arXiv subjects

Faizuddin Ahmed

Publications and source records attributed to Faizuddin Ahmed.

At least 55 records · Page 3Linked to original sources

arXiv2026

Accelerating Bertotti-Robinson Black Holes in a Uniform Magnetic Field

We study the Hawking temperature, geodesic motion, and observable signatures of the accelerating Bertotti-Robinson (BR) spacetime, a vacuum black-hole solution deformed by a uniform magnetic field $B$ and an acceleration parameter $α$. In the timelike sector, we derive the effective potential for massive particles, determine the specific energy and angular momentum for equatorial circular orbits, and determine how $(B,α)$ shifts the ISCO; we also illustrate representative trajectories of massive particles. We then compute the radial and latitudinal epicyclic frequencies for small perturbations about circular orbits, quantifying how the magnetic field and acceleration modify local radial and vertical stability. In the null sector, we derive the photon effective potential and obtain analytical expressions for the photon-sphere radius, critical impact parameter, and shadow radius, complemented by photon trajectories, the effective radial force, and the Lyapunov exponent controlling the instability of circular null orbits; we also provide parameter-space maps for the photon sphere and shadow. Finally, we obtain the energy emission rate emitted from the black hole, showing how the acceleration parameter and the magnetic field affect this.

gr-qc↗

arXiv2026

Shadow and Thermodynamics of deformed Schwarzschild-AdS black hole with a Cloud of Strings embedded in Perfect Fluid Dark Matter

We investigate the optical and thermodynamic properties of a deformed Schwarzschild Anti-de Sitter (AdS) black hole coupled to a cloud of strings and embedded in perfect fluid dark matter. We analyze the photon sphere and the corresponding black hole shadow, determining how these observables are affected by the string cloud, dark matter distribution, and geometric deformation. The thermodynamic behavior is studied through both the quasi-homogeneous fundamental equation within the classical physical approach and the geometric framework of geometrothermodynamics (GTD), showing full consistency between the two descriptions. In the extended phase space, we examine the critical structure and phase behavior of the system. Our analysis reveals that neither the geometric deformation nor the dark matter parameter generates new phase transitions; criticality emerges only in the vicinity of the Reissner--Nordström--AdS solution (RN--AdS), where the deformation parameter effectively plays the role of an electric charge squared. These results clarify the interplay between matter distributions, geometric deformations, and the phase structure of AdS black holes.

gr-qc↗

arXiv2026

$P$--$V$ criticality, Joule--Thomson expansion, and holographic heat engine of charged Hayward-AdS black holes with a cloud of strings and perfect fluid dark matter

We construct the charged Hayward-anti-de Sitter (AdS) black hole (BH) with a cloud of strings (CS) and perfect fluid dark matter (PFDM), and analyze its extended thermodynamic phase structure. The Hayward parameter $g$ replaces the central singularity with a de Sitter (dS) core, while the CS parameter $a$ and the PFDM parameter $β$ encode astrophysically motivated matter content. Treating the cosmological constant as pressure, we derive the thermodynamic quantities, verify the Smarr relation, and establish $P$--$V$ criticality with a van der Waals (vdW)-like small-large BH phase transition and mean-field critical exponents. The Gibbs free energy (GFE) exhibits the characteristic swallowtail below the critical pressure. The Joule-Thomson (JT) expansion yields $T_i^{\rm min}/T_c \approx 0.247$, roughly half the Reissner--Nordström-AdS value. The parameters $g$ and $Q$ contract the cooling region, $β$ expands it, and $a$ reshapes it non-monotonically. A holographic heat engine with a rectangular cycle gives efficiencies $η= 0.362$--$0.396$ and Carnot benchmarking ratios $η/η_C = 0.625$--$0.791$ across six configurations. The CS parameter improves the engine efficiency by reducing the enthalpy at fixed thermodynamic volume, while the PFDM parameter degrades it by adding gravitational enthalpy without contributing to the mechanical work.

gr-qc↗

arXiv2026

Quantum Oppenheimer-Snyder Black Holes with a Cloud of Strings Surrounded by Perfect Fluid Dark Matter

In this study, we examine quantum Oppenheimer-Snyder black holes (BHs) embedded within a cloud of strings and immersed in perfect fluid dark matter. Also, beginning with the underlying spacetime geometry, we determine how quantum corrections, string cloud contributions, and dark matter effects alter the geometrical structure and physical characteristics of the BH. Also, the optical behavior is investigated via a systematic analysis of the photon sphere and the associated BH shadow, emphasizing possible observational features capable of differentiating this configuration from classical models. We also analyze the motion of test particles, focusing on how surrounding matter components affect trajectories, stability conditions, and effective potentials. Scalar field perturbations are considered to investigate the BH response to external excitations and to extract information regarding its dynamical properties. In this case, the thermodynamic behavior of the system is studied, including the role of string clouds and dark matter in modifying BH thermodynamic quantities. Also, the obtained results present a unified description of the combined effects of quantum corrections, nonstandard matter sources, and BH physics, with potential relevance for both observational constraints and theoretical modeling of compact objects.

gr-qc↗

arXiv2026

Dymnikova Black Hole Immersed in Perfect Fluid Dark Matter and a Cloud of Strings: Hawking Temperature, Dynamics and QPOs Analysis

The Dymnikova black hole represents a regular spacetime solution interpolating between a de Sitter core and an asymptotically Schwarzschild geometry. In this work, we investigate a generalized Dymnikova black hole surrounded by perfect fluid dark matter (PFDM) and immersed in a cloud of strings (CS). We analyze how these additional matter sources modify the thermodynamic, optical, and dynamical properties of the spacetime. We derive the Hawking temperature and specific heat capacity and examine the thermal stability and phase structure of the black hole. The results reveal non-monotonic temperature behavior and parameter-dependent phase transitions. We further study photon dynamics, including the photon sphere and black hole shadow, and show that both PFDM and string cloud parameters significantly affect the shadow radius and strong-field structure. Additionally, we investigate the motion of massive test particles, circular orbits, and stability conditions. The corresponding effective potentials, specific energy, and angular momentum are analyzed. Finally, we explore quasi-periodic oscillations (QPOs) by computing the fundamental epicyclic frequencies and discuss how the model parameters encode observable astrophysical signatures.

gr-qc↗

arXiv2026

Some phenomenological aspects of a quantum-corrected Reissner-Nordström black hole: quasi-periodic oscillations, scalar perturbations and thermal fluctuations

In this work, we investigate several phenomenological aspects of a covariant quantum-corrected Reissner-Nordström black hole characterized by the mass $M$, electric charge $Q$, and the quantum correction parameter $ζ$. We first study the motion of neutral test particles and derive the fundamental orbital and epicyclic frequencies, which are then employed to analyze different quasi-periodic oscillation (QPO) models. Using observational QPO data from stellar-mass, intermediate-mass, and supermassive black hole candidates, we perform a Bayesian parameter estimation through a Markov Chain Monte Carlo (MCMC) analysis and obtain constraints on the black hole parameters. The results show that the presence of the quantum correction significantly affects the location of the QPO radii and the separation between the QPO orbit and the ISCO. We then examine the scalar perturbations by deriving the Schrödinger-like radial equation and the corresponding effective potential. The influence of the parameters $Q$ and $ζ$ on the perturbation potential and stability of the spacetime is discussed. Furthermore, we compute the greybody factor and the energy emission rate in the high-frequency (geometric-optics) regime, showing how the quantum correction modifies the absorption probability and radiation spectrum. Finally, we study the effect of thermal fluctuations on the black hole entropy and obtain the logarithmic corrections to the Bekenstein-Hawking area law. We show that these corrections become important for small black holes, while for large horizon radius the standard thermodynamic behavior is recovered. Our analysis demonstrates that the quantum correction parameter leaves observable imprints on both dynamical and thermodynamical properties of the spacetime and can be constrained through QPO observations.

gr-qc↗

arXiv2026

ModMax-AdS Black Hole with Global Monopole as Source in Kalb-Ramond Gravity

In this work, we investigate in detail the thermodynamic properties of a spherically symmetric ModMax-AdS black hole sourced by a global monopole within the Kalb-Ramond gravity. We derive the key thermodynamic quantities, including the Hawking temperature, Gibbs free energy, and specific heat capacity, and analyze how the geometric parameters influence these physical quantities. The first law of thermodynamics and the corresponding Smarr formula are explicitly verified. Furthermore, we study the thermodynamic criticality of the system by deriving the critical points and examining the effects of the space-time geometric parameters. We also obtain the inversion temperature and demonstrate that the minimum inversion temperature is modified by the space-time parameters. In addition, the sparsity of Hawking radiation and thermal fluctuations of the system are investigated, highlighting the effects of the parameters on the entropy corrections. Finally, we analyze the optical properties of the black hole, in particular the photon sphere and shadow radius, showing how these parameters influence these features.

gr-qc↗

arXiv2026

Thermal Analysis, Joule-Thomson Expansion and Hawking Sparsity of Mod(A)Max-AdS Black Hole Immersed in a Cloud of Strings

We investigate the thermodynamic behavior of a spherically symmetric Anti-de Sitter black hole in Mod(A)Max electrodynamics surrounded by a cloud of strings. Within the extended phase-space framework, we treat the cosmological constant as a pressure and interpret the black-hole mass as enthalpy, which enables a unified discussion of local stability, global phase structure, and Joule--Thomson expansion. We analyze the Hawking temperature, Gibbs free energy, and heat capacity, and show how the string-cloud parameter, the Mod(A)Max deformation, and the electric charge reshape the physical domain, the stability windows, and the small/large black-hole transition pattern. We further characterize the critical behavior and demonstrate that a van der Waals--like phase structure arises only in the physical sector, while the alternate branch does not admit a genuine critical point. For the Joule-Thomson process, we determine the inversion curve and the corresponding isenthalpic trajectories, highlighting how the model parameters control the cooling/heating regimes and can generate terminating isenthalpic behavior at sufficiently large charge. Finally, we examine the sparsity of Hawking radiation and discuss how the underlying parameters influence the temporal discreteness of the emitted flux, particularly near extremality and in the large-radius AdS regime.

physics.gen-ph↗

arXiv2026

Geodesic Structure, Thermodynamics and Scalar Perturbations of Mod(A)Max black hole Surrounded by Perfect Fluid Dark Matter

In this work, we investigate the optical properties of a spherically symmetric Mod(A)Max black hole surrounded by perfect fluid dark matter, focusing on key features such as the photon sphere radius, shadow, photon trajectories, and the effective radial force experienced by photons. We also study the dynamics of massive particles around the black hole, deriving the effective potential and, from it, the specific energy and angular momentum of particles moving in circular orbits of fixed radii is discussed. The conditions for marginally stable circular orbits are analyzed, highlighting how the geometric parameters that modify the spacetime curvature influence both the optical and dynamical features. Furthermore, we explore the thermodynamic behavior of the black hole by examining its temperature, Gibbs free energy, and heat capacity, as well as its thermodynamic topology. Finally, scalar field perturbations are considered through the massless Klein-Gordon equation, and the quasinormal modes (QNMs) in the eikonal regime are computed, illustrating how the geometric parameters affect the potential and the QNM spectra.

gr-qc↗

arXiv2026

Observable Optical Signatures, Particle Dynamics and Epicyclic Frequencies of Mod(A)Max Black Holes

In this work, we investigate the observable optical signatures of the Mod(A)Max black hole spacetime. We analyze key optical features, including the photon sphere, black hole shadow, and photon trajectories, and examine how these observables depend on the underlying geometric parameters, such as the electric charge and the Mod(A)Max coupling parameter. We further study the dynamics of neutral test particles in the vicinity of the black hole by deriving the effective potential within the Hamiltonian formalism. Using this potential, we obtain the specific energy and specific angular momentum for test particles on circular orbits of fixed radius, as well as the innermost stable circular orbit (ISCO), and explore how the geometric parameters influence these quantities and the ISCO radius. Finally, we derive the epicyclic (azimuthal, radial, and vertical) frequencies to analyze quasi-periodic oscillations (QPOs) exploring how the geometric parameters influences these and discuss their physical implications.

gr-qc↗

arXiv2026

Probing the Charged Hayward Black Hole in Dark Matter and String Cloud Environments through Shadow, Geodesics, and Quasinormal Spectrum

We construct a charged Bardeen black hole (BH) surrounded by perfect fluid dark matter (PFDM) and coupled to a cloud of strings (CS). The metric function combines the magnetic monopole charge from nonlinear electrodynamics, the PFDM logarithmic correction, and the CS parameter that renders the spacetime asymptotically non-flat. We analyze the horizon structure, identifying parameter ranges yielding non-extremal BHs, extremal configurations, and naked singularities. The null geodesics, photon sphere radius, and shadow are computed, revealing that both CS and PFDM enlarge the shadow. For neutral particle dynamics, we derive the specific energy, angular momentum, and innermost stable circular orbit location. Quasiperiodic oscillations (QPOs) are examined through the azimuthal, radial, and vertical epicyclic frequencies, where notably the azimuthal frequency is independent of the CS parameter. Scalar field perturbations governed by the Klein-Gordon equation yield an effective potential whose peak decreases with both parameters, yet the transmission and reflection probabilities respond oppositely to CS and PFDM variations. The greybody factor bounds are obtained using semi-analytical methods. Our results demonstrate that the distinct effects of $α$ and $β$ on various observables could allow independent constraints on these parameters through shadow measurements, QPO timing, and gravitational wave ringdown observations.

gr-qc↗

arXiv2026

Observational signatures of charged Bardeen black holes in perfect fluid dark matter with a cloud of strings

gr-qc↗

arXiv2026

Effects of spontaneous Lorentz Symmetry breaking on Letelier-AdS charged black holes within Kalb-Ramond gravity

In this study, we investigate the geodesic motion of massless particles -- specifically photons -- in the spacetime of a charged anti-de Sitter (AdS) black hole (BH) surrounded by a cloud of strings (CoS) within the framework of Kalb-Ramond (KR) gravity. We analyze the effective potential that governs photon trajectories, explore the properties and location of the photon sphere (PS), and examine the effective radial force acting on photons. The resulting BH shadow is also studied, highlighting the roles of both the CoS parameter $α$ and the KR field parameter $\ell$ in shaping its geometry. We constrain these parameters using observational data from M87* and Sgr A* obtained by the Event Horizon Telescope (EHT). Furthermore, we extend our investigation to the motion of neutral test particles in the same gravitational background. By examining the impact of the CoS and KR field, we show how these additional fields modify the dynamics relative to standard charged BH scenarios. Finally, we study the fundamental frequencies associated with quasiperiodic oscillations (QPOs) of test particles, demonstrating how these frequencies are affected by the presence of the CoS and KR field. Our results reveal the rich structure of AdS-BH spacetimes influenced by string clouds and antisymmetric tensor fields, with potential observational consequences in gravitational wave and BH imaging astronomy.

gr-qc↗

arXiv2026

Charged Simpson-Visser AdS Black Holes: Geodesic Structure and Thermodynamic Properties

In this article, we apply the Simpson-Visser (SV) regularization scheme to Anti-de Sitter (AdS) charged black holes and investigate the resulting spacetime geometry in detail, with emphasis on both geodesic structure and thermodynamic behavior. In particular, we analyze the motion of massless particle, focusing on key features such as the photon sphere, black hole shadow, photon trajectory and the dynamics of charged particles, including the characteristics of the circular and type of orbits. Furthermore, we compare the theoretical predictions of the charged SV-AdS black hole with recent observations reported by the Event Horizon telescope (EHT) for M87* and Sgr~A*. Beyond the geodesic analysis, we explore the thermodynamics of the regularized charged SV-AdS black hole by deriving essential quantities such as the Hawking temperature, Gibbs free energy, and specific heat capacity. Through a systematic examination of these thermodynamic variables, we demonstrate how the regularization parameter inherent in the SV regularization influences particle dynamics, stability conditions, and the overall thermal properties of the modified black hole solution. This comprehensive study highlights the interplay between regularization effects and the physical observables associated with charged AdS black holes.

gr-qc↗

arXiv2025

Optical appearance, Hawking radiation, and Barrow thermodynamics of Letelier black hole in electromagnetic universe

We present an investigation of a static, spherically symmetric Letelier black hole (BH) immersed in an electromagnetic universe (EMU), characterized by the cloud of strings (CoS) parameter $α$ and the EMU parameter $a$. The photon sphere and shadow radius are derived analytically, revealing how both parameters modify the apparent BH silhouette compared to the Schwarzschild case. We extend the shadow analysis to homogeneous and inhomogeneous plasma environments, demonstrating systematic reductions in the observed shadow size, and compute the weak gravitational lensing deflection angle in plasma using the Gauss-Bonnet theorem. The perturbative dynamics are investigated for scalar, electromagnetic, and Dirac fields, with quasinormal mode frequencies obtained via the sixth-order WKB approximation and greybody factors calculated using the rigorous bounds method. The resulting Hawking radiation spectra reveal distinct signatures for bosonic and fermionic emission channels. We further analyze quasi-periodic oscillations by deriving the fundamental orbital frequencies and applying both parametric resonance and relativistic precession models, obtaining constraints from observations.

gr-qc↗

arXiv2025

Thermodynamic geometry of charged AdS black holes with a string cloud in Lorentz-violating Einstein-Kalb-Ramond gravity

We investigate the thermodynamic microstructure of electrically charged AdS black holes in Einstein-Kalb-Ramond bumblebee gravity in the presence of a spherically symmetric cloud of strings. Employing Weinhold and Ruppeiner thermodynamic geometries in complementary thermodynamic representations, we show that curvature singularities consistently track the spinodal boundaries and the second-order critical point associated with a Van der Waals-like small/large black-hole phase transition. Moreover, the sign structure of the Ruppeiner curvature provides a transparent characterization of the competing microscopic interactions across the coexisting phases. We find that the Lorentz-violating parameter $\ell$ and the string-cloud parameter $α$ shift the critical scales and rescale correlation measures while preserving the universality class of the critical behavior. We further comment on dynamical and holographic implications and contrast the thermodynamic sensitivity to $(\ell,α)$ with thin-disk optical signatures.

hep-th↗

arXiv2025

Letelier black hole immersed in an electromagnetic universe

We investigate a static, spherically symmetric black hole solution surrounded by a cloud of strings and immersed in an electromagnetic universe. By deriving the event horizon from the lapse function, we demonstrate that both the string cloud parameter and the electromagnetic background parameter significantly modify the horizon radius compared to the Schwarzschild case. Consequently, thermodynamic quantities-including the Hawking temperature, Bekenstein-Hawking entropy, and heat capacity-become explicit functions of these additional parameters, with the heat capacity exhibiting divergences that signal phase transitions. We analyze the motion of massive test particles in this spacetime, deriving the effective potential and calculating the innermost stable circular orbit radius, which governs the inner edge of accretion disks and influences orbital stability. Scalar perturbations are examined through the associated effective potential, and quasinormal mode frequencies are computed using the sixth-order WKB approximation; the negative imaginary parts confirm the stability of the black hole under such perturbations. We also study the photon sphere structure, black hole shadow radius, and photon trajectories, showing how the interplay between string clouds and the electromagnetic background shapes the optical properties of this spacetime. Finally, we investigate weak gravitational lensing phenomena by deriving the deflection angle for both massive particles and photons using the Gauss-Bonnet theorem applied to the optical geometry. The results exhibit notable deviations from the Schwarzschild geometry, with the string cloud enhancing the deflection through a $(1-α)^{-1}$ factor while the electromagnetic parameter introduces competing corrections at second order.

gr-qc↗

arXiv2025

Thermodynamics and $P$-$V$ Criticality of Charged AdS Black Holes with a Cloud of Strings in Kalb-Ramond Gravity

We investigate the extended phase-space thermodynamics and $P$--$V$ criticality of electrically charged anti-de Sitter (AdS) black holes in Kalb--Ramond bumblebee gravity in the presence of a spherically symmetric cloud of strings. The background Kalb--Ramond field induces Lorentz symmetry violation through a dimensionless parameter $\ell$, while the string cloud is characterized by a parameter $α$, both entering the lapse function and deforming the geometry. Interpreting the ADM mass as enthalpy, we derive the main thermodynamic quantities, Hawking temperature, entropy, thermodynamic volume, Gibbs free energy, internal energy, and specific heat, and analyze how $(\ell,α)$ jointly affect stability and phase structure. We obtain a Van der Waals--type equation of state, compute the critical point $(P_c,T_c,v_c)$, and show that, although the critical scales depend nontrivially on $\ell$ and $α$, the universal ratio $P_c v_c/T_c = 3/8$ is preserved. Using the thermodynamic topology approach, we determine that the total topological charge of the black hole solution remains $W=1$, placing it in the same topological class as the Reissner--Nordström--AdS black hole. Finally, by studying the Joule--Thomson expansion, we derive the inversion curve and find that the minimal inversion temperature satisfies $T_i^{\rm min}/T_c = 1/2$, as in the Reissner--Nordström--AdS case, indicating that Lorentz violation and the string cloud deform the thermodynamic scales without changing the underlying universality class.

hep-th↗