arXiv subjects

Ainur Urazalina

Publications and source records attributed to Ainur Urazalina.

At least 19 recordsLinked to original sources

arXiv2026

Testing dark matter density profiles based on Padé approximants of different orders

We investigate whether low-order Padé rational functions can be used as empirical density profiles for modeling dark matter halos from galaxy RCs. We introduce three parameterizations determined from Padé series, denoted Padé 02, Padé 12 and Padé 03, and compare them with pseudo-isothermal, Burkert, Beta, Brownstein, exponential-sphere and Persic models. The analysis is performed for eight galaxies under the assumption that RCs are dark matter dominated. The parameters are inferred from RC data and the relative statistical performance of the models is obtained through the Bayesian Information Criterion. We find that the Padé profiles perform well, being comparable with the other profiles, albeit not universally preferred. In most galaxies, conventional two parameter profiles provide fits of comparable or better statistical quality, whereas the clearest improvement occurs for one particular galaxy, where the Padé 03 profile gives the lowest BIC. Even though the Padé profiles are often statistically less strong than other models, they appear plausible in explaining the dark matter nature. Hence, their empirical construction can therefore be used to reconstruct RCs phenomenologically.

astro-ph.GA↗

arXiv2026

Shirokov and Shapiro Effects in the Hartle-Thorne Spacetime

We investigate the influence of rotation and quadrupole deformations of astrophysical compact objects on the Shirokov and Shapiro effects within the Hartle-Thorne spacetime, which describes the exterior gravitational field of slowly rotating, slightly deformed celestial objects. Using geodesic deviation equations, we analyze the oscillatory motion of neighboring test particle trajectories and show how the combined impact of angular momentum $J$ and quadrupole moment $Q$ affects the Shirokov effect. The results are compared with our previous analysis for the Lense-Thirring and Zipoy-Voorhees metrics, revealing consistent trends in the coupling between radial and azimuthal oscillations. Importantly, by evaluating the period splitting in the weak-field regime we show that the dominant contribution to the Shirokov effect is the Newtonian quadrupole moment of the source rather than the relativistic mass term originally identified by Shirokov. For the Shapiro time delay, we examine two limiting configurations: (i) the Lense-Thirring frame -- dragging case with $J^2=0$, $Q=0$ and $J\neq0$, where the effect persists for both positive and negative values of the angular momentum; and (ii) the static quadrupolar case with $J=0$ and $Q\neq0$, where more oblate sources produce a stronger gravitational time delay with increasing distance. We also study these effects in the Hartle-Thorne spacetime without employing the weak-field approximation, performing a full numerical analysis. In particular, we examine the mimicking effects produced by the quadrupole deformation and the angular momentum of the compact object. These results illustrate how the deformation and rotation of compact objects influence the relativistic observables in the surrounding spacetime.

gr-qc↗

arXiv2026

Shadow, Emission, and Strong-Field Lensing of Dilatonic Black Holes

We study the shadow and strong-field optical properties of a static, spherically symmetric dyon-like dilatonic black hole. The photon sphere radius, critical impact parameter, and shadow radius are obtained and analyzed in terms of the charge $Q$ and the dilatonic coupling parameter $a$. We show that increasing these parameters decreases the photon sphere and shadow radii, leading to a smaller apparent shadow. The predicted angular diameter is compared with the observational data for M87$^{*}$ and SgrA$^{*}$, and the model parameters are constrained. We also estimate the high-frequency energy emission rate in the geometric-optics approximation and derive the leading Bozza coefficient $\bar{a}$, which characterizes the logarithmic behavior of the deflection angle in the strong-field regime. Astrophysical implication of the obtained outcomes are discussed.

gr-qc↗

arXiv2025

Accretion Disk Luminosity and Topological Characteristics for a Schwarzschild Black Hole Surrounded by King Dark Matter Halo

This study delves into the intricate properties of a Schwarzschild black hole enveloped by King dark matter in an isotropic configuration. The thermodynamic characteristics of this black hole are meticulously analyzed, and the dynamics of massive and massless particles in its vicinity are investigated. In examining the trajectories of massless particles, the shadow cast in the presence of King dark matter is explored, revealing virtual ranges for the corresponding parameters. For the dynamics of massive particles, the radius of the innermost stable circular orbit, angular momentum, energy, and angular velocity of a test particle within the King dark matter framework surrounding the black hole are calculated. The effect of King dark matter on the accretion disk energy flux, effective radiation temperature, differential luminosity, and spectral luminosity are then investigated. The stability of the photon sphere in the presence of King dark matter is also studied, and finally, the thermodynamic potentials of this black hole are examined from a topological perspective.

gr-qc↗

arXiv2025

Constraints on Kalb-Ramond Gravity from EHT Observations of Rotating Black Holes in Traceless Conformal Electrodynamics

We present a phenomenological study of rotating, charged black holes in Einstein gravity coupled to a traceless (conformal) matter sector formed by ModMax nonlinear electrodynamics and a Kalb-Ramond two-form that spontaneously breaks local Lorentz symmetry. Starting from a family of obtained static, Schwarzschild-like solutions with a traceless Kalb-Ramond sector, we construct the stationary, axisymmetric counterpart via the Newman-Janis algorithm. The resulting Newman-Kerr-like metric depends on four intrinsic parameters: the electric charge $Q$, the ModMax nonlinearity $γ$, the Lorentz-violation amplitude $\ell$ and the spin $a$. We analyze horizon structure and separatrices in parameter space, derive the null geodesic equations and obtain the photon capture boundary that defines the black hole shadow. Using ray-tracing, we compute shadow silhouettes and a suite of shadow observables (areal radius, characteristic radius $R_s$, distortion $δ$, oblateness $D$) and show how $γ$ and $\ell$ produce qualitatively distinct effects: $γ$ acts as a screening factor for the electromagnetic imprint, while $\ell$ introduces angular-dependent metric rescalings that deform shadow shape beyond simple size rescaling. We confront model predictions with EHT angular-radius measurements for M87$^*$ and Sgr A$^*$ and derive conservative bounds on the combinations of $(Q,γ,\ell,a)$. Our results identify an effective charge combination $Q_{\rm eff}\simeq e^{-γ}Q^{2}/(1-\ell)^{2}$ and demonstrate that modest $Q_{\rm eff}$ remains compatible with current EHT images while large $Q_{\rm eff}$ is progressively disfavored.

gr-qc↗

arXiv2025

Constraining quadrupole deformations with relativistic effects

We investigate two general relativistic effects - namely, the Shirokov and Shapiro effects - within the framework of the Zipoy-Voorhees spacetime ($q$-metric), which generalizes the Schwarzschild solution by incorporating a quadrupole moment. By analyzing the geodesic deviation equations, we explore the oscillatory motion of test particles and demonstrate how the source's quadrupole parameter influences the Shirokov effect. Furthermore, we derive an expression for the Shapiro time delay in this deformed spacetime and examine the quadrupole moment's impact on the gravitational time delay experienced by radio waves propagating near a massive object. The first-order approximation reveals a pronounced effect of the quadrupole parameter on the time delay, in contrast to similar recent analyses. These findings deepen our understanding of how deviations from spherical symmetry influence gravitational phenomena, with potential implications for the study of compact astrophysical objects such as neutron stars and naked singularities or ''black hole mimickers'' that exhibit significant multipolar structures.

gr-qc↗

arXiv2025

Geometric properties versus particle motion in the Fan-Wang spacetime

In this work, we explore general relativistic effects and geometric properties of the Fan-Wang spacetime, one of the simplest regular solutions that can be obtained in nonlinear electrodynamics. In particular, we investigate the motion of test particles, the capture cross-section of neutral massive and massless particles, such as neutrinos and photons, and the gravitational redshift. Additionally, using a perturbative approach, we derive analytical expressions for the perihelion shift and gravitational deflection of massless particles. By identifying the one-parameter corrections to the Schwarzschild spacetime, induced by the magnetic charge contained in the Fan-Wang metric, we show that this spacetime can be falsified, since it modifies classical general relativity predictions even at the local level. Moreover, we argue that these modifications could be experimentally tested with advanced observational instrumentation.

gr-qc↗

arXiv2025

Accretion disk luminosity around rotating naked singularities

We explore circular geodesics of neutral test particles in the gravitational field of a rotating deformed mass. The geometry around this source is described by the Quevedo-Mashhoon solution, which corresponds to a naked singularity. To this end, we compute the orbital parameters of test particles in the equatorial plane, such as the angular velocity $Ω$, energy $E$ and angular momentum $L$, per unit mass. We also numerically estimate the radius of the innermost stable circular orbit $r_{ISCO}$. In addition, we consider a simple model for the disk's radiative flux, differential luminosity, and spectral luminosity, employing the well-known Novikov-Page-Thorne model. We mainly focus on the flux of the accretion disk around several rotating naked singularities possessing the same mass and quadrupole moment but different rotation and deformation parameters. We analyze the possibility of distinguishing Kerr black holes from rotating naked singularities. The astrophysical implications of the results obtained are discussed.

gr-qc↗

arXiv2025

Geodesic deviation in the $q$-metric

We consider the tidal forces between test particles falling along geodesics in the exterior spacetime generated by a static and axially symmetric compact matter source with non-vanishing mass quadrupole. Specifically, we analyze the radial and angular geodesic deviation, compare it with that of the Schwarzschild spacetime, and investigate the impact of the deformation parameter $q$, at different polar angles $θ$ with respect to the vertical symmetry axis. Furthermore, we examine the geodesic deviation for the case of non-constant $θ$ during the radial fall. It is shown that the presence of the deformation parameter affects the behavior of the geodesic deviation vectors, depending on its value. In particular, we observe that for arbitrary values of $q$ and $θ$ the behavior of the deviation vector differs as it approaches the singularity at $r = 2m$. Above all, we can witness either stretching or compressing of the deviation vector for various combinations of $q$ and $θ$. These findings provide insight into the effects of quadrupole deformation on the motion of test particles in the vicinity of the central object.

gr-qc↗

arXiv2024

Gravitational capture cross-section in Zipoy-Voorhees spacetimes

We consider geodesics of massive and massless test particles in the gravitational field of a static and axisymmetric compact object described by the quadrupolar metric ($q$-metric), which is the simplest generalization of the Schwarzschild metric, containing an independent quadrupole parameter $q$. We analyze the effective potential profile and calculate the orbital parameters and capture cross-sections of test particles in this spacetime. Moreover, we derive the explicit expression for the escape angle of photons as a function of the quadrupole parameter. All the results reduce in the corresponding limit of vanishing quadrupole to the well-known case of the Schwarzschild spacetime. We argue that our results could be used to investigate realistic compact objects such as white dwarfs and neutron stars.

gr-qc↗

arXiv2024

Stability Analysis of Circular Geodesics in Dyonic Dilatonic Black Hole Spacetimes

This research investigates a non-extreme dyonic-like dilatonic charged black hole solution within a four-dimensional gravity model. This model incorporates two scalar (dilaton) fields and two Abelian vector fields, with interactions between the scalar and vector fields mediated by exponential terms involving two dilatonic coupling vectors. The solution is characterized by a dimensionless parameter $a$ (where $0 < a < 2$), which is specifically defined as a function of the dilatonic coupling vectors. The paper further explores solutions for timelike and null circular geodesics, which are crucial for understanding various astrophysical scenarios, including the quasinormal modes of different test fields in the eikonal approximation. For all values of $a$ the innermost stable circular orbit (ISCO) are found by means of reducing the problem to the solution of fourth order polynomial equation.

gr-qc↗

arXiv2024

Accretion disks properties around regular black hole solutions obtained from non-linear electrodynamics

We investigate a family of spherically symmetric, static, charged regular black hole solutions derived within the framework of Einstein-nonlinear electrodynamics. Our study focuses on examining the characteristics of accretion disks in the spacetimes described by the Dymnikova and Fan-Wang solutions. We explore circular geodesics of test particles and calculate various properties, including the radius of the innermost stable circular orbit, radiant energy, temperature, and conversion efficiency of accretion mass into radiation. We employ the Novikov-Thorne-Page thin accretion disk model as a background. By comparing our findings with those obtained in the Schwarzschild black hole case, we reveal significant modifications in the overall spectral properties. Specifically, we observe an increase in the energy emitted from the disk surface, resulting in higher temperatures for the accretion disks under certain values of the free parameters. Consequently, we note an enhanced efficiency of mass conversion into radiation compared to the Schwarzschild spacetime.

gr-qc↗

arXiv2024

Circular geodesics in the field of double-charged dilatonic black holes

A non-extreme dilatonic charged (by two ``color electric'' charges) black hole solution is examined within a four-dimensional gravity model that incorporates two scalar (dilaton) fields and two Abelian vector fields. The scalar and vector fields interact through exponential terms containing two dilatonic coupling vectors. The solution is characterized by a dimensionless parameter $a$ $(0 < a < 2)$, which is a specific function of dilatonic coupling vectors. The paper presents solutions for timelike and null circular geodesics that may play a crucial role in different astrophysical scenarios, including quasinormal modes of various test fields in the eikonal approximation. For $a = 1/2, 1, 3/2, 2$, the radii of the innermost stable circular orbit are presented and analyzed.

gr-qc↗

arXiv2023

Luminosity of accretion disks around rotating regular black holes

We consider thin accretion disks in the field of a class of rotating regular black holes. For this purpose, we obtain the radius of the innermost stable circular orbit, $r_{ISCO}$ and efficiency of accretion disk in converting matter into radiation $η$ with the aim of modeling the disk's emission spectrum. We consider a simple model for the disk's radiative flux, differential and spectral luminosity and compare the results with those expected from accretion disks around Kerr black holes. As a remarkable result, from our computations we find that both the luminosity of the accretion disk and the efficiency are larger in the geometry of rotating regular black holes for fixed and small values of the spin parameter $j$ with respect to those predicted with the Kerr metric for a black hole of the same mass. These results may have interesting implications for astrophysical black holes.

gr-qc↗

arXiv2023

Accretion disk in the Hartle-Thorne spacetime

We consider the circular motion of test particles in the gravitational field of a rotating deformed object described by the Hartle-Thorne metric. This metric represents an approximate solution to the vacuum Einstein field equations, accurate to second order in the angular momentum $J$ and to first order in the mass quadrupole moment $Q$. We calculate the orbital parameters of neutral test particles on circular orbits (in accretion disks) such as angular velocity, $Ω$, total energy, $E$, angular momentum, $L$, and radius of the innermost stable circular orbit, $R_{ISCO}$, as functions of the total mass, $M$, spin parameter, $j=J/M^2$ and quadrupole parameter, $q=Q/M^3$, of the source. We use the Novikov-Thorne-Page thin accretion disk model to investigate the characteristics of the disk. In particular, we analyze in detail the radiative flux, differential luminosity, and spectral luminosity of the accretion disk, which are the quantities that can be measured experimentally. We compare our results with those obtained in the literature for the Schwarzschild and Kerr metrics, and the $q$-metric. It turns out that the Hartle-Thorne metric and the Kerr metric lead to similar results for the predicted flux and the differential and spectral luminosities, whereas the q-metric predicts different values. We compare the predicted values of $M$, $j$, and $q$ with those of realistic neutron star models. Furthermore, we compare the values of $R_{ISCO}$ with the static and rotating radii of neutron stars.

gr-qc↗

arXiv2023

Adiabatic theory in Kerr spacetimes

We present the main aspects of the adiabatic theory and show that it can be used to study the motion of test particles in general relativity. The theory is based upon the use of vector elements of the orbits and adiabatic invariants. To prove the applicability of the adiabatic theory in Einstein's gravity, we derive a particular representation of the Kerr metric in harmonic coordinates, which allows us to obtain a general formula for the perihelion shift of test particles orbiting on the non-equatorial plane of a rotating central object. We show that the principle of superposition is fulfilled for the individual effects of the gravitational source mass and angular momentum up to the second order. We demonstrate that the adiabatic theory, along with its simplicity, leads to correct results, which in the limiting cases correspond to the ones reported in the literature.

gr-qc↗

arXiv2022

Adiabatic theory of motion of bodies in the Hartle-Thorne spacetime

We study the motion of test particles in the gravitational field of a rotating and deformed object within the framework of the adiabatic theory. For this purpose, the Hartle-Thorne metric written in harmonic coordinates is employed in the post-Newtonian approximation where the adiabatic theory is valid. As a result, we obtain the perihelion shift formula for test particles orbiting on the equatorial plane of a rotating and deformed object. Based on the perihelion shift expression, we show that the principle of superposition is valid for the individual effects of the gravitational source mass, angular momentum and quadrupole moment. The resulting formula was applied to the inner planets of the Solar system. The outcomes are in a good agreement with observational data. It was also shown that the corrections related to the Sun's angular moment and quadrupole moment have little impact on the perihelion shift. On the whole, it was demonstrated that the adiabatic theory, along with its simplicity, leads to correct results, which in the limiting cases correspond to the ones reported in the literature.

gr-qc↗

arXiv2019

Static general relativistic solutions supported by phantom and ordinary scalar fields with higher-order potentials

Domain wall, wormhole, particlelike, and cosmic string general relativistic solutions supported by two interacting phantom or ordinary scalar fields with 4th-, 6th-, and 8th-order potentials are studied. Numerical calculations indicate that regular finite energy solutions exist only for specific values of two free parameters of the potentials. By solving nonlinear eigenvalue problems for some fixed sets of values of the free parameters and of boundary conditions, it is shown that the presence or absence of the solutions depends on a particular symmetry of the problem, on the type of the scalar fields (ordinary or phantom), and on the form of the potential.

gr-qc↗