**Abstract**

## Journal Description

#
*Astronomy*

*Astronomy*is an international, peer-reviewed, open access journal on observational astronomy, theoretical astronomy and other specific subfields published quarterly online by MDPI.

**Open Access**— free for readers, with article processing charges (APC) paid by authors or their institutions.**Rapid Publication:**first decisions in 16 days; acceptance to publication in 5.8 days (median values for MDPI journals in the second half of 2023).**Recognition of Reviewers:**APC discount vouchers, optional signed peer review, and reviewer names published annually in the journal.*Astronomy*is a companion journal of*Universe*and*Aerospace*.

## Latest Articles

Deep Sky Objects Detection with Deep Learning for Electronically Assisted Astronomy
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Electronically Assisted Astronomy is a fascinating activity requiring suitable conditions and expertise to be fully appreciated. Complex equipment, light pollution around urban areas and lack of contextual information often prevents newcomers from making the most of their observations, restricting the field to a
[...] Read more.

Electronically Assisted Astronomy is a fascinating activity requiring suitable conditions and expertise to be fully appreciated. Complex equipment, light pollution around urban areas and lack of contextual information often prevents newcomers from making the most of their observations, restricting the field to a niche expert audience. With recent smart telescopes, amateur and professional astronomers can capture efficiently a large number of images. However, post-hoc verification is still necessary to check whether deep sky objects are visible in the produced images, depending on their magnitude and observation conditions. If this detection can be performed during data acquisition, it would be possible to configure the capture time more precisely. While state-of-the-art works are focused on detection techniques for large surveys produced by professional ground-based observatories, we propose in this paper several Deep Learning approaches to detect celestial targets in images captured with smart telescopes, with a F1-score between 0.4 and 0.62 on test data, and we experimented them during outreach sessions with public in Luxembourg Greater Region.
Full article

Open AccessBrief Report

Constraining the Inner Galactic DM Density Profile with H.E.S.S.
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**Jaume Zuriaga-Puig****Abstract**

In this short review, corresponding to a talk given at the conference “Cosmology 2023 in Miramare”, we combine an analysis of five regions observed by H.E.S.S. in the Galactic Center, intending to constrain the Dark Matter (DM) density profile in a WIMP annihilation
[...] Read more.

In this short review, corresponding to a talk given at the conference “Cosmology 2023 in Miramare”, we combine an analysis of five regions observed by H.E.S.S. in the Galactic Center, intending to constrain the Dark Matter (DM) density profile in a WIMP annihilation scenario. For the analysis, we include the state-of-the-art Galactic diffuse emission Gamma-optimized model computed with DRAGON and a wide range of DM density profiles from cored to cuspy profiles, including different kinds of DM spikes. Our results are able to constrain generalized NFW profiles with an inner slope $\gamma \gtrsim 1.3$ . When considering DM spikes, the adiabatic spike is completely ruled out. However, smoother spikes given by the interactions with the bulge stars are compatible if $\gamma \lesssim 0.8$ , with an internal slope of ${\gamma}_{\mathrm{sp-stars}}=1.5$ .
Full article

(This article belongs to the Special Issue Current Trends in Cosmology)

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Open AccessArticle

Cosmography of the Minimally Extended Varying Speed-of-Light Model
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Cited by 1
**Seokcheon Lee****Abstract**

Cosmography, as an integral branch of cosmology, strives to characterize the Universe without relying on pre-determined cosmological models. This model-independent approach utilizes Taylor series expansions around the current epoch, providing a direct correlation with cosmological observations and the potential to constrain theoretical models.
[...] Read more.

Cosmography, as an integral branch of cosmology, strives to characterize the Universe without relying on pre-determined cosmological models. This model-independent approach utilizes Taylor series expansions around the current epoch, providing a direct correlation with cosmological observations and the potential to constrain theoretical models. Various observable quantities in cosmology can be described as different combinations of cosmographic parameters. Furthermore, one can apply cosmography to models with a varying speed of light. In this case, the Hubble parameter can be expressed by the same combination of cosmographic parameters for both the standard model and varying speed-of-light models. However, for the luminosity distance, the two models are represented by different combinations of cosmographic parameters. Hence, luminosity distance might provide a method to constrain the parameters in varying speed-of-light models.
Full article

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Open AccessReview

Refracted Gravity Solutions from Small to Large Scales
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**Valentina Cesare****Abstract**

If visible matter alone is present in the Universe, general relativity (GR) and its Newtonian weak field limit (WFL) cannot explain several pieces of evidence, from the largest to the smallest scales. The most investigated solution is the cosmological model $\mathsf{\Lambda}$ cold dark
[...] Read more.

If visible matter alone is present in the Universe, general relativity (GR) and its Newtonian weak field limit (WFL) cannot explain several pieces of evidence, from the largest to the smallest scales. The most investigated solution is the cosmological model $\mathsf{\Lambda}$ cold dark matter ($\mathsf{\Lambda}$ CDM), where GR is valid and two dark components are introduced, dark energy (DE) and dark matter (DM), to explain the ∼70% and ∼25% of the mass–energy budget of the Universe, respectively. An alternative approach is provided by modified gravity theories, where a departure of the gravity law from $\mathsf{\Lambda}$ CDM is assumed, and no dark components are included. This work presents refracted gravity (RG), a modified theory of gravity formulated in a classical way where the presence of DM is mimicked by a gravitational permittivity $\u03f5\left(\rho \right)$ monotonically increasing with the local mass density $\rho $ , which causes the field lines to be refracted in small density environments. Specifically, the flatter the system the stronger the refraction effect and thus, the larger the mass discrepancy if interpreted in Newtonian gravity. RG presented several encouraging results in modelling the dynamics of disk and elliptical galaxies and the temperature profiles of the hot X-ray emitting gas in galaxy clusters and a covariant extension of the theory seems to be promising.
Full article

(This article belongs to the Special Issue Current Trends in Cosmology)

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Open AccessArticle

A Critical Examination of the Standard Cosmological Model: Toward a Modified Framework for Explaining Cosmic Structure Formation and Evolution
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**Robert Nyakundi Nyagisera**,**Dismas Wamalwa**,**Bernard Rapando**,**Celline Awino**and**Maxwell Mageto****Abstract**

This paper explores the fundamental cosmological principle, with a specific focus on the homogeneity and isotropy assumptions inherent in the Friedmann model that underpins the standard model. We propose a modified redshift model that is based on the spatial distribution of luminous matter,
[...] Read more.

This paper explores the fundamental cosmological principle, with a specific focus on the homogeneity and isotropy assumptions inherent in the Friedmann model that underpins the standard model. We propose a modified redshift model that is based on the spatial distribution of luminous matter, examining three key astronomical quantities: light intensity, number density, and the redshift of galaxies. Our analysis suggests that the model can account for cosmic accelerated expansion without the need for dark energy in the equations. Both simulations and analytical solutions reveal a unique pattern in the formation and evolution of cosmic structures, particularly in galaxy formation. This pattern shows a significant burst of activity between redshifts 0 < z < 0.4, which then progresses rapidly until approximately z ≈ 0.9, indicating that the majority of cosmic structures were formed during this period. Subsequently, the process slows down considerably, reaching a nearly constant rate until around z ≈ 1.6, after which a gradual decline begins. We also observe a distinctive redshift transition around z ≈ 0.9 before the onset of dark-matter-induced accelerated expansion. This transition is directly related to the matter density and is dependent on the geometry of the universe. The model’s ability to explain cosmic acceleration without requiring fine tuning of the cosmological constant highlights its novelty, providing a fresh perspective on the dynamic evolution of the universe.
Full article

(This article belongs to the Special Issue Current Trends in Cosmology)

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Open AccessArticle

Gravity on a Large Scale—Does It Necessarily Look like It Does on a Small Scale?
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**Jerzy Kijowski****Abstract**

The notion of a local inertial reference frame is thoroughly analyzed. Dynamics of a field of such frames is derived from the variational principle. It is shown that the resulting theory splits naturally into three sectors, one of which is purely gravitational. Field
[...] Read more.

The notion of a local inertial reference frame is thoroughly analyzed. Dynamics of a field of such frames is derived from the variational principle. It is shown that the resulting theory splits naturally into three sectors, one of which is purely gravitational. Field dynamics in this sector, equivalent to Einstein’s vacuum equations, is obtained

*unambiguously*and admits no*ad hoc*corrections. The cosmological constant is an essential element of this construction and cannot be removed. It has been shown that the second sector of this theory corresponds to electrodynamics, while the last sector could possibly describe dark matter. Full articleOpen AccessBrief Report

Possible Tests of Fundamental Physics with GINGER
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**Giuseppe Di Somma**,**Carlo Altucci**,**Francesco Bajardi**,**Andrea Basti**,**Nicolò Beverini**,**Salvatore Capozziello**,**Giorgio Carelli**,**Simone Castellano**,**Donatella Ciampini**,**Gaetano De Luca**,**Angela D. V. Di Virgilio**,**Francesco Fuso**,**Francesco Giovinetti**,**Enrico Maccioni**,**Paolo Marsili**,**Antonello Ortolan**,**Alberto Porzio**,**Matteo Luca Ruggiero**and**Raffaele Velotta****Abstract**

The GINGER (gyroscopes in general relativity) project foresees the construction of an array of large frame ring laser gyroscopes, rigidly connected to the Earth. Large frame ring laser gyroscopes are high-sensitivity instruments used to measure angular velocity with respect to the local inertial
[...] Read more.

The GINGER (gyroscopes in general relativity) project foresees the construction of an array of large frame ring laser gyroscopes, rigidly connected to the Earth. Large frame ring laser gyroscopes are high-sensitivity instruments used to measure angular velocity with respect to the local inertial frame. In particular, they can provide sub-daily variations in the Earth rotation rate, a measurement relevant for geodesy and for fundamental physics at the same time. Sensitivity is the key point in determining the relevance of this instrument for fundamental science. The most recent progress in sensitivity evaluation, obtained on a ring laser prototype, indicates that GINGER should reach the level of 1 part in ${10}^{11}$ of the Earth’s rotation rate. The impact on fundamental physics of this kind of apparatus is reviewed.
Full article

(This article belongs to the Special Issue Current Trends in Cosmology)

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Open AccessConference Report

Beyond mirkwood: Enhancing SED Modeling with Conformal Predictions
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**Sankalp Gilda****Abstract**

Traditional spectral energy distribution (SED) fitting techniques face uncertainties due to assumptions in star formation histories and dust attenuation curves. We propose an advanced machine learning-based approach that enhances flexibility and uncertainty quantification in SED fitting. Unlike the fixed NGBoost model used in
[...] Read more.

Traditional spectral energy distribution (SED) fitting techniques face uncertainties due to assumptions in star formation histories and dust attenuation curves. We propose an advanced machine learning-based approach that enhances flexibility and uncertainty quantification in SED fitting. Unlike the fixed NGBoost model used in mirkwood, our approach allows for any scikit-learn-compatible model, including deterministic models. We incorporate conformalized quantile regression to convert point predictions into error bars, enhancing interpretability and reliability. Using CatBoost as the base predictor, we compare results with and without conformal prediction, demonstrating improved performance using metrics such as coverage and interval width. Our method offers a more versatile and accurate tool for deriving galaxy physical properties from observational data.
Full article

Open AccessTechnical Note

Generating Stellar Spectra Using Neural Networks
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**Marwan Gebran****Abstract**

A new generative technique is presented in this paper that uses Deep Learning to reconstruct stellar spectra based on a set of stellar parameters. Two different Neural Networks were trained allowing the generation of new spectra. First, an autoencoder is trained on a
[...] Read more.

A new generative technique is presented in this paper that uses Deep Learning to reconstruct stellar spectra based on a set of stellar parameters. Two different Neural Networks were trained allowing the generation of new spectra. First, an autoencoder is trained on a set of BAFGK synthetic data calculated using ATLAS9 model atmospheres and SYNSPEC radiative transfer code. These spectra are calculated in the wavelength range of Gaia RVS between 8400 and 8800 Å. Second, we trained a Fully Dense Neural Network to relate the stellar parameters to the Latent Space of the autoencoder. Finally, we linked the Fully Dense Neural Network to the decoder part of the autoencoder and we built a model that uses as input any combination of ${T}_{\mathrm{eff}}$ , $logg$ , ${v}_{e}sini$ , $[M/H]$ , and ${\xi}_{t}$ and output a normalized spectrum. The generated spectra are shown to represent all the line profiles and flux values as the ones calculated using the classical radiative transfer code. The accuracy of our technique is tested using a stellar parameter determination procedure and the results show that the generated spectra have the same characteristics as the synthetic ones.
Full article

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Open AccessArticle

Testing Cosmic Acceleration from the Late-Time Universe
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**Jose Agustin Lozano Torres****Abstract**

We investigate the accelerated cosmic expansion in the late universe and derive constraints on the values of the cosmic key parameters according to different cosmologies such as $\mathsf{\Lambda}$ CDM, ${w}_{0}{w}_{a}$ CDM. We select 24 baryon acoustic oscillation
[...] Read more.

*w*CDM, and
We investigate the accelerated cosmic expansion in the late universe and derive constraints on the values of the cosmic key parameters according to different cosmologies such as $\mathsf{\Lambda}$ CDM, ${w}_{0}{w}_{a}$ CDM. We select 24 baryon acoustic oscillation (BAO) uncorrelated measurements from the latest galaxy surveys measurements in the range of redshift $z\in [0.106,2.33]$ combined with the Pantheon SNeIa dataset, the latest 33 $H\left(z\right)$ measurements using the cosmic chronometers (CCs) method, and the recent Hubble constant value measurement measured by Riess 2022 (R22) as an additional prior. In the $\mathsf{\Lambda}$ CDM framework, the model fit yields ${\mathsf{\Omega}}_{m}=0.268\pm 0.037$ and ${\mathsf{\Omega}}_{\mathsf{\Lambda}}=0.726\pm 0.023$ . Combining BAO with Pantheon plus the cosmic chronometers datasets we obtain ${H}_{0}=69.76\pm 1.71$ km s${}^{-1}$ Mpc${}^{-1}$ and the sound horizon result is ${r}_{d}=145.88\pm 3.32$ Mpc. For the flat $w=-1.001\pm 0.040$ . For the dynamical evolution of the dark energy equation of state, ${w}_{0}{w}_{a}$ CDM cosmology, we obtain ${w}_{a}=-0.848\pm 0.180$ . We apply the Akaike information criterion approach to compare the three models, and see that all cannot be ruled out from the latest observational measurements.
Full article

*w*CDM, and*w*CDM model, we obtainFigure 1

Open AccessArticle

Frequency–Redshift Relation of the Cosmic Microwave Background
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Cited by 1
**Ralf Hofmann**and**Janning Meinert****Abstract**

We point out that a modified temperature–redshift relation (

*T*-*z*relation) of the cosmic microwave background (CMB) cannot be deduced by any observational method that appeals to an a priori thermalisation to the CMB temperature*T*of the excited states in [...] Read more.
We point out that a modified temperature–redshift relation ($x=\frac{h\nu}{{k}_{B}T}$ . Since the literature on extractions of the CMB’s $\nu \left(z\right)=(1+z)\nu (z=0)$ , where $\nu (z=0)$ is the observed frequency in the heliocentric rest frame, the finding (ii) $T\left(z\right)=(1+z)T(z=0)$ just confirms the expected blackbody nature of the interacting CMB at $z>0$ . In contrast to the emission of isolated, directed radiation, whose frequency–redshift relation ($\nu $ -$\nu $ -$\nu \left(z\right)=f\left(z\right)\nu (z=0)$ of pure, isotropic blackbody radiation, subject to adiabatically slow cosmic expansion, necessarily has to follow that of the $T\left(z\right)=f\left(z\right)T(z=0)$ and vice versa. In general, the function $f\left(z\right)$ is determined by the energy conservation of the CMB fluid in a Friedmann–Lemaitre–Robertson–Walker universe. If the pure CMB is subject to an SU(2) rather than a U(1) gauge principle, then $f\left(z\right)={\left(1/4\right)}^{1/3}(1+z)$ for $z\gg 1$ , and $f\left(z\right)$ is non-linear for $z\sim 1$ .
Full article

*T*-*z*relation) of the cosmic microwave background (CMB) cannot be deduced by any observational method that appeals to an a priori thermalisation to the CMB temperature*T*of the excited states in a probe environment of independently determined redshift*z*. For example, this applies to quasar-light absorption by a damped Lyman-alpha system due to atomic as well as ionic fine-splitting transitions or molecular rotational bands. Similarly, the thermal Sunyaev-Zel’dovich (thSZ) effect cannot be used to extract the CMB’s*T*-*z*relation. This is because the relative line strengths between ground and excited states in the former and the CMB spectral distortion in the latter case both depend, apart from environment-specific normalisations, solely on the dimensionless spectral variable*T*-*z*relation always assumes (i)*z*relation) is subject to (i), a non-conventional*z*relation*T*-*z*relationGraphical abstract

Open AccessArticle

Schwarzschild Black Holes in Extended Spacetime with Two Time Dimensions
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**Mechid Paiman**,**Horia Cornean**and**Christoph Köhn****Abstract**

Black holes are one of the most extreme phenomena in the Universe, bridging the gap between the realms of general relativity and quantum physics. Any matter that crosses the event horizon moves towards the core of the black hole, creating a singularity with
[...] Read more.

Black holes are one of the most extreme phenomena in the Universe, bridging the gap between the realms of general relativity and quantum physics. Any matter that crosses the event horizon moves towards the core of the black hole, creating a singularity with infinite mass density—a phenomenon that cannot be comprehended within present theories of relativity and quantum physics. In this study, we undertake an investigation of non-rotating, non-charged Schwarzschild black holes in an extended spacetime framework with two time dimensions. To accomplish this, we extend Einstein’s field equations by one more temporal dimension. We solve the corresponding equations for a spherical central mass, which leads to an Abel-type equation for the 5D Schwarzschild metric. By exploring distinct solution classes, we present an approximate solution for the 5D metric. Our proposed solution maintains consistency with Schwarzschild’s 4D solution. Finally, we address the central black hole singularity and demonstrate a potential breakthrough, as our solution effectively avoids the singularity quandary, providing valuable insight into the fundamental properties of black holes in this augmented framework.
Full article

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Open AccessArticle

The Upgraded Planck System of Units That Reaches from the Known Planck Scale All the Way Down to Subatomic Scales
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**Dimitris M. Christodoulou**and**Demosthenes Kazanas****Abstract**

Natural systems of units $\left\{{U}_{i}\right\}$ need to be overhauled to include the dimensionless coupling constants $\left\{{\mathsf{\alpha}}_{{U}_{i}}\right\}$ of the natural forces. Otherwise, they cannot quantify all the forces of nature in a unified manner. Thus, each force
[...] Read more.

Natural systems of units $\left\{{U}_{i}\right\}$ need to be overhauled to include the dimensionless coupling constants $\left\{{\mathsf{\alpha}}_{{U}_{i}}\right\}$ of the natural forces. Otherwise, they cannot quantify all the forces of nature in a unified manner. Thus, each force must furnish a system of units with at least one dimensional and one dimensionless constant. We revisit three natural systems of units (atomic, cosmological, and Planck). The Planck system is easier to rectify, and we do so in this work. The atomic system discounts $\{G,{\mathsf{\alpha}}_{G}\}$ , thus it cannot account for gravitation. The cosmological system discounts $\{\overline{)h},{\mathsf{\alpha}}_{\overline{)h}}\}$ , thus it cannot account for quantum physics. Here, the symbols have their usual meanings; in particular, ${\mathsf{\alpha}}_{G}$ is the gravitational coupling constant and ${\mathsf{\alpha}}_{\overline{)h}}$ is Dirac’s fine-structure constant. The speed of light ${Z}_{0}$ are resistive properties imposed by the vacuum itself; thus, they must be present in all systems of units. The upgraded Planck system with fundamental units $\mathrm{UPS}\phantom{\rule{0.166667em}{0ex}}:=\phantom{\rule{0.166667em}{0ex}}\{c,{Z}_{0},G,{\mathsf{\alpha}}_{G},\overline{)h},{\mathsf{\alpha}}_{\overline{)h}},\phantom{\rule{0.166667em}{0ex}}\dots \phantom{\rule{0.166667em}{0ex}}\phantom{\rule{-0.166667em}{0ex}}\}$ describes all physical scales in the universe—it is nature’s system of units. As such, it reveals a number of properties, most of which have been encountered previously in seemingly disjoint parts of physics and some of which have been designated as mere coincidences. Based on the UPS results, which relate (sub)atomic scales to the Planck scale and the fine-structure constant to the Higgs field, we can state with confidence that no observed or measured physical properties are coincidental in this universe. Furthermore, we derive from first principles Koide’s $K=2/3$ enigmatic constant and additional analogous quark and vector boson constants. These are formal mathematical proofs that justify a posteriori the use of geometric means in deriving the quark/boson mass ladder. This ladder allows us to also calculate the Higgs couplings to the vector bosons and the Weinberg angle in terms of

*c*and the impedance of free space*K*only, and many of the “free” parameters of the Standard Model of particle physics were previously expected to be determined only from experiments. Full articleOpen AccessCommunication

Radio Pulsars Resonantly Accelerating Electrons
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**Zaza N. Osmanov**and**Swadesh M. Mahajan****Abstract**

Based on the recently demonstrated resonant wave–wave process, it is shown that electrons can be accelerated to ultra-relativistic energies in the magnetospheres of radio pulsars. The energization occurs via the resonant interaction of the electron wave (described by the Klein–Gordon (KG) equation) moving
[...] Read more.

Based on the recently demonstrated resonant wave–wave process, it is shown that electrons can be accelerated to ultra-relativistic energies in the magnetospheres of radio pulsars. The energization occurs via the resonant interaction of the electron wave (described by the Klein–Gordon (KG) equation) moving in unison with an intense electromagnetic (EM) wave; the KG wave/particle continuously draws energy from EM. In a brief recapitulation of the general theory, the high-energy (resonantly enhanced) electron states are investigated by solving the KG equation, minimally coupled to the EM field. The restricted class of solutions that propagate in phase with EM radiation (functions only of $\zeta =\omega t-kz$ ) are explored to serve as a possible basis for the proposed electron energization in the radio pulsars. We show that the wave–wave resonant energization mechanism could be operative in a broad class of radio pulsars with periods ranging from milliseconds to normal values (∼1 s); this could drive the magnetospheric electrons to acquire energies from 100 s of TeVs (millisecond pulsars) to 10 ZeVs (normal pulsars).
Full article

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Open AccessArticle

Emergent Strings at an Infinite Distance with Broken Supersymmetry
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Cited by 3
**Ivano Basile****Abstract**

We investigate the infinite-distance properties of families of unstable flux vacua in string theory with broken supersymmetry. To this end, we employ a generalized notion of distance in the moduli space and we build a holographic description for the non-perturbative regime of the
[...] Read more.

We investigate the infinite-distance properties of families of unstable flux vacua in string theory with broken supersymmetry. To this end, we employ a generalized notion of distance in the moduli space and we build a holographic description for the non-perturbative regime of the tunneling cascade in terms of a renormalization group flow. In one limit, we recover an exponentially-light tower of Kaluza-Klein states, while in the opposite limit, we find a tower of higher-spin excitations of D1-branes, realizing the emergent string proposal. In particular, the holographic description includes a free sector, whose emergent superconformal symmetry resonates with supersymmetric stability, the CFT distance conjecture and S-duality. We compute the anomalous dimensions of scalar vertex operators and single-trace higher-spin currents, finding an exponential suppression with the distance which is not generic from the renormalization group perspective, but appears specific to our settings.
Full article

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Open AccessArticle

New Strong Constraints on the Central Behaviour of Spherical Galactic Models
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**Marco Roncadelli**and**Giorgio Galanti****Abstract**

First of all, we show that $M\left(r\right)\to 0$ as $r\to 0$ is physically correct close to the centre

*any spherically symmetric*galactic model with integrated mass profile*only*provided that the circular velocity [...] Read more.
First of all, we show that $M\left(r\right)\to 0$ as $r\to 0$ is physically correct close to the centre ${v}_{c}\left(r\right)\to 0$ and the gravitational field $g\left(r\right)\to 0$ as $r\to 0$ . Next, we apply this statement to a broad class of five-parameter spherical galactic models, including most of those used in astrophysics and cosmology. Specifically, we show that the Jaffe and Hernquist models $r\gtrsim 0.2\phantom{\rule{0.166667em}{0ex}}{R}_{e}$ (${R}_{e}$ being the effective radius), while the Navarro–Frank–White (NFW) model cannot describe galaxies in the central region of regular clusters. We also briefly discuss the relevance of our result for the NFW profile of pure dark matter halos. However, we are unable to tell at which central distance the NFW model breaks down in either case, and this is a challenge for future investigations.
Full article

*any spherically symmetric*galactic model with integrated mass profile*only*provided that the circular velocity*can be trusted only*forFigure 1

Open AccessReview

Quantum Astronomy at the University and INAF Astronomical Observatory of Padova, Italy
by

**Cesare Barbieri**,**Giampiero Naletto**and**Luca Zampieri****Abstract**

Twenty years ago, we started to apply quantum optics to the astronomical research carried out inside the Department of Physics and Astronomy and the INAF Astronomical Observatory in Padova, Italy. The initial activities were stimulated by the project of the European Southern Observatory
[...] Read more.

Twenty years ago, we started to apply quantum optics to the astronomical research carried out inside the Department of Physics and Astronomy and the INAF Astronomical Observatory in Padova, Italy. The initial activities were stimulated by the project of the European Southern Observatory (ESO) to build a 100 m diameter telescope, the Overwhelmingly Large (OWL) telescope. The enormous photon flux expected from such an aperture suggested that quantum optics concepts be utilized in order to obtain novel astrophysical results. Following initial successful attempts to utilize the orbital angular momentum of the light beam to enhance the visibility of faint companions to bright stars, the Padova team concentrated its efforts on very high time resolution, in order to measure and store the arrival time of celestial photons to better than one nanosecond. To obtain observational results, we built two photon counting photometers (AquEye and IquEye) to be used with our telescopes of the Asiago Observatory and with 4 m class telescopes such as the ESO New Technology Telescope (NTT) in Chile. This paper firstly describes these two instruments and then expounds the results obtained on pulsar light curves, lunar occultations and the first photon counting intensity interferometry measurements of the bright star Vega. Indeed, the correlation of photon arrival times on two or more apertures can lead to extremely high angular resolutions, as shown around 1970 by Hanbury Brown and Twiss. Prospects for quantum intensity interferometry with arrays of Cherenkov light telescopes will also be described.
Full article

(This article belongs to the Special Issue Quantum Astronomy)

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Open AccessArticle

Space Weather Effects on Satellites
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Cited by 2
**Rositsa Miteva**,**Susan W. Samwel**and**Stela Tkatchova****Abstract**

The study presents a concise overview on the main effects on satellites due to space weather drivers compared to the well-known interplanetary, magnetospheric and ground-based consequences. The solar-activity-driven influences include specific physics-based effects on the spacecraft surface and on-board electronics due to electromagnetic
[...] Read more.

The study presents a concise overview on the main effects on satellites due to space weather drivers compared to the well-known interplanetary, magnetospheric and ground-based consequences. The solar-activity-driven influences include specific physics-based effects on the spacecraft surface and on-board electronics due to electromagnetic emission and energetic particles as well as complex effects due to geomagnetic storms which may endanger the mission performance and spacecraft longevity. We select as test examples the Starlink satellites in the period 2019–2022 and present the temporal correspondence between their launches and the space weather phenomena. Based on comparative analysis, we discuss whether the occurrence vs. the intensity of solar and interplanetary drivers of space weather can be considered as a cause for orbital stability problems and satellite loss. The results suggest that a sequence of geomagnetic disturbances together with multiple weak space weather events could lead to severe levels of atmospheric drag ending in a service or satellite loss.
Full article

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Open AccessArticle

Simulation of Dynamic Evolution of Ring Current Ion Flux by a Lunar Base Energetic Neutral Atom (ENA) Imaging
by

Cited by 1
**Li Lu**,**Qinglong Yu**,**Shuai Jia**,**Zhong Xie**,**Jian Lan**and**Yuan Chang****Abstract**

The distribution of energetic ion flux in the ring current region, such as a meteorological cumulonimbus cloud, stores up the particle energy for a geomagnetic substorm. It is helpful to study the geomagnetic substorm mechanism by using a lunar base ENA imaging simulation
[...] Read more.

The distribution of energetic ion flux in the ring current region, such as a meteorological cumulonimbus cloud, stores up the particle energy for a geomagnetic substorm. It is helpful to study the geomagnetic substorm mechanism by using a lunar base ENA imaging simulation of the dynamic evolution of the ring current, and establishing the corresponding relationship between key node events of the substorm. Based on the previous observation experience and our simulation results of the dynamic evolution of the ring current, we propose a macroscopic model of substorms related to the dynamic evolution of ring currents and present the possibility of confirming the causal sequence of some of those critical node events of substorms with the lunar base ENA imaging measurement. IBEX, operating in the ecliptic plane, may even give examples of the telemetry of ring current ion fluxes through ENA measurements during substorms/quiets.
Full article

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Open AccessCommunication

Spins of Supermassive Black Holes M87* and SgrA* Revealed from the Size of Dark Spots in Event Horizon Telescope Images
by

Cited by 2
**Vyacheslav Ivanovich Dokuchaev****Abstract**

We reconstructed dark spots in the images of supermassive black holes SgrA* and M87* provided by the Event Horizon Telescope (EHT) collaboration by using the geometrically thin accretion disk model. In this model, the black hole is highlighted by the hot accretion matter
[...] Read more.

We reconstructed dark spots in the images of supermassive black holes SgrA* and M87* provided by the Event Horizon Telescope (EHT) collaboration by using the geometrically thin accretion disk model. In this model, the black hole is highlighted by the hot accretion matter up to the very vicinity of the black hole event horizon. The existence of hot accretion matter in the vicinity of black hole event horizons is predicted by the Blandford–Znajek mechanism, which is confirmed by recent general relativistic MHD simulations in supercomputers. A dark spot in the black hole image in the described model is a gravitationally lensed image of an event horizon globe. The lensed images of event horizons are always projected at the celestial sphere inside the awaited positions of the classical black hole shadows, which are invisible in both cases of M87* and SgrA*. We used the sizes of dark spots in the images of SgrA* and M87* for inferring their spins, $0.65<a<0.9$ and $a>0.75$ , accordingly.
Full article

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