Nada Belhadj Ltaief, Mohammadkarim Shafieian, Messaoud Ahmed Ouameur
et al.
Practical real-time experiments are an essential step in demonstrating the effectiveness of optimizing Reconfigurable Intelligent Surface (RIS)-aided communication, a pivotal technology for future 5G and 6G networks. In this paper, we present a comprehensive approach to RIS optimization using multiple algorithms, including Genetic Algorithms (GA), Deep Reinforcement Learning (DRL), Adversarial Bandit (AB), GA-aided DRL, and a novel GA-aided AB framework. The experiments are conducted on a custom-built 96-element RIS operating at 3.75 GHz, specifically designed to enhance signal strength and adaptability in realistic propagation conditions. Across diverse receiver placements, the proposed GA-aided AB consistently accelerates convergence and optimizes RIS configurations more effectively than the learning and heuristic baselines, while maintaining real-time operation. GA–aided AB achieves a peak gain of 7.1 dB and remains the top performer even at the most challenging receiver positions like wider angles and longer paths in NLoS settings. Compared to DRL and GA benchmarks, the method delivers higher signal improvements without introducing meaningful computational overhead, underscoring its suitability for embedded deployment. These results demonstrate that a training-free hybrid search combining evolutionary proposals with adversarial bandit selection scales to large RIS configuration spaces and improves practical deployability. This study serves as a foundation for future RIS research, emphasizing the practical benefits of integrating optimization algorithms in real-world deployments.
Abstract In this paper, we present a detailed study of axial gravitational perturbations of the regular black hole solution in asymptotically safe gravity, as proposed in Bonanno et al. (Phys Rev Lett 132:031401, https://doi.org/10.1103/PhysRevLett.132.031401 , arXiv:2308.10890 [gr-qc], 2024). We analyze the quasinormal mode (QNM) spectrum of this black hole using two numerical techniques: the Bernstein spectral method and the asymptotic iteration method (AIM). These approaches allow us to compute QNM frequencies with high accuracy, even for higher overtones. Our results show that the fundamental mode is only weakly affected by the deviation parameter, whereas notable deviations from the Schwarzschild case emerge for higher overtones. Additionally, we examine the correspondence between grey-body factors and QNMs by comparing the WKB grey-body factors with accurate QNM frequencies, finding excellent agreement, especially for larger multipole numbers $$\ell $$ ℓ .
Astrophysics, Nuclear and particle physics. Atomic energy. Radioactivity
Abstract Can models that are degenerate in electromagnetic observations (i.e., having identical shadows) be distinguished by their dynamical behaviors and quantum radiation properties? To address this question, this paper considers a unique charged black hole with logarithmic term corrections in NED (Mazharimousavi in Phys Lett B 841:137948, 2023, https://doi.org/10.1016/j.physletb.2023.137948 , arXiv:2305.01048 [gr-qc]). It is found that for models degenerate with the shadow of a Schwarzschild black hole, although the oscillation frequencies (real parts) of their quasinormal modes (QNMs) are almost indistinguishable, their decay rates (imaginary parts) exhibit significant sensitivity. This provides the primary criterion for breaking the observational degeneracy. Furthermore, to investigate the effect of parameters, it is observed that when the parameter $$\zeta $$ ζ is fixed, the deviation behavior of QNM frequencies from those of the Schwarzschild black hole follows a more distinct pattern: the oscillation frequency decreases almost linearly with the increase in charge q, while the decay rate shows a stronger nonlinear dependence. In addition, the analysis of greybody factors (GFs) offers a second approach for distinction. It reveals a more complex non-monotonic behavior: in the low-frequency region, the transmittance of the Schwarzschild black hole is higher; however, above a cross frequency, the transmittance of the NED model rapidly surpasses that of the former. These results indicate that the damping time of QNMs and the precise shape of the Hawking radiation spectrum, rather than the shadow size, are more sensitive physical observables for detecting such logarithmic corrections. This study provides specific and distinguishable theoretical criteria for testing and constraining such NED models using gravitational wave astronomy and high-precision radiation observations in the future.
Astrophysics, Nuclear and particle physics. Atomic energy. Radioactivity
A recent study [Annals Phys. 455 (2023) 169394, e-Print: 2204.01901 [gr-qc]] examined the thermodynamic behavior of an axially symmetric black hole within a non-commutative framework that mimics the effect of an angular momentum. However, the analysis presents notable computational inconsistencies. In that analysis, the event horizon was miscalculated, and this error propagated through and compromised all subsequent results. In addition, an incorrect definition of surface gravity was used -- the spherically symmetric case was invoked for an axially symmetric spacetime -- rendering the thermodynamic results invalid. In other words, all the results presented in the paper require a thorough reexamination.
Souhaila Khalfallah, Borhen Louhichi, Sasan Sattarpanah Karganroudi
et al.
Epilepsy is a widespread neurological disorder affecting approximately 50 million people worldwide, significantly impacting quality of life and placing a heavy burden on healthcare systems. Early and reliable seizure detection remains a critical challenge, often hindered by limited availability of high-quality electroencephalogram (EEG) data and the suboptimal performance of existing classification methods. In this work, we propose a novel two-stage framework that addresses both data scarcity and classification accuracy. The first stage involves generating synthetic EEG signals that realistically mimic epileptic patterns using the Variable Structure Model Neuron (VSMN) with multidendrites, providing an effective means of data augmentation. In the second stage, we introduce a hybrid LSTM-VSMN model, where the VSMN activation function is integrated within the Long Short-Term Memory (LSTM) network gates, replacing conventional activations such as tanh. This integration improves the model’s ability to capture complex temporal dependencies in EEG sequences. To the best of our knowledge, this is the first study to leverage VSMN both for EEG signal synthesis and as an activation function within a deep recurrent neural network for seizure detection. The proposed model is rigorously evaluated against conventional activation functions, achieving an accuracy of 98.16% in single-fold validation and 97.59% under 3-fold cross-validation. Furthermore, it achieves a Mean Absolute Error (MAE) as low as 0.0241 and a Mean Absolute Percentage Error (MAPE) of 2.41%, substantially outperforming baseline approaches. These results demonstrate the effectiveness of the hybrid LSTM-VSMN architecture in enhancing automated seizure detection, offering a promising tool for clinical decision support and real-time monitoring applications.
Mattia Damia Paciarini, Manuel Del Piano, Stefan Hohenegger
et al.
Abstract Charged black holes arise as solutions of General Relativity (GR) coupled to Maxwell theory. As functions of the mass and charge, they can exhibit extremal behavior, in which case they are stable against thermal decay. (Quantum) corrections to GR are expected to alter the classical features of these objects, especially near extremality. To capture such effects in a model-independent way, we extend the Effective Metric Description (EMD) previously introduced in (Del Piano et al. in Phys Rev D 109(2):024045, 2024, https://doi.org/10.1103/PhysRevD.109.024045 , arXiv:2307.13489 [gr-qc]; Eur Phys J C 84(12):1273, 2024, https://doi.org/10.1140/epjc/s10052-024-13609-5 , arXiv:2403.12679 [gr-qc]) for spherically symmetric and static black holes. The EMD parametrizes deformations of the metric in terms of physical quantities, such as the radial spatial distance to the event horizon. While the latter is still viable for non-extremal charged black holes, we argue that the proper time of a free-falling observer is better suited in the extremal case: we derive the necessary conditions for the parameters of such an EMD for constructing a consistent space-time in the vicinity of the (extremal) horizon. Finally, we illustrate our framework through a concrete example, and mention implications of the Weak Gravity Conjecture on the effective metric parameters.
Astrophysics, Nuclear and particle physics. Atomic energy. Radioactivity
Anne E. Peck, William Roberson, Eric L. Nielsen
et al.
HD 143811 AB is the host star to the directly imaged planet HD 143811 AB b, which was recently discovered using data from the Gemini Planet Imager and Keck NIRC2. A member of the Sco-Cen star-forming region with an age of 13 ± 4 Myr, HD 143811 AB is somewhat rare among hosts of directly imaged planets, as it is a close stellar binary, with an ∼18-day period. Accurate values for the orbital and stellar parameters of this binary are needed to understand the formation and evolutionary history of the planet in orbit. We utilize archival high-resolution spectroscopy from FEROS on the MPG/ESO 2.2 m telescope to fit the orbit of the binary, and we combine with unresolved photometry to derive the basic stellar properties of the system. From the orbit, we derive precise values of orbital period of 18.59090 ± 0.00007 days and mass ratio of 0.886 ± 0.003. When combined with stellar evolutionary models, we find masses of both components of ${M}_{{\rm{A}}}=1.3{0}_{-0.05}^{+0.03}$ M _⊙ and ${M}_{{\rm{B}}}=1.1{5}_{-0.04}^{+0.03}$ M _⊙ . While the current data are consistent with the planet and stellar orbits being coplanar, the 3D orientations of both systems are currently poorly constrained, with additional observations required to more rigorously test for coplanarity.
Recently Alexeyev et al. published paper (J. Theor. Exper. Phys. v. 165, N 4, p. 508 in Russian; arXiv:2404.16079 [gr-qc], the reference is given also in [1]). In the paper the authors discussed an opportunity of estimating spins from the analysis of the shadow reconstruction of black holes, theoretically considered using the nonlocal gravity model proposed earlier for the description of "quantum" black holes. However, in essence, this paper considered circular photon orbits, and the fact that the corresponding motion parameters determine the shape and size of shadows, similarly to Kerr black holes, remained unproven. It is also remained unproven the statement that for an equatorial observer the shadow size in the direction of rotation of quantum black holes remains independent of spin. A long time ago the shadow property was established for the Kerr black hole case.
This thesis embarks on a comprehensive investigation of modified gravity theories and their implications on the properties of compact objects. Our primary objective is to shed light on the fundamental nature of gravity by exploring potential departures from General Relativity (GR) through a combination of theoretical analyses and observational techniques. On the theoretical side, we consider black hole thermodynamics, stability of compact objects, presence of black hole hairs, and the issue of causality that may provide valuable input towards the ultimate quantum theory of gravity. Moreover, on the observational side, we employ gravitational wave observations and black hole perturbation theory to explore new aspects of gravity and put stringent bounds on the beyond-GR parameters. To provide a structured overview of the thesis, we have organized it into chapters that progressively delve deeper into these diverse aspects of modified gravity and compact objects. Each chapter is dedicated to a specific facet of our investigation, building a coherent narrative that spans both theoretical and observational explorations. We aspire to achieve nothing less than imparting valuable insights and novel perspectives that may significantly enhance our understanding of the fundamental nature of gravitation.
We present a tight inequality to test the dynamical nature of spacetime. A general-relativistic violation of that inequality certifies change of curvature, in the same sense as a quantum-mechanical violation of a Bell inequality certifies a source of entanglement. The inequality arises from a minimal generalization of the Bell setup. It represents a limit on the winning chance of a collaborative multi-agent game played on the Möbius graph. A long version of this Letter including other games and how these games certify the dynamical character of the celebrated quantum switch is accessible as arXiv:2309.15752 [gr-qc].
In this work, we extend the investigation of the consequences of thermal fluctuations on the Casimir effect within the context of a traversable wormhole, recently proposed by Garattini & Faizal, arXiv:2403.15174 [gr-qc], subject to charge contributions. Specifically, we focus on scenarios where the plates exhibit both constant and radial variations. In our analysis, we initially concentrate on the high temperature approximation, considering solely the influence of charge on the thermal Casimir wormholes. Additionally, upon incorporating Generalized Uncertainty Principle (GUP) corrections to the Casimir energy, we obtain a new class of wormhole solutions. Notably, we establish that the flare-out condition remains consistently satisfied. Intriguingly, our findings reveal that both the charge and GUP contributions serve to further enlarge the throat's size in the radial variation.
Nuclear and particle physics. Atomic energy. Radioactivity
Abstract A connection between regular black holes and horizonless ultracompact objects was proposed in Carballo-Rubio et al. (JHEP 08:046, 2023, arXiv:2211.05817 [gr-qc]). In this paper, we construct a model of a horizonless compact object, specifically an anisotropic gravastar with continuous pressure, that corresponds to regular black hole spacetime in the appropriate limit. The construction begins by modeling an equation of state that satisfies the anisotropic gravastar conditions and transitions to the de Sitter ( $$p=-\epsilon $$ p = - ϵ ) upon horizon formation. The spacetime structure is similar to the Quantum Horizonless Compact Object (QHCO) described in Chen and Yokokura (Phys Rev D 109:104058, 2024, arXiv:2403.09388 [gr-qc]). Within this model, we also generate images of the corresponding objects surrounded by a thin accretion disk. The resulting images reveal that assuming that the emitting matter exists only outside the object, the inner light ring structure closely resembles that of the horizonless configuration of a regular black hole and the QHCO, yet it exhibits a distinct light ring structure compared to the thin-shell gravastar model. However, the opposite occurs when emitting matter is taken into account inside the object.
Astrophysics, Nuclear and particle physics. Atomic energy. Radioactivity
Abstract We derive a class of Taub-NUT metrics in the presence of a scalar field (TNS) by using Ernst equations and potential, as well as using Ehlers transformations on the exact solutions that was recently introduced in Azizallahi et al. (Nucl Phys B 998:116414, https://doi.org/10.1016/j.nuclphysb.2023.116414 , arXiv:2307.09328 [gr-qc], 2023) and Mirza et al. (Eur Phys J C 83:1161, https://doi.org/10.1140/epjc/s10052-023-12255-7 , arXiv:2307.13588 [gr-qc], 2023). Furthermore, we investigate the effective potential, geodesics, topological charge, quasinormal modes (QNMs) and the deflection angle of light in a gravitational lensing for the obtained class of TNS metrics. We also use conformal transformations to generate a new class of exact solutions of the Einstein-conformal-scalar theory by using the obtained TNS solutions as seed metrics. Finally we compare QNMs of the class of exact solutions.
Astrophysics, Nuclear and particle physics. Atomic energy. Radioactivity
Svetlana Jorstad, Maciek Wielgus, Rocco Lico
et al.
We report on the observations of the quasar NRAO 530 with the Event Horizon Telescope (EHT) on 2017 April 5−7, when NRAO 530 was used as a calibrator for the EHT observations of Sagittarius A*. At z = 0.902, this is the most distant object imaged by the EHT so far. We reconstruct the first images of the source at 230 GHz, at an unprecedented angular resolution of ∼20 μ as, both in total intensity and in linear polarization (LP). We do not detect source variability, allowing us to represent the whole data set with static images. The images reveal a bright feature located on the southern end of the jet, which we associate with the core. The feature is linearly polarized, with a fractional polarization of ∼5%–8%, and it has a substructure consisting of two components. Their observed brightness temperature suggests that the energy density of the jet is dominated by the magnetic field. The jet extends over 60 μ as along a position angle ∼ −28°. It includes two features with orthogonal directions of polarization (electric vector position angle), parallel and perpendicular to the jet axis, consistent with a helical structure of the magnetic field in the jet. The outermost feature has a particularly high degree of LP, suggestive of a nearly uniform magnetic field. Future EHT observations will probe the variability of the jet structure on microarcsecond scales, while simultaneous multiwavelength monitoring will provide insight into the high-energy emission origin.
Sara Issaoun, Maciek Wielgus, Svetlana Jorstad
et al.
The blazar J1924–2914 is a primary Event Horizon Telescope (EHT) calibrator for the Galactic center’s black hole Sagittarius A*. Here we present the first total and linearly polarized intensity images of this source obtained with the unprecedented 20 μ as resolution of the EHT. J1924–2914 is a very compact flat-spectrum radio source with strong optical variability and polarization. In April 2017 the source was observed quasi-simultaneously with the EHT (April 5–11), the Global Millimeter VLBI Array (April 3), and the Very Long Baseline Array (April 28), giving a novel view of the source at four observing frequencies, 230, 86, 8.7, and 2.3 GHz. These observations probe jet properties from the subparsec to 100 pc scales. We combine the multifrequency images of J1924–2914 to study the source morphology. We find that the jet exhibits a characteristic bending, with a gradual clockwise rotation of the jet projected position angle of about 90° between 2.3 and 230 GHz. Linearly polarized intensity images of J1924–2914 with the extremely fine resolution of the EHT provide evidence for ordered toroidal magnetic fields in the blazar compact core.
Maciek Wielgus, Nicola Marchili, Iván Martí-Vidal
et al.
The Event Horizon Telescope (EHT) observed the compact radio source, Sagittarius A* (Sgr A*), in the Galactic Center on 2017 April 5–11 in the 1.3 mm wavelength band. At the same time, interferometric array data from the Atacama Large Millimeter/submillimeter Array and the Submillimeter Array were collected, providing Sgr A* light curves simultaneous with the EHT observations. These data sets, complementing the EHT very long baseline interferometry, are characterized by a cadence and signal-to-noise ratio previously unattainable for Sgr A* at millimeter wavelengths, and they allow for the investigation of source variability on timescales as short as a minute. While most of the light curves correspond to a low variability state of Sgr A*, the April 11 observations follow an X-ray flare and exhibit strongly enhanced variability. All of the light curves are consistent with a red-noise process, with a power spectral density (PSD) slope measured to be between −2 and −3 on timescales between 1 minute and several hours. Our results indicate a steepening of the PSD slope for timescales shorter than 0.3 hr. The spectral energy distribution is flat at 220 GHz, and there are no time lags between the 213 and 229 GHz frequency bands, suggesting low optical depth for the event horizon scale source. We characterize Sgr A*’s variability, highlighting the different behavior observed just after the X-ray flare, and use Gaussian process modeling to extract a decorrelation timescale and a PSD slope. We also investigate the systematic calibration uncertainties by analyzing data from independent data reduction pipelines.
This is the Part III paper of our series of papers on a gauge-invariant perturbation theory on the Schwarzschild background spacetime. After reviewing our general framework of the gauge-invariant perturbation theory and the proposal on the gauge-invariant treatments for $l=0,1$ mode perturbations on the Schwarzschild background spacetime in [K.~Nakamura, arXiv:2110.13508 [gr-qc]], we examine the problem whether the $l=0,1$ even-mode solutions derived in the Part II paper [K.~Nakamura, arXiv:2110.13512 [gr-qc]] are physically reasonable, or not. We consider the linearized versions of the Lemaître-Tolman-Bondi solution and the non-rotating C-metric. As the result, we show that our derived even-mode solutions to the linearized Einstein equations actually realize above two linearized solutions. This fact supports that our derived solutions are physically reasonable, which implies that our proposal on the gauge-invariant treatments for $l=0,1$ mode perturbations are also physically reasonable. We also briefly summarize our conclusions of our series of papers.
This is the Part II paper of our series of papers on a gauge-invariant perturbation theory on the Schwarzschild background spacetime. After reviewing our general framework of the gauge-invariant perturbation theory and the proposal on the gauge-invariant treatments for $l=0,1$ mode perturbations on the Schwarzschild background spacetime in the Part I paper [K.~Nakamura, arXiv:2110.13508 [gr-qc]], we examine the linearized Einstein equations for even-mode perturbations. We discuss the strategy to solve the linearized Einstein equations for these even-mode perturbations including $l=0,1$ modes. Furthermore, we explicitly derive the $l=0,1$ mode solutions to the linearized Einstein equations in both the vacuum and the non-vacuum cases. We show that the solutions for $l=0$ mode perturbations includes the additional Schwarzschild mass parameter perturbation, which is physically reasonable. Then, we conclude that our proposal of the resolution of the $l=0,1$-mode problem is physically reasonable due to the realization of the additional Schwarzschild mass parameter perturbation and the Kerr parameter perturbation in the Part I paper.
P. T. Chruściel, Sk J. Hoque, M. Maliborski
et al.
Abstract We analyse the canonical energy of vacuum linearised gravitational fields on light cones on a de Sitter, Minkowski, and Anti de Sitter backgrounds in Bondi gauge. We derive the associated asymptotic symmetries. When $$\varLambda >0$$ Λ > 0 the energy diverges, but a renormalised formula with well defined flux is obtained. We show that the renormalised energy in the asymptotically off-diagonal gauge coincides with the quadratisation of the generalisation of the Trautman–Bondi mass proposed in Chruściel and Ifsits (Phys Rev D 93:124075, arXiv:1603.07018 [gr-qc], 2016).
Astrophysics, Nuclear and particle physics. Atomic energy. Radioactivity