galaxies 2025, 13(5), 103; https://doi.org/10.3390/galaxies13050103 - 3 Sep 2025

Abstract

The Moon’s unique environment, strategic position, and resource abundance make it a key target for deep space exploration. As lunar missions evolve from research to long-term habitation, leveraging local resources is essential to reduce dependence on Earth-based supply chains. Despite significant studies on [...] Read more.

The Moon’s unique environment, strategic position, and resource abundance make it a key target for deep space exploration. As lunar missions evolve from research to long-term habitation, leveraging local resources is essential to reduce dependence on Earth-based supply chains. Despite significant studies on the lunar environment and in-situ resource utilization (ISRU), a unified framework that integrates these findings remains lacking. This article addresses this gap by systematically reviewing and synthesizing current research to support sustainable lunar development. It first explores the use of extreme lunar environmental factors such as thermal gradients, weak magnetic fields, subsurface cavities, and geographic advantages. It then examines lunar water and mineral resource development, highlighting methods for detection, extraction, purification, and storage, alongside strategies for utilizing various minerals. The article further reviews recent progress in in-situ manufacturing, construction technologies, energy regeneration, and closed-loop life-support systems vital for lunar base establishment. These advances are crucial for creating sustainable infrastructure and maintaining life on the Moon. Finally, the paper outlines the challenges and limitations associated with ISRU and offers perspectives on future directions, aiming to inform the design of next-generation lunar missions and facilitate permanent human presence on the Moon. Full article

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46 pages, 3727 KB

Open AccessReview

Jet Feedback on kpc Scales: A Review

by Dipanjan Mukherjee

Galaxies 2025, 13(5), 102; https://doi.org/10.3390/galaxies13050102 - 2 Sep 2025

Abstract

Relativistic jets from AGN are an important driver of feedback in galaxies. They interact with their environments over a wide range of physical scales during their lifetime, and an understanding of these interactions is crucial for unraveling the role of supermassive black holes [...] Read more.

Relativistic jets from AGN are an important driver of feedback in galaxies. They interact with their environments over a wide range of physical scales during their lifetime, and an understanding of these interactions is crucial for unraveling the role of supermassive black holes in shaping galaxy evolution. The impact of such jets has been traditionally considered in the context of heating large-scale environments. However, in the last few decades, there has been additional focus on the immediate impact of jet feedback on the host galaxy itself. In this review, we outline the development of various numerical simulations from the onset of research on jets to the present day, where sophisticated numerical techniques have been employed to study jet feedback, including a range of physical processes. The jets can act as important agents of energy injection into a host’s ISM, as confirmed in both observations of multi-phase gas as well as in simulations. Such interactions have the potential to impact the kinematics of the gas as well as star formation. We summarize recent results from simulations of jet feedback on kpc scales and outline the broader implications for observations and galaxy evolution. Full article

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17 pages, 3444 KB

Open AccessArticle

Determination of Orbital Parameters of Binary Star Systems Using the MCMC Method

by Nadezhda L. Vaidman, Shakhida T. Nurmakhametova, Anatoly S. Miroshnichenko, Serik A. Khokhlov, Aldiyar T. Agishev, Azamat A. Khokhlov, Yeskendyr K. Ashimov and Berik S. Yermekbayev

Galaxies 2025, 13(5), 101; https://doi.org/10.3390/galaxies13050101 - 2 Sep 2025

Abstract

We present new spectroscopic orbits for the bright binaries Mizar B, 3 Pup,

ν

Gem, 2 Lac, and

ϕ

Aql. Our analysis is based on medium-resolution (

R \approx

12,000) échelle spectra obtained with the 0.81-m telescope and fiber-fed eShel spectrograph of the [...] Read more.

We present new spectroscopic orbits for the bright binaries Mizar B, 3 Pup,

ν

Gem, 2 Lac, and

ϕ

Aql. Our analysis is based on medium-resolution (

R \approx

12,000) échelle spectra obtained with the 0.81-m telescope and fiber-fed eShel spectrograph of the Three College Observatory (Greensboro, NC, USA) between 2015 and 2024. Orbital elements were inferred with an affine-invariant Markov-chain Monte-Carlo sampler; convergence was verified through the integrated autocorrelation time and the Gelman–Rubin statistic. Errors quote the 16th–84th-percentile credible intervals. Compared with previously published orbital solutions for the studied stars, our method improves the root-mean-square residuals by 25–50% and bring the 1

σ

uncertainties on the radial velocity (RV) semi-amplitudes down to 0.02–0.15 km

s^{- 1}

. These gains translate into markedly tighter mass functions and systemic RVs, providing a robust dynamical baseline for future interferometric and photometric studies. A complete Python analysis pipeline is openly available in a GitHub repository, ensuring full reproducibility. The results demonstrate that a Bayesian RV analysis with well-motivated priors and rigorous convergence checks yields orbital parameters that are both more precise and more reproducible than previous determinations, while offering fully transparent uncertainty budgets. Full article

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11 pages, 260 KB

Open AccessArticle

Energy of Photons in Expanding Spacetime: Comparing FLRW and Conformal Cosmology Metrics

by Václav Vavryčuk

Galaxies 2025, 13(5), 100; https://doi.org/10.3390/galaxies13050100 - 2 Sep 2025

Abstract

We investigate the behaviour of photons in Riemann spacetime, focusing on how their velocity and energy are affected by cosmic expansion. Specifically, we examine the differences in energy conservation depending on the cosmological model. Our findings indicate that photons exhibit fundamentally different behaviour [...] Read more.

We investigate the behaviour of photons in Riemann spacetime, focusing on how their velocity and energy are affected by cosmic expansion. Specifically, we examine the differences in energy conservation depending on the cosmological model. Our findings indicate that photons exhibit fundamentally different behaviour based on the chosen metric. In the standard

Λ

CDM model, which relies on the Friedmann–Lemaître–Robertson–Walker (FLRW) metric, the energy conservation law for redshifted photons is violated. However, in a cosmological model based on the conformal cosmology (CC) metric, this law remains valid. The CC metric offers additional advantages, as it accurately reproduces the cosmological redshift, cosmic time dilation observed in Type Ia supernova light curves, and flat galaxy rotation curves without requiring the introduction of dark matter. These findings underscore the potential significance of the CC metric in cosmological applications. Full article

30 pages, 18915 KB

Open AccessReview

The Astronomical Hub: A Unified Ecosystem for Modern Astronomical Research

by Yerlan Aimuratov, Vitaliy Kim, Aleksander Serebryanskiy, Denis Yurin, Maxim Krugov, Chingiz Akniyazov, Saule Shomshekova, Maxim Makukov, Gaukhar Aimanova, Rashit Valiullin, Raushan Kokumbaeva, Alan Kazkenov and Chingis Omarov

Galaxies 2025, 13(5), 99; https://doi.org/10.3390/galaxies13050099 - 1 Sep 2025

Abstract

We present the conceptual framework of the Astronomical Hub (AstroHub), a unified platform combining various optical instruments at a single observatory. Its major approach lies in arranging conditions for research groups to install telescopes and equipment and participate in joint projects. AstroHub is [...] Read more.

We present the conceptual framework of the Astronomical Hub (AstroHub), a unified platform combining various optical instruments at a single observatory. Its major approach lies in arranging conditions for research groups to install telescopes and equipment and participate in joint projects. AstroHub is planned to integrate Virtual Observatory (VO) tools, FAIR data principles, and a telescope network to create a powerful and attractive ecosystem for both robust near-Earth object (NEO) monitoring and diverse deep space research. We provide an overview of the AstroHub development directions in the case study of the Assy-Turgen Observatory. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

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18 pages, 30275 KB

Open AccessArticle

RAD@home Citizen Science Discovery of Two Spiral Galaxies Where the 30–220 kpc Radio Lobes Are Possibly Shaped by Ram Pressure Stripping

by Prakash Apoorva, Ananda Hota, Pratik Dabhade, P. K. Navaneeth, Dhruv Nayak and Arundhati Purohit

Galaxies 2025, 13(5), 98; https://doi.org/10.3390/galaxies13050098 - 22 Aug 2025

Abstract

We report the RAD@home citizen science discovery of two rare spiral-host radio galaxies (NGC 3898 and WISEA J221656.57-132042434.1 or RAD-“Thumbs up” galaxy), both exhibiting asymmetric radio lobes extending over 30 to 220 kiloparsec scales. We present a multi-wavelength image analysis of these two [...] Read more.

We report the RAD@home citizen science discovery of two rare spiral-host radio galaxies (NGC 3898 and WISEA J221656.57-132042434.1 or RAD-“Thumbs up” galaxy), both exhibiting asymmetric radio lobes extending over 30 to 220 kiloparsec scales. We present a multi-wavelength image analysis of these two sources using radio, optical, and ultraviolet data. Both host galaxies are young, star-forming systems with asymmetric or distorted stellar disks. These disks show similarities to those in galaxies undergoing ram pressure stripping, and the radio morphologies resemble those of asymmetric or bent FR-II and wide-angle-tailed radio galaxies. We suggest that non-uniform gas density in the environment surrounding the ram pressure-stripped disks may contribute to the observed asymmetry in the size, shape, and brightness of bipolar radio lobes. Such environmental effects, when properly accounted for, could help explain many of the non-standard radio morphologies observed in Seyfert galaxies and in recently identified populations of galaxies with galaxy-scale radio jets, which are now being revealed through deep and sensitive radio surveys with uGMRT, MeerKAT, LOFAR, and, in the future, SKAO. These findings also underscore the potential of citizen science to complement professional research and data-driven approaches involving machine learning and artificial intelligence in the analysis of complex radio sources. Full article

(This article belongs to the Special Issue Recent Advances in Radio Astronomy)

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16 pages, 1035 KB

Open AccessArticle

Light Variability from UV to Near-Infrared in the Ap Star CU Vir Induced by Chemical Spots

by Yury Pakhomov, Ilya Potravnov and Tatiana Ryabchikova

Galaxies 2025, 13(4), 97; https://doi.org/10.3390/galaxies13040097 - 21 Aug 2025

Cited by 1

Abstract

Multiwavelength modelling of the light variations in the chemically peculiar star CU Vir is presented. The modelling is based on the recent Doppler Imaging of CU Vir, which provides maps of the surface distribution of Si, Fe, He, and Cr. Intensity maps in [...] Read more.

Multiwavelength modelling of the light variations in the chemically peculiar star CU Vir is presented. The modelling is based on the recent Doppler Imaging of CU Vir, which provides maps of the surface distribution of Si, Fe, He, and Cr. Intensity maps in both individual photometric filters and in the wide wavelength range from UV to NIR were calculated, taking into account the individual chemical abundances on the stellar surface. Comparison with observations revealed good agreement of both the light curves and their amplitude along the spectrum. Additionally, we analysed changes in the photometric period of the CU Vir from 1955 to 2022, including TESS measurements. The data of the last decades clearly indicate a gradual decrease in this period. Measurements of the CU Vir period over the next two decades will be crucial for verifying or refuting the periodic nature of its variations. Full article

(This article belongs to the Special Issue Stellar Spectroscopy, Molecular Astronomy and Atomic Astronomy)

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Graphical abstract

10 pages, 343 KB

Open AccessArticle

Distant Resolved Spectroscopic Binaries: Orbital Parallaxes Contradict Trigonometric Parallaxes

by Oleg Y. Malkov and Arseniy M. Sachkov

Galaxies 2025, 13(4), 96; https://doi.org/10.3390/galaxies13040096 - 21 Aug 2025

Abstract

Resolved spectroscopic binaries (RSB) are the only way (besides trigonometric parallax) to determine the dynamical, hypothesis-free distances to the stars of the galaxy. Analyzing the most comprehensive up-to-date data on RSB, we found that trigonometric parallaxes of all distant (

d > \approx

[...] Read more.

Resolved spectroscopic binaries (RSB) are the only way (besides trigonometric parallax) to determine the dynamical, hypothesis-free distances to the stars of the galaxy. Analyzing the most comprehensive up-to-date data on RSB, we found that trigonometric parallaxes of all distant (

d > \approx

0.5 kpc) binaries overestimate the distance by 10–50%. Such objects appear as single stars in Gaia and Hipparcos data, but their binarity can be detected/suspected by comparing trigonometric parallaxes in different data releases from these space missions. Full article

(This article belongs to the Special Issue Stellar Spectroscopy, Molecular Astronomy and Atomic Astronomy)

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33 pages, 489 KB

Open AccessReview

Multi-TeV Gamma Rays from GRB 221009A: Challenges for Emission Mechanisms, EBL Opacity, and Fundamental Physics

by Hassan Abdalla

Galaxies 2025, 13(4), 95; https://doi.org/10.3390/galaxies13040095 - 19 Aug 2025

Abstract

The detection of gamma-ray burst GRB 221009A has attracted significant attention due to its record brightness and first-ever detection of multi-TeV

γ

-rays from a GRB. Located at redshift

z = 0.151

, this event is relatively nearby by GRB standards yet remains [...] Read more.

The detection of gamma-ray burst GRB 221009A has attracted significant attention due to its record brightness and first-ever detection of multi-TeV

γ

-rays from a GRB. Located at redshift

z = 0.151

, this event is relatively nearby by GRB standards yet remains cosmologically distant, making the survival of multi-TeV photons surprising. The Large High Altitude Air Shower Observatory detected photons with energies up to ∼13 TeV during the early afterglow phase, challenging standard EBL models. We investigate whether several theoretical frameworks can explain this anomalous emission: reduced EBL opacity due to cosmic voids along the line of sight, novel emission mechanisms within the GRB environment, secondary

γ

-ray production through cosmic-ray cascades, and new physics scenarios involving Lorentz invariance violation or axion-like particles. Our analysis reveals areas of consensus regarding the exceptional nature of this event, while highlighting ongoing theoretical tensions about the dominant physical processes. We discuss the limitations of current models and identify specific observational signatures that future multi-wavelength and multi-messenger observations could provide to discriminate between competing explanations. The continued study of similar events with next-generation facilities will be crucial for resolving these theoretical challenges and advancing our understanding of extreme particle acceleration processes in astrophysical environments. Full article

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12 pages, 1683 KB

Open AccessArticle

An Array of Bulk Acoustic Wave Sensors as a High-Frequency Antenna for Gravitational Waves

by Giorgia Albani, Matteo Borghesi, Lucia Canonica, Rodolfo Carobene, Federico De Guio, Marco Faverzani, Elena Ferri, Raffaele Gerosa, Alessio Ghezzi, Andrea Giachero, Claudio Gotti, Danilo Labranca, Leonardo Mariani, Angelo Nucciotti, Gianluigi Pessina, Davide Rozza and Tommaso Tabarelli de Fatis

Galaxies 2025, 13(4), 94; https://doi.org/10.3390/galaxies13040094 - 15 Aug 2025

Abstract

In their simplest form, bulk acoustic wave (BAW) devices consist of a piezoelectric crystal between two electrodes that transduce the material’s vibrations into electrical signals. They are adopted in frequency control and metrology, with well-established standards at frequencies of 5 MHz and above. [...] Read more.

In their simplest form, bulk acoustic wave (BAW) devices consist of a piezoelectric crystal between two electrodes that transduce the material’s vibrations into electrical signals. They are adopted in frequency control and metrology, with well-established standards at frequencies of 5 MHz and above. Their use as a resonant-mass strain antenna for high-frequency gravitational waves has been recently proposed (Goryachev and Tobar, 2014). The estimated power spectral density sensitivity at the resonant frequencies is of the order of

10^{- 21} strain / \sqrt{Hz}

. In this paper, after introducing the science opportunity and potential of gravitational wave detection with BAWs, we describe the two-stage BAUSCIA project plan to build a multimode antenna based on commercial BAWs, followed by an optimized array of custom BAWs. We show that commercially available BAWs already provide sensitivity comparable to current experiments around 10 MHz. Finally, we outline options for optimization of custom devices to improve sensitivity in an unexplored region, probe multiple frequencies between 0.1 and 10 MHz, and target specific signals, such as post-merger emission from neutron stars or emission from various dark matter candidates. Full article

(This article belongs to the Special Issue Challenges for Third-Generation Gravitational Wave Detectors and Beyond)

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22 pages, 5394 KB

Open AccessArticle

Unveiling the Variability and Chemical Composition of AL Col

by Surath C. Ghosh, Santosh Joshi, Samrat Ghosh, Athul Dileep, Otto Trust, Mrinmoy Sarkar, Jaime Andrés Rosales Guzmán, Nicolás Esteban Castro-Toledo, Oleg Malkov, Harinder P. Singh, Kefeng Tan and Sarabjeet S. Bedi

Galaxies 2025, 13(4), 93; https://doi.org/10.3390/galaxies13040093 - 14 Aug 2025

Abstract

In this study, we present analysis of TESS photometry, spectral energy distribution (SED), high-resolution spectroscopy, and spot modeling of the

α^{2}

CVn-type star AL Col (HD 46462). The primary objective is to determine its fundamental physical parameters and investigate its surface activity [...] Read more.

In this study, we present analysis of TESS photometry, spectral energy distribution (SED), high-resolution spectroscopy, and spot modeling of the

α^{2}

CVn-type star AL Col (HD 46462). The primary objective is to determine its fundamental physical parameters and investigate its surface activity characteristics. Using TESS short-cadence (120 s) SAP flux, we identified a rotational frequency of 0.09655

d^{- 1}

(

P_{rot} = 10.35733

d). Wavelet analysis reveals that while the amplitudes of the harmonic components vary over time, the strength of the primary rotational frequency remains stable. A SED analysis of multi-band photometric data yields an effective temperature (

T_{eff}

) of 11,750 K. High-resolution spectroscopic observations covering wavelengthrange 4500–7000 Å provide refined estimates of

T_{eff}

= 13,814 ± 400 K,

log g

= 4.09 ± 0.08 dex, and

υ sin i

= 16 ± 1 km s⁻¹. Abundance analysis shows solar-like composition of O ii, Mg ii, S ii, and Ca ii, while helium is under-abundant by 0.62 dex. Rare earth elements (REEs) exhibit over-abundances of up to 5.2 dex, classifying the star as an Ap/Bp-type star. AL Col has a radius of

R = 3.74 \pm 0.48 R_{⊙}

, with its H–R diagram position estimating a mass of

M = 4.2 \pm 0.2 M_{⊙}

and an age of

0.12 \pm 0.01

Gyr, indicating that the star has slightly evolved from the main sequence. The TESS light curves were modeled using a three-evolving-spot configuration, suggesting the presence of differential rotation. This star is a promising candidate for future investigations of magnetic field diagnostics and the vertical stratification of chemical elements in its atmosphere. Full article

(This article belongs to the Special Issue Stellar Spectroscopy, Molecular Astronomy and Atomic Astronomy)

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