arXiv subjects

Grigoris Panotopoulos

Publications and source records attributed to Grigoris Panotopoulos.

At least 19 recordsLinked to original sources

arXiv2026

Chromatic Weak Lensing by Charged Black Holes with Two Lorentz-Violating Kalb-Ramond Couplings

We study weak gravitational lensing and steady spherical test--fluid accretion by a static charged black hole in a Lorentz--violating Kalb--Ramond background with two nonminimal curvature couplings, assuming minimally coupled probe radiation. Using the Gauss--Bonnet theorem with the correct boundary term and perturbed ray boundary, we obtain the complete local deflection angle through second post--Minkowskian order. In a homogeneous cold plasma, the mass and charge sectors acquire different frequency dependences, producing distinct chromatic signatures. The two Lorentz--violating couplings are also separated: one controls the conical geometry and mass normalization, while the other first enters through the effective charge. For neutral adiabatic accretion, we derive the conserved fluxes, Bernoulli relation, Hamiltonian flow, and sonic--point conditions. These results disentangle local, global, dispersive, and accretion effects and establish the calibrations required for phenomenological constraints.

gr-qc↗

arXiv2026

Slowly rotating condensate dark stars beyond the mean-field approximation

We investigate rotational properties and universal relations of slowly rotating Bose-Einstein condensate dark stars in the context of General Relativity, both at the mean-field level and when the leading beyond-mean-field Lee-Huang-Yang correction is retained self-consistently. Adopting the polytropic $n=1$ equation of state appropriate to a dilute, self-interacting Bose gas, parameterised by the boson mass $m$ and the $s$-wave scattering length $a_s$, we integrate the Tolman-Oppenheimer-Volkoff equations together with Hartle's dipole equation for the frame-dragging angular velocity, and we compute the moment of inertia, the gravito-electric tidal Love number and the dimensionless tidal deformability. The resulting equilibrium sequences yield gravitational masses in the $1$--$2\,M_{\odot}$ range with radii of $10$--$20\,\mathrm{km}$, squarely within the window presently probed by NICER and the LIGO-Virgo-KAGRA network. We observe that the LHY term produces a measurable reduction of the dimensionless moment of inertia at fixed compactness, whilst the I-$\Lambda$ universal relation is preserved to within a few per cent. We supply polynomial fits for the I-$\Lambda$ and I-$C$ relations, and show that the LHY footprint is large enough to serve as a clean diagnostic of beyond-mean-field quantum physics in a putative dark star population, complementing existing dark matter constraints from pulsar masses and from the equation-of-state interpretation of the unusually light compact remnant HESS~J1731-347.

gr-qc↗

arXiv2026

Black hole physics within $f(R)$ gravity: Quasi-normal spectra and greybody factors

We investigate several astrophysical motivated properties of a four-dimensional black hole solution in the framework of $f(R)$ gravity. The model is characterized by a single additional parameter, $\alpha$, which encodes nontrivial deviations from General Relativity. Using this black hole spacetime as the background geometry, we analyze: (i) the quasinormal modes of massless scalar perturbations (employing several complementary methods), and (ii) the greybody factors associated to the propagation of massless test scalar fields. Regarding quasinormal modes, we study the response of black holes to massless scalar perturbations using three independent approaches: the well-established sixth-order WKB semi-analytic method, analytic expressions, and the analytical expression in the eikonal limit. We examine the behavior of both the real and imaginary parts of the quasinormal frequencies as functions of the parameter $\alpha$, the overtone number $n$, and the multipole number $\ell$. In addition to that, we compute the greybody factors within the WKB approximation for various combinations of the relevant parameters. In particular, we focus on the propagation of test massless scalar fields and investigate how the greybody factors depend on the multipole number $\ell$, and the free parameter $\alpha$. The impact of the aforementioned parameters on the absorption cross-section and the reflection and transmission coefficients is studied in detail.

gr-qc↗

arXiv2026

Condensate Dark Stars beyond the Mean-Field Approximation: The Lee-Huang-Yang correction

We study structural properties of self-gravitating fluid spheres made of a dilute, homogeneous and ultracold Bose gas assuming repulsive, short-range interactions. For the first time we include the Lee-Huang-Yang correction to the usual polytropic equation-of-state of index $n=1$, which goes beyond the Hartree mean-field approximation taking into account quantum fluctuations. We find that the correction has a considerable impact on the M-R relationships and other properties of condensate dark stars, such as factor of compactness and tidal Love numbers. The impact is more significant for equation-of-states that support larger highest stellar masses.

gr-qc↗

arXiv2025

Quantum Black Holes: Perihelion Advance, Quasi Normal Modes and Classical/ Topological Thermodynamics

We report on some properties of a quantum black hole obtained recently. The correction to the Newtonian gravitational potential is proportional to a coupling $\alpha$, which is the only free parameter of the theory. We constrain the coupling using the perihelion advance, we compute the quasi-normal modes for scalar (both massless and massive) and electromagnetic perturbations. We find that all modes computed here are complex numbers characterized by a positive real part and a negative imaginary part, while both parts increase with the mass of the test scalar field. Also thermodynamics properties are investigated from the classical and topological point of view. In this regard, the quantum black hole exhibits the same behavior as the classical Reissner-Nordstr\"om space-time, that is, it presents stable/unstable branches in the Gibbs potential, one generating point and a topological charge $W=0$.

gr-qc↗

arXiv2025

Dark matter admixed relativistic stars: Structural properties and tidal Love numbers

We study the impact of bosonic, self-interacting dark matter on structural properties and tidal deformabilities of compact stars. As far as the gravitational theory is concerned, we assume Einstein's gravity in four dimensions with a vanishing cosmological constant. Regarding matter content, we consider a state-of-matter to a linear form of equation-of-state (EoS), while for dark matter we assume a quartic scalar potential, which implies a certain non-linear EoS obtained long time ago. Adopting the two-fluid formalism we integrate the structure equations as well as the Riccati equation for the metric even perturbations imposing appropriate initial conditions at the center of the stars and matching conditions at their surface. We compute the stellar mass and radius, factor of compactness and dimensionless deformability varying several free parameters of the model studied here. Tidal deformability and the corresponding tidal Love number determine the imprint of the underlying EoS within the signals emitted during binary coalescences, and it is expected to be altered due to the presence of dark matter inside the objects. We find that in all cases considered here, the dimensionless deformability of the canonical stellar mass remains lower than the upper bound, $\Lambda_{1.4} < 800$. We also look at the stability of these stars based on the Harrison-Zeldovich-Novikov criterion under various conditions. It is observed that the presence of dark matter implies significantly lower highest stellar mass, and also smaller and more compact stars for a given stellar mass.

gr-qc↗

arXiv2025

Radial Oscillations of the HESS J1731-347 Compact Object via the Karmarkar Condition in Gravity

We model the light HESS J1731-347 compact object (of known stellar mass and radius) within Einstein's General Relativity imposing the Karmarkar condition in gravity for anisotropic stars. The three free parameters of the analytic solution are determined imposing the matching conditions at the surface of the star for objects of known stellar mass and radius. Finally, using well established criteria it is shown that the solution is compatible with all requirements for well behaved and realistic solutions. Furthermore, we study the radial oscillation modes, and we compare to the ones corresponding to an isotropic star modeled by the Tolman IV exact analytic solution obtained long time ago. A comparison between the large frequency separations is made as well.

gr-qc↗

arXiv2025

Quasinormal modes and absorption cross-section of a Bardeen black hole surrounded by perfect fluid dark matter in four dimensions

In this paper we study quasinormal modes and absorption cross sections for the $(1+3)$-dimensional Bardeen black hole surrounded by perfect fluid dark matter. Studies of the massless scalar field is already done in \cite{Sun:2023slzl}. Hence, in this paper we will focus on the massive scalar field perturbations and massless Dirac field perturbations. To compute the quasinormal modes we use the semi-analytical 3rd-order WKB method, which has been shown to be one of the best approaches when the effective potential is adequate and when $n < \ell$ and $n < \lambda$. We have also utilized the P\"oschl-Teller method to compare the valus obtained using the WKB approach. We have computed quasinormal frequencies by varying various parameters of the theory such as the mass of the scalar field $\mu$, dark matter parameter $\alpha$ and the magnetic charge $g$. We have summarized our solutions in tables and figures for clarity. As for the absorption cross section, we used third order WKB approach to compute reflection, transmission coefficients and partial absorption cross sections. Graphs are presented to demonstrate the behavior of the above quantities when the dark matter parameter and mass of the massive scalar field are varied.

gr-qc↗

arXiv2024

Radial Oscillations in Hybrid Stars with Slow Quark Phase Transition

This study investigates the radial oscillations of hybrid neutron stars, characterized by a composition of hadronic external layers and a quark matter core. Utilizing a density-dependent relativistic mean-field model that incorporates hyperons and baryons for describing hadronic matter, and a density-dependent quark model for quark matter, we analyze the ten lowest eigenfrequencies and their corresponding oscillation functions. Our focus lies on neutron stars with equations-of-state involving N, N + $\Delta$, N + H, and N + H + $\Delta$, featuring a phase transition to quark matter. Emphasizing the effects of a slow phase transition at the hadron-quark interface, we observe that the maximum mass is attained before the fundamental mode's frequency decreases for slow phase transitions. This observation implies the stability of stellar configurations with higher central densities than the maximum mass, called Slow Stable Hybrid Stars (SSHSs), even under small radial perturbations. The length of these SSHS branch depends upon the energy density jump between two phases and the stiffness of the quark EoS.

nucl-th↗

arXiv2024

Impact of hyperons on structural properties of neutron stars and hybrid stars within the regularized four-dimensional Einstein-Gauss-Bonnet gravity

We investigate the impact of hyperons and phase transition to quark matter on the structural properties of neutron stars within the regularized four-dimensional Einstein-Gauss-Bonnet gravity (4DEGB). We employ the density-dependent relativistic mean-field model (DDME2) for the hadronic phase and the density-dependent quark mass (DDQM) model for the quark phase to construct hadronic and hybrid equations-of-state (EoSs) that are consistent with the astrophysical constraints. The presence of hyperons softens the EoS and with a phase transition, the EoS further softens, and the speed of sound squared drops to around 0.2 for the maximum mass configuration, which lies in the pure quark phase. Adjusting the Gauss-Bonnet coupling constant, $\alpha$, within its allowed range results in a decrease in the mass-radius relationship for negative $\alpha$, and an increase for positive $\alpha$. In addition, functions are fitted to the maximum mass and its associated radius as a function of the constant $\alpha$ to observe its impact on these properties. We find that positive values of $\alpha$ support massive stars consistent with the 2\,$M_{\odot}$ constraint and NICER measurements, while negative values, although compatible with low-mass radius observations, fail to reach the observed maximum mass, particularly for EoSs involving phase transitions. Therefore, astrophysical observations may be used to effectively constrain the allowed range of $\alpha$.

nucl-th↗

arXiv2024

On new regular charged black hole solutions: Limiting Curvature Condition, Quasinormal modes and Shadows

We introduce two new static, spherically symmetric regular black hole solutions that can be obtained from non-linear electrodynamics models. For each solution, we investigate the dynamic stability with respect to arbitrary linear fluctuations of the metric and electromagnetic field, and also examine the energy conditions that those black holes satisfy. Moreover, based on those solutions, we present two additional ones that satisfy the Limiting Curvature Condition. Finally, we make a comparison between the two solutions exploring their null geodesics and circular photon orbits.

gr-qc↗

arXiv2024

Properties of white dwarf with anisotropic pressure in Rainbow gravity

We investigate the properties of anisotropic white dwarf stars within the rainbow gravity adopting for matter content the Chandrasekhar model based on an ideal Fermi gas at zero temperature. We study in detail the effects of the anisotropic factor on stellar mass and radius, the speed of sound, and the relativistic adiabatic index in both radial and tangential directions. We find that causality is never violated, whereas the stability criterion based on the relativistic adiabatic index is not met when the objects are characterized by a positive anisotropic factor close to the Chandrasekhar limit. We present this significant observation here for the first time, to the best of our knowledge.

gr-qc↗

arXiv2024

Tidal Love numbers of anisotropic stars within the complexity factor formalism

We compute the quadrupolar gravitoelectric tidal Love numbers of spherical configurations made of anisotropic matter. Anisotropies are introduced within the vanishing complexity factor, while interior solutions are obtained adopting the Extended Chaplygin gas equation-of-state. A comparison with a more conventional approach is made as well.

gr-qc↗

arXiv2024

Strange Quark Stars and Condensate Dark Stars in Bumblebee Gravity

In this paper, we investigate the properties of relativistic stars made of isotropic matter within the framework of the minimal Standard Model Extension, where a Bumblebee field (BF) coupled to spacetime induces spontaneous Lorentz symmetry breaking. We adopt analytic equations-of-state describing either condensate dark stars or strange quark stars. We solve the structure equations numerically, and we compute the mass-to-radius relationships. The influence of the Bumblebee parameter $\mathbf{l}$ is examined in detail, and an upper bound is obtained using the massive pulsar (PSR) J0740+6620 and the strangely light High Energy Stereoscopic System (HESS) J1731-347 compact object.

gr-qc↗

arXiv2024

Stellar modeling via the Tolman IV solution: The cases of the massive pulsar J0740+6620 and the HESS J1731-347 compact object

We model compact objects of known stellar mass and radius made of isotropic matter within Einstein's gravity. The interior solution describing hydrostatic equilibrium we are using throughout the manuscript corresponds to the Tolman IV exact analytic solution obtained long time ago. The three free parameters of the solutions are determined imposing the matching conditions for objects of known stellar mass and radius. Finally, using well established criteria it is shown that contrary to the Kohler Chao solution, the Tolman IV solution is compatible with all requirements for well behaved and realistic solutions. except for the relativistic adiabatic index that diverges at the surface of the stars. The divergence of the index $\Gamma$ may be resolved including a thin crust assuming a polytropic equation-of-state, which is precisely the case seen in studies of neutron stars. To the best of our knowledge, we model here for the first time the recently discovered massive pulsar PSR J0740+6620 and the strangely light HESS compact object via the Tolman IV solution. The present work may be of interest to model builders as well as a useful reference for future research.

gr-qc↗

arXiv2024

Thermodynamics of the quantum Schwarzschild black hole

We discuss some thermodynamic properties as well as the stability of a quantum Schwarzschild black hole, comparing the results with those obtained within a bumblebee gravity model. In particular, the Hawking temperature, $T_H$, the entropy, $S$, the heat capacity, $C$, and the Gibbs free energy, $G$, are computed for both cases. In addition to that, we compute the Brown-York quasilocal energy and compare the solution with the Schwarzschild case. We find that in both cases (quantum Schwarzschild and bumblebee gravity model) the temperature, the entropy, and the heat capacity show the same functional form, under the replacement $\lambda^2 \rightarrow \ell$ and vice versa. Specifically, the temperature is found to be lower compared to the classical (Schwarzschild) solution, whereas the entropy is computed to be larger. Moreover, the heat capacity becomes more negative. Notably, a distinct contrast emerges in obtaining the Gibbs free energy between these two cases, and this distinction appears to stem from the ADM mass.

gr-qc↗

arXiv2024

Radial Oscillations of Hybrid Stars and Neutron Stars including Delta baryons: The Effect of a Slow Quark Phase Transition

We study radial oscillations of hybrid neutron stars composed of hadronic external layers followed by a quark matter core. We employ a density-dependent relativistic mean-field model including hyperons and ${\Delta}$ baryons to describe hadronic matter, and a density-dependent quark model for quark matter. We obtain the ten lowest eigenfrequencies and the corresponding oscillation functions of N, N+${\Delta}$, N+H, and N+H+${\Delta}$ equations-of-state with a phase transition to the quark matter at 1.4 and 1.8 ${M_{\odot}}$, focusing on the effects of a slow phase transition at the hadron-quark interface. We observe that the maximum mass is reached before the fundamental mode's frequency vanishes for slow phase transitions, suggesting that some stellar configurations with higher central densities than the maximum mass remain stable even when they undergo small radial perturbations. Future gravitational wave detectors and multi-messenger astronomy, complemented by robust microscopic models enabling exploration of various neutron star compositions, including hyperon content, are anticipated to impose precise limitations on the equation of state of baryonic matter under high-density conditions.

nucl-th↗

arXiv2023

Quark Models and Radial Oscillations: Decoding the HESS J1731-347 Compact Object's Equation of State

We investigate the peculiar nature of strange stars through an analysis of different quark models, i.e. vBag model and CFL model equation of states at different parameter sets, and focus on understanding the equation of state governing the intriguing central compact object (CCO) within the supernova remnant HESS J1731-347, with a mass and radius of $M = 0.77^{+0.20}_{-0.17} M_{\odot}$ and $R = 10.4^{+0.86}_{-0.78}$ km, respectively. Additionally, we compare the radial oscillations of two models to determine the frequency of the HESS J1731-347 compact object at its maximum mass. The frequencies of radial oscillations are computed for each of the four EoSs considered. In total, the 10 lowest radial frequencies for each of those EoSs have been computed. By delving into these aspects, we aim at deepening our understanding of strange stars and their connection to the observed HESS J1731-347 mass-radius relationship.

astro-ph.HE↗