arXiv subjects

Sanjar Shaymatov

Publications and source records attributed to Sanjar Shaymatov.

At least 37 records · Page 2Linked to original sources

arXiv2026

Gravitational wave signatures from periodic orbits around a Schwarzschild-Bertotti-Robinson black hole

In this paper, we investigate periodic bound orbits and gravitational wave (GW) emission in the Schwarzschild-Bertotti-Robinson (Schwarzschild-BR) spacetime-an exact electrovacuum solution describing a static black hole (BH) immersed in a uniform magnetic field. We explore how the background magnetic field qualitatively alters the BH's gravitational dynamics, affecting timelike geodesics such as the marginally bound orbit (MBO) and the innermost stable circular orbit (ISCO). We then analyze periodic bound orbits using the frequency ratio ${ω_φ}/{ω_{r}}$, which characterizes the orbits by their azimuthal and radial motions. Based on the numerical kludge method we further compute the gravitational waveforms emitted from periodic orbits around a supermassive Schwarzschild-BR BH. We show that the background magnetic field significantly changes orbital frequencies, resonance conditions, zoom-whirl structures, and the resulting waveforms. Finally, we examine the frequency spectra in the mHz range and the detectability of these GW signals by computing the characteristic strain via a discrete Fourier transform on the time-domain waveforms, comparing the results with the sensitivity curves of space-based GW detectors such as LISA, Taiji, and TianQin. Our results show that intrinsically magnetic fields modify spacetime and leave observable imprints on extreme mass-ratio inspiral GWs, which may be tested by future observations.

gr-qc↗

arXiv2026

Optical Signatures of a Schwarzschild Black Hole in a Dehnen-Type Dark Matter Halo

In this paper, the optical effects that occur near a Schwarzschild-like black hole (BH) with a Dehnen-type $(1,4,2)$ dark matter (DM) halo are explored. We first derive the photon sphere radius and obtain an analytical expression for the deflection angle in the weak-field regime by applying the Gauss-Bonnet theorem (GBT). For the strong-field regime, we perform ray-tracing calculations to examine the behavior of light trajectories and determine the corresponding number of orbits. We further compute the BH shadow and gravitational lensing in a plasma medium and provide constraints arising from the DM halo parameters. We also extend our analysis to weak gravitational lensing within plasma environments, considering both uniform and singular isothermal sphere (SIS) distributions. We find the analytical expressions for the deflection angle in the presence of plasma and examine the resulting effects on image magnification. The overall results highlight how DM halo properties and plasma characteristics jointly alter observable lensing signatures.

gr-qc↗

arXiv2026

Non-Monotonic Enhancement of the Magnetic Penrose Process in Kerr-Bertotti-Robinson Spacetime and its Implication for Electron Acceleration

We studied the magnetic Penrose process (MPP) in the Kerr-Bertotti-Robinson (KBR) spacetime, an exact rotating electrovacuum solution describing a black hole (BH) immersed in an intrinsic, uniform electromagnetic field. We analyze the behavior of charged particles in this geometry and find that the spacetime structure itself responds non-monotonically to the background magnetic field $B$. Specifically, both the event horizon and the static limit surface first expand as $B$ increases, reach a maximum size at an intermediate field strength, and then contract toward the extremal limit. Although the ergoregion itself shrinks monotonically with $B$, this structural feature gives rise to a pronounced non-monotonic dependence of the energy extraction efficiency on the magnetic field $B$, i.e., the efficiency initially rises, attains a maximum value, and subsequently falls as the extremal condition is approached. This contrasts sharply with the monotonic trends usually associated with magnetic enhancements in the Kerr geometry. We further explore an astrophysical application of the MPP by estimating the maximum energy of electrons escaping from the ergoregion of the KBR BH. Modeling neutron beta decay occurring near the event horizon, we derive an analytical expression for the energy gained by electrons accelerated by the magnetic field. Applying our results to the supermassive BH at the Galactic center, $\mathrm{SgrA}^*$, we find that electrons can be accelerated up to energies of $\sim 10^{15}\,\mathrm{eV}$ for realistic values of the spin and magnetic field. Although these energies exceed the observed upper range of cosmic-ray electrons, radiative losses such as synchrotron emission and inverse-Compton scattering can efficiently reduce them to the observed $\mathrm{TeV}$ scale.

gr-qc↗

arXiv2026

Epicyclic motion and accretion disk around a charged black hole in Einstein-ModMax theory with a quintessence field

We investigate the epicyclic motion of charged test particles and the associated quasi-periodic oscillations (QPOs) around a weakly magnetized black hole surrounded by quintessence within the framework of Einstein-ModMax theory. We analyze the dynamics of charged particles on circular orbits and derive the corresponding radial and vertical epicyclic frequencies. The influence of the nonlinear electrodynamics parameter, magnetic coupling, dyonic charge, and quintessence state parameter on the innermost stable circular orbit and epicyclic frequencies is examined in detail. Using the forced resonance model, we compare the theoretical predictions of high-frequency QPOs with observational data from several X-ray binary systems. A Markov Chain Monte Carlo analysis is employed to constrain the black hole parameters and assess the role of weak magnetization and nonlinear electrodynamics effects. This analysis indicates that QPO observations tightly constrain the black hole mass and orbital radius while placing stringent upper bounds on the ModMax coupling, magnetic interaction, and dyonic charge. In addition, we study the radiative properties of the accretion disk and analyze the effects of the model parameters on the disk flux and temperature profiles. These findings suggest that the observed QPOs are consistent with general relativity in the strong-field regime, allowing only small deviations associated with Einstein-Maxwell theory in the presence of a quintessence field.

gr-qc↗

arXiv2025

Nonlinear electrodynamic black holes and their role in testing modified theories of gravity

The nature of black holes (BHs) and their potential deviations from classical General Relativity (GR) remain central questions in modern astrophysics. Nonlinear electrodynamics (NED) offers a promising mechanism to construct regular BHs that avoid singularities while retaining key astrophysical features. In this work, we analyze NED-inspired BHs and derive constraints on the magnetic parameter using Bayesian parameter estimation from Event Horizon Telescope (EHT) observations. We obtain $q = 0.63 ^{+0.68}_{-0.44} \times 10^9 M_{\odot}$ for M87* and $q = 0.33 ^{+0.36}_{-0.23} \times 10^6 M_{\odot}$ for Sgr A*, affecting their horizon structure. A comparison with the Schwarzschild solution highlights deviations in geometry and horizon properties, emphasizing the impact of NED. We also examine observational signatures, including BH shadows and gravitational lensing. Using $R_{\text{sh}} = r_{\text{ph}} \sqrt{1/f(r_{\text{ph}})}$, we explore shadow features under uniform and non-uniform plasma conditions, revealing deviations from GR predictions due to NED effects. By analyzing photon sphere shifts and lensing patterns, we identify observable features that distinguish these BHs from classical ones. Our results suggest that NED-induced modifications may leave detectable imprints in strong-field regimes, opening pathways to test alternative gravity models. Future missions such as the Laser Interferometer Space Antenna (LISA) and next-generation X-ray observatories, along with ongoing observations from the EHT and gravitational wave detections by the Laser Interferometer Gravitational-Wave Observatory and Virgo (LIGO--Virgo), will be essential for refining constraints on such models.

gr-qc↗

arXiv2025

Lensing Stability and Scattering Phenomena in Anisotropic Black Hole Spacetimes in Plasma

We investigate the physical and observational features of static, spherically symmetric black hole spacetimes surrounded by anisotropic fluid and embedded in a plasma environment. Motivated by recent advances in black hole imaging and precision measurements in strong gravity, we explore light propagation, wave dynamics, and observational signatures in such geometries. We begin by analyzing the background spacetime and matter content, examining the horizon structure and verifying the energy conditions associated with the anisotropic fluid. We then study photon trajectories in both vacuum and plasma environments, deriving the equations of motion and computing deflection angles and image magnifications under weak gravitational lensing. Both uniform and power-law plasma profiles are considered to model realistic astrophysical settings. In the wave optics regime, we first analyze the linear stability of the spacetime under axial (odd-parity) perturbations using Chandrasekhar's method, and then investigate scalar field scattering by solving the wave equation in the curved background with plasma. Using Born and WKB approximations, we compute the differential scattering cross-sections and examine how anisotropy and plasma affect interference features. Finally, we perform parameter estimation using Markov Chain Monte Carlo methods to constrain the black hole mass and anisotropic fluid parameters, utilizing EHT and GRAVITY data for Sgr A* and EHT-only data for M87*. These results present a unified theoretical framework that links anisotropic matter effects with lensing and scattering observables providing a firm basis for future comparisons with high-resolution astrophysical data sets in diverse contexts.

gr-qc↗

arXiv2025

Gravitational waveforms from periodic orbits around a dyonic ModMax black hole

In this work, we study the gravitational waveforms from the periodic orbits of a massive particle around a dyonic ModMax black hole. We begin with a brief analysis of the spacetime and then examine how its parameters influence the dynamics of a massive neutral particle using the Lagrangian formalism. In particular, we compute the characteristics of marginally bound orbits and innermost stable circular orbits. Our results show that the values of these quantities increase with the black hole charge $Q$ and the screening parameter $γ$. We then plot various periodic orbits, characterized by the integers ($z$,$w$,$v$). Finally, we present the gravitational waveforms associated with extreme mass ratio inspirals, consisting of a stellar-mass compact object orbiting a supermassive black hole.

gr-qc↗

arXiv2025

Periodic orbits and observational accretion disk around a Schwarzschild-like black hole surrounded by dark matter halo

In this work, we investigate the dynamics of periodic orbits and the properties of accretion disks around a Schwarzschild-like black hole (BH) immersed in a King-type dark matter (DM) halo. Our analysis focuses on how the presence of the King DM halo influences both the behavior of periodic orbits and the radiative characteristics of the accretion disk. We begin by examining time-like periodic geodesic orbits for various configurations characterized by different energy and angular momentum values, represented by the integers $(z, w, v)$. Furthermore, we explore the effects of the King DM halo on time-like periodic geodesics, marginally bound orbits, and innermost stable circular orbits, thereby providing a deeper understanding of how the DM halo environment modifies the behavior of these stable orbits and timelike particle geodesics. Finally, we analyze the null geodesics and the accretion disk properties by studying their direct and secondary images, redshift distributions, and radiation fluxes as observed at infinity for a range of inclination angles. This approach allows us to gain valuable insights into the spacetime geometry of a Schwarzschild-like BH within the King-type DM halo, its physical and radiative properties in the accretion disk, and the corresponding observational implications.

gr-qc↗

arXiv2025

Probing the Schwarzschild black hole immersed in a dark matter halo through astrophysical tests

We investigate a recently derived Schwarzschild-like black hole immersed in a Dehnen-type $(α,β,γ)=(1,4,5/2)$ dark matter (DM) halo. We obtain constraints on the two model parameters, i.e., the halo core radius $r_s$ and the DM density parameter $ρ_s$ in both the weak and the strong field regimes. In the weak field, we model test particle geodesics and match the predicted perihelion shift to Mercury (Solar System) and the orbit of the S2 star data. We obtain upper limits on $r_s$ and $ρ_s$ and highlight that the DM halo effects become observable only around supermassive BHs. In the strong field, we analyse twin high frequency quasiperiodic oscillations (QPOs) from four microquasars (e.g., GRO~J1655-40, GRS~1915+105, XTE~J1859+226, and XTE~J1550-564). Because QPO frequencies depend only on the local spacetime curvature, they can serve as a probe of halo-induced deviations from general relativity. Our MCMC analysis produces posterior distributions for model parameters, revealing close agreement between the theoretical QPO frequencies and the observations for GRS 1915+105 and GRO J1655-40. The same analysis also yielded best-fit values and upper bounds for each parameter. Our combined geodesic and QPO analysis demonstrates that timelike orbits and epicyclic oscillations can act as sensitive probes of DM halos around BHs, offering a pathway to distinguish Dehnen-type profiles from alternative DM distributions in future analysis and observations.

gr-qc↗

arXiv2025

Regular black hole's impact on the gravitational waveforms from periodic orbits

In this paper, we investigate periodic orbits exhibiting zoom-whirl behavior around a magnetically charged black hole (MCBH) within the framework of the regular black hole. We examine how the magnetic charge influences orbital dynamics by modifying the background spacetime geometry, thereby affecting the energy and angular momentum of particles. In particular, we calculate the radii of the marginally bound orbits (MBOs) and innermost stable circular orbits (ISCOs), demonstrating that the magnetic charge parameter reduces both radii. This provides valuable insight into the role of the charge parameter in shaping orbital behavior and altering spacetime geometry. We model the complex motion of a stellar-mass object as a timelike particle, inspiraling into a supermassive black hole (SMBH) in the MCBH background, with its trajectory described using periodic geodesic orbits. Based on this analysis of such periodic orbits, we further analyze the gravitational waveforms generated by extreme mass ratio inspirals (EMRIs), in which the SMBH's spacetime dominates the dynamics of the stellar-mass object. By combining particle trajectory analysis with waveform modeling in a semi-analytical approach, we show that the charge parameter significantly alters the zoom-whirl orbital dynamics and induces notable changes in the waveform structure. These results illustrate that future gravitational wave (GW) observations may constrain the properties of MCBHs, thereby deepening our understanding of the gravitational imprint of regular black holes.

gr-qc↗

arXiv2025

Observable thin accretion disk around a self-dual black hole in loop quantum gravity

In this paper, we study a self-dual black hole (BH) in Loop Quantum Gravity (LQG), analyzing both timelike and null geodesics. Using observational data from Mercury's perihelion shift and the orbit of the S2 star around Sagittarius A$^{\star}$ (Sgr A$^{\star}$), we derive constraints on the polymeric function $P$. We further investigate photon trajectories near the self-dual BH under various scenarios to explore their observational relevance. Finally, we examine the properties of accretion disks around the self-dual BH in LQG, including their direct and secondary images, and study the redshift and the observed energy flux distribution across the accretion disk as measured by distant observers for different inclination angles. Our findings provide new insights into the physical nature and accretion properties of self-dual BHs in LQG and their possible observational consequences.

gr-qc↗

arXiv2025

Epicyclic oscillations and accretion disk around a special Buchdahl-inspired spacetime

In this paper, we consider the Buchdahl-inspired spacetime metric and investigate its aspects using the quasiperiodic oscillations and the accretion disk due to the accreting matter. First, we focus on analyzing the geodesics of particles around the Buchdahl-inspired spacetime, together with the conserved quantities such as specific energy and angular momentum for massive particles orbiting on the innermost stable circular orbits (ISCOs). We show that the effect of the Buchdahl parameter $\tilde{k}$ increases as the radii of the ISCO orbits decrease, resulting in shifting orbits toward the central object compared to the Schwarzschild black hole case. We also consider astrophysical epicyclic oscillations and derive their general expressions using implications of circular motion of massive particles around the Buchdahl-inspired spacetime. Further, we explore the astrophysical implications of observational higher-frequency QPOs of the selected galactic microquasars of X-ray binary systems to obtain the best-fit constraints on the Buchdahl-inspired spacetime parameters. Finally, we consider the accretion disk around the Buchdahl-inspired spacetime using implications of the ISCO parameters that define the accretion disk's inner edge. We explore radiation properties of the accretion disk and redshifted image and intensity of a lensed accretion disk around the Buchdahl-inspired spacetime.

gr-qc↗

arXiv2025

Magnetized Letelier black hole in AdS spacetime

In this study, we investigate the motion of charged, neutral, and light-like particles in a magnetized black hole solution surrounded by a cloud of strings in an anti-de Sitter (AdS) background. This spacetime admits several well-known solutions as special cases, including the Letelier-AdS black hole, the Melvin spacetime, and the Schwarzschild-AdS black hole. We demonstrate that key parameters characterizing the geometry-such as the cloud of strings parameter, the magnetic field strength, and the AdS radius-significantly affect the trajectories of these particles. Our analysis shows that increasing the cloud of strings parameter weakens the gravitational influence, while the magnetic field introduces additional attractive components that can destabilize particle orbits. We examine the photon sphere and calculate the black hole shadow, showing that the shadow radius decreases with increasing magnetic field parameter but increases with the cloud of strings parameter. These findings provide potential observables for distinguishing between different black hole models in realistic astrophysical environments.

gr-qc↗

arXiv2025

Exploring the Effects of Generalized Entropy onto Bardeen Black Hole Surrounded by Cloud of Strings

This work explores the thermodynamic characteristics and geothermodynamics of a Bardeen black hole (BH) that interacts with a string cloud and is minimally connected to nonlinear electrodynamics. To avoid the singularities throughout the cosmic evolution, we consider an entropy function which comprises five parameters. In addition, by employing this entropy function for the specific range of parameters, we obtain the representations of BH entropy based on the holographic principle. Moreover, we employ this entropy function to investigate its impact on the thermodynamics of the BH by studying various thermodynamic properties like mass, temperature, heat capacity, and Gibbs free energy for numerous scalar charge and string cloud values. To support our investigation, we use various geothermodynamics formalisms to evaluate the stable behavior and identify different physical scenarios. Furthermore, in this analysis, we observe that only one entropy formalism provides us with better results regarding the thermodynamic behavior of the BH. Moreover, it is shown that one of the entropy models provides a thermodynamic geometric behavior compared to the other entropy models.

gr-qc↗

arXiv2025

Gravitational waveforms from periodic orbits around a Schwarzschild black hole embedded in a Dehnen-type dark matter halo

In this paper, we study the periodic orbits, characterized by zoom-whirl behavior, around a Schwarzschild-like black hole (BH) embedded within a Dehnen-type dark matter (DM) halo. We demonstrate how the DM halo modifies the gravitational dynamics of the black hole, influencing the energy and angular momentum of timelike particle geodesics and enhancing their interaction with the BH. We determine the radii of the marginally bound orbits (MBOs) and innermost stable circular orbits (ISCOs), showing that the DM halo increases both. This provides a deeper understanding of how the DM alters the behavior, energy, and angular momentum of timelike particle geodesics. Furthermore, we explore the gravitational waveforms emitted by a timelike particle in periodic orbits around a supermassive black hole (SMBH) within this BH-DM system. Using a semi-analytical approach, we calculate particle trajectories and derive the corresponding waveforms, demonstrating that the DM halo modifies the zoom-whirl orbital behavior, leading to distinct changes in the waveform structure. Our findings suggest that future gravitational wave (GW) observations could constrain the properties of DM halos surrounding BHs, providing new insights into the gravitational wave signatures arising from the interaction between BH gravity and DM.

gr-qc↗

arXiv2025

Energy Extraction from Loop Quantum Black Holes: The Role of Magnetic Penrose Process and Quantum Gravity Effects with Astrophysical Insights

In this study, we explore the influence of quantum gravitational corrections, derived from Loop Quantum Gravity (LQG), on the efficiency of the magnetic Penrose process (MPP) in black hole (BH) environments. We begin by analyzing the rotating Loop Quantum Black Hole (LQBH) metric, describing the structure of the event horizon and ergosphere as functions of the quantum parameter $ε= γδ$, with $γ$ representing the Immirzi parameter and $δ$ the polymeric parameter, and the spin parameter $a$. These modifications provide a novel setting for exploring the dynamics of charged particles near the LQBH and evaluating the resultant energy extraction through the MPP. Interestingly, for a given value of the LQBH parameter $a$, we observe that the ergosphere region of the LQBH exhibits a more intricate structure compared to its classical counterpart, the Kerr BH, as $ε$ increases. Furthermore, we find that the overall efficiency of the process decreases with $ε$ that decreases $a_{max}$, again in contrast to the Kerr BH, where efficiency rises with an increasing $a$. Our analysis also extends to astrophysical contexts, applying constraints on the mass and magnetic field of LQBHs for astrophysical BH candidates, including SgrA*, M87*, NGC 1052, and BZ (Blandford and Znajek sources, i.e., supermassive BHs with masses around $10^9 M_\odot$ and magnetic fields in the range $10^3-10^4 \text{G}$). We assess these sources as potential accelerators of high-energy protons across different values of the quantum parameter $ε$. Additionally, we examine how variations in the magnetic field strength $B$ and quantum corrections impact the energy of protons accelerated from M87$^{\star}$ and Sgr A$^{\star}$ following beta decay.

gr-qc↗

arXiv2025

Shadow properties and orbital dynamics around an effective quantum-modified black hole surrounded by quintessential dark energy

In this study, we investigate black holes (BHs) surrounded by a quintessence field (QF) within the framework of effective quantum gravity (EQG). We analyze the spacetime metric characterized by quantum correction parameter $ξ$ and quintessence parameters $(c,w)$, revealing a rich three-horizon structure whose properties depend on both quantum effects and dark energy. Our main focus is on understanding the observational signatures and dynamical behavior of these modified BHs. We derive analytical expressions for the photon sphere and shadow radius for various values of the state parameter, finding that quintessence fields increase the shadow radius while quantum corrections decrease it-a distinctive interplay that creates potentially observable effects. Using Event Horizon Telescope data from M87*, we establish constraints on the quantum correction parameter, showing that the allowable range for $ξ$ increases with the quintessence parameter $c$. We conduct a comprehensive analysis of null and timelike geodesics, demonstrating how quantum corrections enhance interactions between photons and the gravitational field while modifying the energy, angular momentum, and stability properties of massive particle orbits. Our investigation extends to periodic orbits characterized by zoom-whirl behavior, finding that quintessence allows such orbits to occur at lower energies compared to purely quantum-corrected BHs. We further analyze scalar perturbations and their effective potentials, which reveal how both quantum and quintessence effects shape the response characteristics of these BHs. Throughout our study, we find that quantum corrections and quintessence frequently produce counteracting effects on observables that could provide simultaneous tests of quantum gravity theories and dark energy models through future high-precision astronomical observations.

gr-qc↗

arXiv2025

Electric Penrose process and collisions of particles near five-dimensional weakly charged Schwarzschild black hole

The particle dynamics and the electric Penrose process for the five-dimensional weakly charged Schwarzschild black hole are studied. Firstly, the horizon structure and the effective potential for the test particle are explored. The radial profile of the effective potential is plotted for different values of the BH charge. Then, we studied energy efficiency using the conservation laws for energy, angular momentum, and charge of particles. The appropriate plots were obtained and compared with the results for the four-dimensional Schwarzschild BH. Moreover, we obtained the constraints on mass and charge of black hole to accelerate protons of different energies ($1$-$10^{9}$ GeV). Finally, collisions of electrically charged particles near the horizon of the five-dimensional Schwarzschild BH are studied. We demonstrated the radial dependence of the center of mass energy for different values of the angular momentum and charge of the particles together with the BH charge.

gr-qc↗