arXiv subjects

Uktamjon Uktamov

Publications and source records attributed to Uktamjon Uktamov.

11 recordsLinked to original sources

arXiv2026

Perturbations and quasinormal modes of black holes in general relativity coupled to nonlinear electrodynamics

We investigate the quasinormal spectra and time-domain evolution of scalar, electromagnetic, and gravitational perturbations of the Einstein-Bronnikov black hole in Einstein gravity coupled to nonlinear electrodynamics. The effective potentials associated with all perturbation sectors are shown to form positive potential barriers that vanish at the event horizon and spatial infinity. Their height increases with the magnetic charge, while the scalar perturbation possesses the largest potential barrier. Unlike the Reissner-Nordström black hole, axial and polar electromagnetic perturbations are characterized by distinct effective potentials, demonstrating the breaking of isospectrality due to nonlinear electrodynamics. Quasinormal frequencies are computed using the sixth-order WKB approximation and the asymptotic iteration method, yielding excellent agreement for both fundamental and higher overtone modes. The oscillation frequencies increase monotonically with the magnetic charge, whereas the damping rates exhibit only moderate variations, confirming the linear stability of the Einstein-Bronnikov black hole under all perturbations considered. Time-domain profiles display exponentially damped ringdown signals consistent with the frequency-domain analysis. These results demonstrate that nonlinear electrodynamics leaves measurable imprints on black hole perturbations and may provide observational signatures for testing regular black hole geometries through future gravitational wave observations.

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arXiv2026

Horizon-Brightened Acceleration Radiation and the Deflection Angle Near a Degenerate Photon Sphere of Schwarzschild-like Quantum-Corrected Black Hole

We investigate horizon-brightened acceleration radiation (HBAR) and a strong-deflection expansion for the deflection angle of light rays scattered in the vicinity of a degenerate photon sphere, within the context of a quantum-corrected black hole spacetime. We characterize the horizon structure and thermodynamics, and we extract the divergent part of the deflection-angle integral from the near-marginal-orbit contribution using a nonsingular prescription at marginality, obtaining a unique leading power-law term. In terms of the closest-approach radius, the strong-deflection leading coefficient factorizes into a universal branch constant and a local factor involving the third derivative of the effective potential at the degenerate photon sphere. On the quantum side, we develop the near-horizon reduction relevant to HBAR, demonstrating that the dominant sector governing the detector response exhibits conformal behavior and yields a thermal excitation spectrum characterized by the horizon temperature. We adopt a Lindblad master-equation framework for the radiation field, establish the existence of a thermal steady state, and obtain an HBAR entropy-energy relation that satisfies a Clausius-type first-law structure. Also, we derive a Wien-type displacement law for the HBAR spectrum, connecting the peak wavelength to horizon thermodynamics and thereby providing an additional observable probe of quantum gravity via near-horizon radiation.

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arXiv2026

Spinning particle dynamics, epicyclic frequencies, and transient QPO signatures in Schwarzschild spacetime

We study the motion of spinning test particles in Schwarzschild spacetime within the Mathisson--Papapetrou--Dixon pole--dipole approximation, imposing the Tulczyjew--Dixon spin supplementary condition. Restricting to equatorial orbits with the particle spin aligned with the orbital angular momentum, and retaining terms through linear order in the specific spin $s$, we derive the spin-corrected radial potential, circular-orbit conditions, bound periodic trajectories, epicyclic frequencies, and Lyapunov exponents of unstable circular orbits. The spin--curvature coupling shifts the circular-orbit energy and angular momentum and moves the innermost stable circular orbit to $r_{\rm ISCO}=6M-2\sqrt{2/3}\,s+\mathcal{O}(s^2)$ in the sign convention adopted here. We construct bound periodic orbits using the Levin--Perez-Giz zoom--whirl taxonomy and show how the particle spin deforms the corresponding energy--angular-momentum map. We then obtain the coordinate-time azimuthal and radial epicyclic frequencies and use them as kinematical inputs for relativistic-precession and resonance prescriptions for quasi-periodic oscillations. Finally, we relate the Lyapunov exponent of unstable circular orbits to the local separatrix structure governing near-homoclinic zoom--whirl motion. The resulting formulation provides a compact analytic connection between linear-in-spin MPD dynamics, periodic-orbit taxonomy, epicyclic-frequency shifts, and transient strong-field phenomenology in a nonrotating black-hole background. Also, we study the gravitational waveforms from the periodic orbits of a massive spinning particle around a black hole, presenting those associated with extreme mass-ratio inspirals involving a stellar-mass compact spinning object orbiting a supermassive black hole.

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arXiv2026

Chaotic motion and power spectral density in Schwarzschild Bertotti-Robinson black hole spacetime

In this paper, we show that in weak field limit Schwarzschild Bertotti-Robinson black hole (Schwarzschild-BR BH) turns into Schwarzschild black hole immersed in external uniform magnetic field which is given in 1. The dynamics of both magnetized and electrically charged particles in the vicinity of a Schwarzschild-BR black hole are investigated. The innermost stable circular orbits (ISCOs) for both magnetized and electrically charged particles are examined in detail, revealing that the magnetic field parameter B exerts a considerable influence, leading to an increase in the ISCO radius. The orbital and epicyclic motion of test particles in Schwarzschild-BR black hole spacetime was analyzed, including both circular orbits and their oscillatory perturbations. Additionally, the trajectories of both magnetized and electrically charged particles are analyzed for various configurations of the magnetic parameter B. We also demonstrate how the magnetic field B, electric charge q, and magnetic moment μ influence the dynamics of charged particles, specifically affecting the chaotic behavior, Poincare' sections, oscillatory frequencies and power spectral density.

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arXiv2026

New analytical model of static black hole with a dark matter halo and parametric constraints through quasiperiodic oscillations

A novel analytical Schwarzschild-like black hole (BH) solution is derived. It exhibits a static BH with a dark matter (DM) halo characterized by a Dehnen-type density profile. This solution could represent an alternative perspective on the interaction of black hole-dark matter systems, providing new insights into the fundamental properties of DM halos. We study the properties of the newly derived BH solution by examining its spacetime curvature characteristics and energy conditions, providing insights into how the DM halo influences these fundamental characteristics. Additionally, we analyze the timelike geodesics of test particles in the obtained BH-DM spacetime, highlighting how the presence of the novel Dehnen-type DM halo alters the gravitational dynamics and modifies particle trajectories. Increase of the DM halo's density $ρ_s$ and characteristic scale $r_s$ leads to an outward shift of both stable and unstable circular orbits. Finally, we test our model by fitting it to real data from the microquasars GRO J1655-40, GRS 1915+105, and XTE J1550-564 using a statistical Markov Chain Monte Carlo (MCMC) method. This allows us to find the best estimates for the properties of the DM halo surrounding these systems.

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arXiv2026

Horizon-Brightened Acceleration Radiation and Optical Signatures of Generic Regular Black Holes from Nonlinear Electrodynamics

We investigate horizon-brightened acceleration radiation (HBAR) and optical signatures for a broad class of regular black holes sourced by nonlinear electrodynamics. The spacetimes considered are static, spherically symmetric, and nonsingular, and they include Bardeen-like, and Hayward-like regular black-hole limits as spacial cases. We characterize the horizon structure and thermodynamics properties, and we compute key optical observables by determining the photon-sphere location and the corresponding shadow size as seen by distant observers, including controlled perturbative limits and full numerical solutions. Using angular-size constraints for SgrA* and M87* from the Event Horizon Telescope and the GRAVITY collaboration, we perform a Markov Chain Monte Carlo analysis to infer the admissible parameter ranges of the model and to quantify degeneracies among the black-hole mass and nonlinear-electrodynimcs parameters. On the quantum side, we develop the near-horizon reduction relevant for HBAR, showing that the dominant sector governing the detector response exhibits conformal behavior and leads to a thermal excitation spectrum governed by the horizon temperature. We formulate a Lindblad master-equation description of the radiation field, identify the thermal steady state, and derive an HBAR entropy-energy relation consistent with a Clausius-type first law. Finally, we establish a Wien-type displacement law for the HBAR spectrum, expressing the peak wavelength in terms of the horizon thermodynamics, thereby providing an additional observable link between nonlinear electrodynamics, regularity, and near-horizon quantum radiation.

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arXiv2026

New analytical model of rotating black hole with dark matter halo: constraints from EHT observations and accretion disk

In this paper, we start from a static black hole (BH) immersed in a Deanne-type dark matter (DM) halo and employ the Newman-Janis algorithm (NJA) to generate the rotating black hole solution with a dark matter halo. Also, we have checked the validity of the obtained space-time. Then we study optical properties of newly obtained rotating BH in DM halo, including the shadow's geometrical shape, deflection angle of light based Ono, Ishihara and Asada (OID) method, photon sphere and the dependence of the shadow radius on DM parameters. Additionally, assuming that spacetime of a supermassive black hole (SMBH) is described by the newly obtained rotating BH solution, we analyze the parameters of the model with shadow size estimates based on the Event Horizon Telescope (EHT) and Gravity collaboration observations of M87* and Sgr A* SMBHs. Then we have used Markov Chain Monte Carlo (MCMC) analysis to constrain DM parameters $ρ_s$, $r_s$ and BH mass M, BH spin a, also we show that best-fit values for the parameters $ρ_s$, $r_s$ are well agreement with previous results which indicate physically reasonability of the our model. Finally, we have analyzed the electromagnetic radiation flux of the rotating BH in the DM halo employing a ray tracing code.

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arXiv2026

Charged particle bound orbits around magnetized Schwarzschild black holes: S2 star and hotspot applications

The dynamics of charged particles around magnetized black holes provide valuable insights into astrophysical processes near compact objects. In this work, we investigate the bound and unbound trajectories of charged particles in the vicinity of a Schwarzschild black hole immersed in an external, uniform magnetic field. By analyzing the effective potential and solving the corresponding equations of motion, we classify the possible orbital configurations and identify the critical parameters governing the transition between stable and escape trajectories. The influence of the magnetic field strength and particle charge on the orbital structure, energy, and angular momentum is systematically explored. Applications of the obtained results are discussed in the context of the S2 star orbiting Sagittarius A* and the motion of bright hotspots detected near the event horizon, offering a potential interpretation of recent observations in terms of magnetized dynamics. The study contributes to a deeper understanding of charged-particle motion around black holes and its relevance to high-energy astrophysical phenomena in the galactic center. Finally, we test our model by fitting it to real data from the observed trajectory of the S2 star using a statistical Markov Chain Monte Carlo (MCMC) method. This allows us to find the best estimates for magnetic field and charge of the S2 star.

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arXiv2026

Acceleration Radiation of Freely Falling Atoms: Nonlinear Electrodynamic Effects

Motivated by the work of Scully \textit{et al.} [ \textcolor{blue}{Proc. Nat. Acad. Sci. 115, 8131 (2018)}] and Camblong \textit{et al.}[ \textcolor{blue}{Phys. Rev. D 102, 085010 (2020)}], we investigate horizon-brightened acceleration radiation (HBAR) for freely falling two-level atoms in the geometry of a Bardeen regular black hole. Building on the quantum-optics approach to acceleration radiation and its near-horizon conformal quantum mechanics (CQM) structure, we show that the dominant physics is again governed by an inverse-square potential in the radial Klein-Gordon equation, with an effective coupling fixed by the Bardeen surface gravity. Using geodesic expansions and a near-horizon CQM reduction of the scalar field, we derive the excitation probability for atoms falling through a Boulware-like vacuum in the presence of a stretched-horizon mirror. The resulting spectrum is Planckian in the mode frequency, with a temperature determined by the Bardeen Hawking temperature. We analyze how the regular core parameter controls the strength of the radiation and demonstrate that the excitation probability is strongly suppressed as the geometry approaches the extremal (cold remnant) limit. Numerical results illustrate the dependence of the spectrum on the Bardeen parameter and on the atomic transition frequency.

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arXiv2024

Particle dynamics and the accretion disk around a Self-dual Black Hole immersed in a magnetic field in Loop Quantum Gravity

In this paper, we study the motion of magnetic dipoles and electrically charged particles in the vicinity of a self-dual black hole in Loop Quantum Gravity (LQG) immersed in an external asymptotically uniform magnetic field. We explore the effects of the quantum correction parameter and electromagnetic interactions on the particle geodesics. We derive the field equations and determine the electromagnetic four-vector potential for the case of a self-dual black hole in LQG. We investigate the innermost stable circular orbits (ISCOs) for both magnetic dipoles and electrically charged particles in detail, demonstrating that the quantum correction parameter significantly influences on the ISCO radius, causing it to shrink. Additionally, we show that the ISCO radius of magnetic dipoles is greater than that of electrically charged particles due to the magnetic field interaction. We investigate the ISCO parameters (i.e., $r_{ISCO}$, $l_{ISCO}$, $\mathcal{E}_{ISCO}$, $v_{ISCO}$, and $Ω_{ISCO}$) for magnetic dipoles and electrically charged particles, providing detailed values. Furthermore, we examine the trajectories of charged particles under various scenarios resulting from the quantum correction parameter $P$. Finally, analyzing the ISCO parameters that define the inner edge of the accretion disk, we explore the accretion disk around a self-dual black hole in LQG. We delve into the electromagnetic radiation flux, temperature, and differential luminosity as radiation properties of the accretion disk in detail. We show that the quantum correction parameter shifts the profile of the electromagnetic flux and accretion disk temperature towards the central object, leading to a slight increase in these quantities.

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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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