arXiv subjects

Mohsen Fathi

Publications and source records attributed to Mohsen Fathi.

At least 37 records · Page 2Linked to original sources

arXiv2025

Quasinormal modes of a static black hole in nonlinear electrodynamics

We investigate the axial electromagnetic quasinormal modes of a static, asymptotically Anti--de Sitter (AdS) black hole sourced by a nonlinear electrodynamics model of Plebański type. Starting from the master equation governing axial perturbations, we impose ingoing boundary conditions at the event horizon and normalizable (Dirichlet) behavior at the AdS boundary. Following the approach of Jansen, we recast the radial equation into a linear generalized eigenvalue problem by using an ingoing Eddington--Finkelstein formulation, compactifying the radial domain, and regularizing the asymptotic coefficients. The resulting problem is solved using a Chebyshev--Lobatto pseudospectral discretization. We compute the fundamental quasinormal mode frequencies for both the purely electric ($Q_m=0$) and purely magnetic ($Q_e=0$) sectors, emphasizing the role of the nonlinearity parameter $β$ and the effective charge magnitude $Q$. Our results show that increasing either $β$ or $Q$ raises both the oscillation frequency $ω_R$ and the damping rate $-ω_I$, leading to faster but more rapidly decaying ringdown profiles. Nonlinear electrodynamics breaks the isospectrality between electric and magnetic configurations: magnetic modes are systematically less oscillatory and more weakly damped than their electric counterparts. For sufficiently large $β$ and small $Q_m$, the fundamental mode becomes purely imaginary ($ω_R \approx 0$), in agreement with the absence of a trapping potential barrier in this regime. These findings reveal qualitative signatures of nonlinear electromagnetic effects on black hole perturbations and may have implications for high-field or high-charge astrophysical environments.

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arXiv2025

Astrophysical Constraints on Charged Black Holes in Scalar--Tensor--Vector Gravity

We explore charged black holes in Scalar-Tensor-Vector Gravity (STVG), unveiling their distinctive features across multiple physical domains. Our topological analysis reveals that the STVG coupling parameter $α$ bolsters thermal stability while electromagnetic charge $Q$ weakens it. Using the Gauss-Bonnet theorem, we find that $α$ amplifies light deflection and enlarges shadow silhouettes, with $Q$ generating opposite effects. Our quantum-corrected models with exponential entropy terms pinpoint phase transitions in the microscopic regime, modifying conventional thermodynamic relationships. Calculations of strong gravitational lensing, shadow geometry, and Hawking emission show clear STVG signatures that diverge from Einstein's predictions. Notably, our accretion disk analysis uncovers an intriguing phenomenon: specific combinations of $α$ and $Q$ can produce radiation patterns resembling spinning Kerr black holes, creating potential identification challenges for observers. These findings establish concrete observational tests for STVG theory through next generation astronomical imaging and lensing campaigns. By connecting theoretical predictions to measurable quantities, we outline specific pathways to confirm or constrain STVG using data from current and future space telescopes.

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arXiv2025

Cosmological evolution driven by polytropic fluids in an inhomogeneous spacetime

Addressing the late-time accelerated expansion of the universe, known as the "dark energy problem", remains a central challenge in cosmology. While the cosmological constant is the standard explanation, alternative models such as quintessence, phantom fluids, and Chaplygin gas have been proposed. This work investigates the generalized Chaplygin gas (GCG) model, which is characterized by a polytropic equation of state. We explore this model within the framework of an anisotropic fluid, by means of a metric that reduces to the standard form of the Friedmann-Lemaître-Robertson-Walker (FLRW) spacetime at cosmological scales. To assess the model's viability, we derive analytical expressions for the scale factor, the Hubble parameter, and the deceleration parameter. Finally, the model is tested against observational data to constrain its parameters and evaluate its consistency.

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arXiv2025

Effect of primary scalar hair on black hole's strong lensing in Beyond Horndeski Gravity

We study the gravitational lensing effects of a static, asymptotically flat black hole with primary scalar hair in Beyond Horndeski gravity, focusing on the strong lensing regime. Recently, in Ref. [1], we placed constraints on the scalar hair parameter by analyzing its thermodynamic stability and the black hole shadow. In this work, we further investigate the strong lensing properties of the black hole, which form the basis of shadow formation, and employ observational data from the Event Horizon Telescope to derive more precise constraints on the scalar hair parameter. Additionally, we compute the shape and position of different lensed images of a thin accretion disk and argue that the observed black hole shadow corresponds to the secondary image of the emitting disk. Using this interpretation, we perform a new set of constraints on the scalar hair. Furthermore, we discuss why higher-order images are not suitable for astrophysical constraints, highlighting the limitations posed by their faintness and observational challenges. Finally, we find that models satisfying these constraints exhibit local instabilities.

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arXiv2025

Shadow of a nonlinear electromagnetic generalized Kerr-Newman-AdS black hole

In this work, we investigate the shadow properties of the Kerr-Newman-Anti-de Sitter black hole coupled to a nonlinear electrodynamics. We construct the shadow by employing the celestial coordinate approach for observers located at finite distances, accounting for the non-asymptotically flat nature of the spacetime. The size, distortion, area, and oblateness of the shadow are analyzed in terms of the black hole parameters, including the spin, effective charge, and nonlinearity parameter. We further explore observational constraints using the Event Horizon Telescope results for M87* and Sgr~A*. The angular diameters inferred from the observed images allow us to bound the parameter space of the model, showing that both spin and nonlinearity play key roles in shaping the shadow and in restricting the effective charge. Additionally, we examine the energy emission rate derived from the shadow radius and Hawking temperature, highlighting the influence of rotation and nonlinear electrodynamics on black hole evaporation. Our results demonstrate that the black hole model provides a physically consistent and observationally viable extension of the standard Kerr paradigm, with potential implications for near-horizon quantum processes and tests of gravity beyond general relativity.

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arXiv2025

Shedding Light on Gravity: Black Hole Shadows and Lensing Signatures in Lorentz Gauge Theory

Recent advances, including gravitational wave detections and imaging of black hole shadows, have strongly validated general relativity. Nevertheless, ongoing cosmological observations suggest potential limitations of general relativity, spurring interest in modified theories of gravity. This work investigates the Lorentz-gauge formulation of gravity-a novel framework that addresses key conceptual challenges in quantum gravity and cosmology by leveraging the recent black hole solutions presented in Ref. \cite{Koivisto:2024asr}. By analyzing black hole shadow structures and gravitational lensing effects-both weak and strong deflection regimes-we highlight unique observational signatures of Lorentz gauge gravity. Our findings provide valuable tools for future observational tests, potentially distinguishing these modified gravity models from general relativity and advancing our understanding of spacetime geometry and fundamental gravitational interactions.

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arXiv2025

Shadows aspect of rotating black holes in the Einstein-AdS SU(N)-nonlinear sigma model

Recent observations of the supermassive black holes M87* and Sgr A* by the Event Horizon Telescope (EHT) have opened new avenues for testing gravity theories through black hole shadow observables. These observations offer a means to distinguish between general relativity and modified gravity theories while providing insights into the astrophysical properties of the observed black holes. In this work, we investigate photon orbits and shadow characteristics of rotating black holes within the Einstein-$\mathrm{SU}(N)$ nonlinear sigma model. We first analyze the static, spherically symmetric black hole solution, focusing on its asymptotically Anti-de Sitter behavior and causal structure. The Modified Newman-Janis Algorithm is then applied to obtain the rotating counterpart, followed by an examination of its geometric properties, ergoregions, and causal structure. Utilizing the Lagrangian formalism, we derive the equations of motion for photons and study the resulting black hole shadow on the celestial plane. We explore the dependence of the shadow's size and shape on the black hole parameters $K$ and $N$, imposing constraints using EHT observational data for M87* and Sgr A*. Finally, we analyze the black hole's evaporation rate under different scenarios.

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arXiv2025

Black hole with global monopole charge in self-interacting Kalb-Ramond field

In this study, we explore a static, spherically symmetric black hole solution in the context of a self-interacting Kalb-Ramond field coupled with a global monopole. By incorporating the effects of Lorentz-violating term $\ell$ and the monopole charge $η$ in the KR field, we derive the modified gravitational field equations and analyze the resulting black hole spacetime. The obtained solution exhibits deviations from the Schwarzschild metric with topological defect, as it is influenced by the monopole charge and self-interaction potential. We investigate the thermodynamic properties of the black hole, including its Hawking temperature, entropy, and specific heat, revealing novel stability conditions. Additionally, we perform solar system tests such as perihelion precession, gravitational redshift, light deflection, and time delay of signals to impose constraints on the Lorentz-violating parameter and monopole charge. Our findings suggest that these parameters have to be significantly small, although there are different constraints imposed by individual tests, ranging from $10^{-9}\leq|\ell|\leq 10^{-4}$ and $10^{-9}\leqη\leq 10^{-6}\, \mathrm{m}^{-1}$.

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arXiv2025

Shadow analysis of an approximate rotating black hole solution with weakly coupled global monopole charge

We investigate the shadow properties of a rotating black hole with a weakly coupled global monopole charge, using a modified Newman-Janis algorithm. This study explores how this charge and rotational effects shape the black hole's shadow, causal structure, and ergoregions, with implications for distinguishing it from Kerr-like solutions. Analysis of null geodesics reveals observable features that may constrain the global monopole charge and weak coupling parameters within nonminimal gravity frameworks. Observational data from M87* and Sgr A* constrain the global monopole charge and coupling constant to $0 \leq γ\lesssim 0.036$ and $-0.2 \lesssim α\leq 0$, respectively.

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arXiv2025

Thermodynamic and observational constraints on black holes with primary hair in Beyond Horndeski Gravity: Stability and shadows

In this paper, we find that unlike in General Relativity, the shift-symmetric subclass of Beyond Horndeski theories permits black holes with primary hair that are thermodynamically stable and align with current Event Horizon Telescope observations of the M87* and Sgr A* black holes. This work begins by investigating thermodynamic properties, analyzing how primary hair influences thermodynamic quantities and local stability, which imposes strict constraints on the allowed range of primary hair values. The null geodesics near this black hole are then examined, demonstrating how scalar hair affects the shadow diameter. Specifically, when the parameter of the Beyond Horndeski function $F_4$ is negative, increasing scalar hair enlarges the shadow; in contrast, when this parameter is positive, greater scalar hair reduces the shadow size. Further constraints on the scalar hair are derived using observational data, highlighting its sensitivity to other black hole parameters. To explore additional observational features, face-on two-dimensional images of spherically infalling accretion disks are simulated, revealing how primary scalar hair shapes the black hole's shadow. Finally, all relevant constraints are combined to identify black holes that are both stable and consistent with observational data.

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arXiv2024

Null geodesics around a black hole with weakly coupled global monopole charge

In this paper, we study an asymptotically flat black hole spacetime with weakly nonminimally coupled monopole charge. We analytically and numerically investigate light ray propagation around such a black hole by employing the common Lagrangian formalism. Our analysis encompasses both radial and angular geodesics, for which we present analytical solutions in terms of incomplete Lauricella hypergeometric functions. Additionally, we explore the impact of the coupling constant on geodesic motion. Based on observations from the Event Horizon Telescope, we constrain the black hole parameters, resulting in a coupling constant range of $-0.5\lesssim α\lesssim 0.5$. Throughout our analysis, we simulate all possible trajectories and, where necessary, perform numerical inversion of the included integrals.

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arXiv2024

Orbits of particles with magnetic dipole moment around magnetized Schwarzschild black holes: Applications to S2 star orbit

This study provides a comprehensive analytical investigation of the bound and unbound motion of magnetized particles orbiting a Schwarzschild black hole immersed in an external asymptotically uniform magnetic field, which includes all conceivable types of bounded and unbounded orbits. In particular, for planetary orbits, we perform a comparative analysis of our findings with the observed position of the S2 star carrying magnetic dipole moment around Sagittarius A* (Sgr A*). We found maximum and minimum values for the parameter of magnetic interaction between the magnetic dipole of the star and the external magnetic field, as well as the energy and angular momentum of the S2 star. As a result, we obtain estimations of the magnetic dipole of the star in order of $10^6 \rm \ G\cdot cm^{3}$. Additionally, we explore deflecting trajectories akin to gravitational Rutherford scattering. In obtaining the solutions for the orbital equations, we articulate the elliptic integrals and Jacobi elliptic functions, and our study is augmented by illustrative figures and simulations.

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arXiv2024

Light propagation around a Kerr-like black hole immersed in an inhomogeneous anisotropic plasma in Rastall gravity: Analytical solutions to the equations of motion

In this paper, we explore the behavior of light ray trajectories in the exterior geometry of a rotating black hole within the Rastall theory of gravity, which is surrounded by an inhomogeneous anisotropic electronic cold plasma. By specifying the plasma's frequency profile, we derive fully analytical solutions for the temporal evolution of spacetime coordinates using elliptic integrals and Jacobi elliptic functions. These solutions illustrate various possible orbits. Throughout the study, we compare the results with those in the vacuum case, emphasizing the influence of plasma. Additionally, we utilize the analytical solutions to establish the lens equation for the considered spacetime. The investigation also addresses the significance of spherical photon orbits on critical trajectories, by presenting several examples.

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arXiv2024

Black hole in a generalized Chaplygin-Jacobi dark fluid: shadow and light deflection angle

We investigate a generalized Chaplygin-like gas with an anisotropic equation of state, characterizing a dark fluid within which a static spherically symmetric black hole is assumed. By solving the Einstein equations for this black hole spacetime, we explicitly derive the metric function. The spacetime is parametrized by two critical parameters, $\mathcal{B}$ and $α$, which measure the deviation from the Schwarzschild black hole and the extent of the dark fluid's anisotropy, respectively. We explore the behavior of light rays in the vicinity of the black hole by calculating its shadow and comparing our results with the Event Horizon Telescope observations. This comparison constrains the parameters to $0 \leq \mathcal{B} < 0.03$ and $0 < α< 0.1$. Additionally, we calculate the deflection angles to determine the extent to which light is bent by the black hole. These calculations are further utilized to formulate possible Einstein rings, estimating the angular radius of the rings to be approximately $37.6\,\mathrm{μas}$. Throughout this work, we present analytical solutions wherever feasible, and employ reliable approximations where necessary to provide comprehensive insights into the spacetime characteristics and their observable effects.

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arXiv2024

Observational Signatures: Shadow cast by the effective metric of photons for black holes with rational non-linear electrodynamics

This study explores spherically symmetric non-linear electrodynamics black holes and their effects on light propagation. We derive the governing metric, revealing radial coordinate dynamics within the event horizon. We analyze photon trajectories, finding that increasing magnetic charge expands the horizon and emission range. Furthermore, with the help of the Event Horizon Telescope results, we constrain parameters and emission profiles. Direct emission dominates, while lensing rings play a lesser role. Comparing with Schwarzschild black holes, we observe higher intensity but a wider emission region in non-linear electrodynamics black holes. This work enhances our understanding of modified spacetimes and their impact on black hole properties.

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arXiv2023

Observational signatures of a static $f(R)$ black hole with thin accretion disk

In this study, we focus on a static spherically symmetric $f(R)$ black hole spacetime characterized by a linear dark matter-related parameter. Our investigation delves into understanding the influence of different assumed values of this parameter on the observable characteristics of the black hole. To fulfill this task, we investigate the light deflection angles, which are inferred from direct analytical calculations of null geodesics.} To examine the black hole's properties further, we assume an optically thin accretion disk and explore various emission profiles. Additionally, we investigate the shadow cast by the illuminated black hole when affected by the disk. Furthermore, we simulate the brightness of an infalling spherical accretion in the context of silhouette imaging for the black hole. Our findings indicate that, except for some specific cases, the observed brightness of the accretion disk predominantly arises from direct emission, rather than lensing and photon rings. Moreover, we reveal that the linear dark parameter of the black hole significantly influences the shadow size and brightness. Our discussion covers both analytical and numerical approaches, and we utilize ray-tracing methods to produce accurate visualizations.

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arXiv2023

Spherical particle orbits around a rotating black hole in massive gravity

In this paper, we present a rotating de Rham-Gabadadze-Tolley black hole with a positive cosmological constant in massive gravity, achieved by applying a modified Newman-Janis algorithm. The black hole exhibits stable orbits of constant radii, prompting a numerical study on the behavior of the solutions to a nonic equation governing the radii of planar orbits around the black hole. Additionally, we investigate the stability of orbits near the event horizon and provide a comprehensive analytical examination of the solutions to the angular equations of motion. This is followed by simulating some spherical particle orbits around the black hole.

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arXiv2023

Analytical study of particle geodesics around a scale-dependent de Sitter black hole

We give a fully analytical description of radial and angular geodesics for massive particles that travel in the spacetime provided by a (3+1)-dimensional scale-dependent black hole in the cosmological background, for which, the quantum corrections are assumed to be small. We show that the equations of motion for radial orbits can be solved by means of Lauricella hypergeometric functions with different numbers of variables. We then classify the angular geodesics and argue that no planetary bound orbits are available. We calculate the epicyclic frequencies of circular orbits at the potential's maximum and the deflection angle of scattered particles is also calculated. Finally, we resolve the raised Jacobi inversion problem for the angular motion by means of a genus-2 Riemannian theta function, and the possible orbits are derived and discussed.

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