arXiv subjects

Javlon Rayimbaev

Publications and source records attributed to Javlon Rayimbaev.

At least 55 records · Page 3Linked to original sources

arXiv2024

Optical features of rotating quintessential charged black holes in de-Sitter spacetime

One of the most important and actual issues in relativistic astrophysics is testing gravity theories and obtaining constraint values for the parameters of black holes using observational data. In this research, we aimed to explore the optical features of a Kerr--Newman black hole model in the presence of a quintessential field, which may be a candidate for a dark-energy model with a nonzero cosmological constant. First, we obtain the equations of motion for photons using the Hamilton--Jacobi formalism. We also study the horizons and shapes of the apparent regions of the photon region around the said black hole. In various scenarios, we investigate shadows cast by the black hole using celestial coordinates. Furthermore, we analyze the effects of the quintessential field and black hole charge on the shadow radius and distortion. Furthermore, we look into the constraints on the spin and charge of supermassive black holes M87$^*$ and Sagittarius A$^*$ for different values of the quintessential field using their shadow size measured by the Event Horizon Telescope Collaboration. Finally, we study the effects of the quintessential field, black hole spin and charge on its energy emission rate by Hawking radiation and compare our results with those of the available literature.

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arXiv2024

Charged particle motion and acceleration around Kerr-MOG black hole

One of the most important issues in relativistic astrophysics is to explain the origin mechanisms of (ultra)high energy charged particle components of cosmic rays. Black holes (BHs) being huge reservoirs of (gravitational) energy can be candidates for such particle sources. The main idea of this work is to study the effects of scalar-tensor-vector gravity (STVG) on particle acceleration by examining charged particle dynamics and their acceleration through the magnetic Penrose process (MPP) near magnetized Kerr-MOG BHs. First, we study the horizon structure of the BH. Also, we study the effective potential to gain insight into the stability of circular orbits. Our results show that the magnetic field can extend the region of stable circular orbits, whereas the STVG parameter reduces the {instability} of the circular orbit. The motion of charged particles around the magnetized BH reveals various feasible regimes of the ionized Keplerian disk behavior. Thus, from the examination of particle trajectories we observe that at fixed values of other parameters, the Schwarzschild BH captures the test particle; in the case of Kerr BH, the test particle escapes to infinity or is captured by the BH, while in Kerr-MOG BH, the test particle is trapped in some region around BH and starts orbiting it. On investigating the MPP, we found that with increasing magnetic field, the behavior of orbits becomes more chaotic. As a result, the particle escapes to infinity more quickly.

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arXiv2024

Bouncing behavior in $f(R,L_m)$ gravity: Phantom crossing and energy conditions

In this work, we investigate the bouncing behavior of the universe within the framework of $f(R,L_m)$ gravity, using a simple form of $f(R,L_m)=\frac{R}{2}+L_m^γ$ (where $γ$ is a free model parameter) as previously studied. The model predicts a vanishing Hubble parameter in the early and late times, with the deceleration parameter approaching a specific limit at the bouncing point. The EoS parameter is observed to cross the phantom divide line ($ω=-1$) near the bouncing point, indicating a significant transition from a contracting to an expanding phase. The model satisfies the necessary energy conditions for a successful bouncing scenario, with violations indicating exotic matter near the bouncing point. Stability conditions are satisfied for certain values of $γ$ near the bouncing point, but potential instabilities in late-time evolution require further investigation. Finally, we conclude that the $f(R,L_m)$ gravity model is promising for understanding the universe's dynamics, especially during events like the bouncing phase.

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arXiv2024

Phase structures and critical behaviour of rational non-linear electrodynamics AdS black holes in Rastall gravity

This research paper presents a black hole solution with a rational non-linear electrodynamics source in the Rastall gravity framework. The paper analyzes the thermodynamic properties of the solution in normal phase space and explores its critical behavior. The phase structure is examined using the extended first law of thermodynamics, with the cosmological constant $Λ$ serving as pressure $P$. The isotherms exhibit van der Waals behavior at small values of horizon $r_+$. The paper also investigates the Gibbs free energy behavior and finds two critical points with two pressures where the reentrant phase transition occurs and disappears. On the other hand, we explore the prevalent microstructure of black holes in Ruppeiner geometry, uncovering significant deviations in the nature of particle interactions from conventional practice. Moreover, the thermodynamic geometry is analyzed using the Ruppeiner formalism, with the normalized Ricci scalar indicating possible point-phase transitions of the heat capacity, and the normalized extrinsic curvature having the same sign as the normalized Ricci scalar. The three-phase transitions of the heat capacity are those that we find for the normalized Ruppeiner curvatures. Thus, there is an absolute correspondence.

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arXiv2024

Dark matter effects of a black hole with nonsingular Yukawa-modified potential in Einstein-Gauss-Bonnet Gravity

This paper investigates the contribution of the nonsingular Yukawa-modified potential in the context of four-dimensional Einstein-Gauss-Bonnet (EGB) gravity modeling by a static and spherically symmetric black hole solution. These Yukawa-type corrections are essentially described along two parameters, $β$ and $λ$, affecting Newton's law of gravity at large distances, and a deformation parameter $\ell_0$, which is essential at short distances. Primarily, the strongest effect is encoded in $β$, which alters the total mass of the black hole with additional mass proportional to $β\mathcal{M}$, imitating the effects of dark matter at large distances from the black hole. In contrast, the effect due to $λ$ is small for astrophysical values. On the other hand, the EGB gravity is ruled by the Gauss-Bonnet (GB) coupling constant $α$, a fundamental parameter of the theory. We pay particular attention to thermodynamic stability, critical orbits, geodesics and quasinormal modes. The results demonstrate stability of the black hole solution for a range of values of the GB coupling constant $α$. Furthermore, this study investigates the null geodesic motion, namely the shadow behavior, providing intriguing results in relation to the size of the black hole shadow.

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arXiv2023

Shadow of novel rotating black holes in GR coupled to nonlinear electrodynamics and constraints from EHT results

We study the optical properties of spacetime around a novel regular black hole (BH) in general relativity (GR) coupled to nonlinear electrodynamics (NED), which is asymptotically flat. First, we study the angular velocity and Lyapunov exponent in unstable photon circular orbits in the novel spherically symmetric BH spacetime. Later, the rotating regular BH solution is obtained using the Newmann-Janis algorithm, and the event horizon properties of the BH are determined. We analyze the effective potential for the circular motion of photons in the spacetime of the novel rotating BH. Also, we analyze the photon sphere around the novel BH and its shadow using celestial coordinates. We obtain that an increase of the BH spin and charge as well as NED field nonlinearity parameters causes an increase in the distortion parameter of the BH shadow, while, the area of the shadow and its oblateness decrease. Moreover, we also obtain the constraint values for the BH charge and the nonlinearity parameters using Event Horizon Telescope data from shadow sizes of supermassive BHs Sgr A* and M87*. Finally, the emission rate of BH evaporation through Hawking radiation is also studied.

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arXiv2023

Dynamics of particles with electric charge and magnetic dipole moment near Schwarzschild-MOG black hole

Investigations of electromagnetic interactions between test-charged and magnetized particles are important in the dynamics of the particles in strong gravitational fields around black holes. Here, we study the dynamics of a particle having an electric charge and a magnetic dipole moment in the spacetime of a Schwarzschild black hole in modified gravity (MOG), called Schwarzschild-MOG black hole. First, we provide a solution of Maxwell equations for the angular component of electromagnetic four potentials in the Schwarzschild-MOG spacetime. Then, we derive equations of motion and effective potential for circular motion of such particles using a hybrid form of the Hamilton-Jacobi equation which includes both interactions of electric charge and magnetic dipole moment with the external magnetic field assumed as asymptotically uniform, and interaction between the particles and the MOG field. Also, we investigate the effects of the three types of interactions on the radius of innermost stable circular orbits (ISCOs) and the energy \& angular momentum of the particles at their corresponding ISCOs. Finally, we provide detailed analyses of the effects of the three interactions mentioned above on the center of mass energy in the collisions between neutral, electrically charged, and magnetized particles.

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arXiv2023

Spinning magnetized particles orbiting magnetized Schwarzschild black holes

A way to test electromagnetic field and spacetime properties around black holes is by considering the dynamics of test particles. In fact, in real astrophysical scenarios, it is hard to determine spacetime geometry which is dominating due to degeneracy gravitational effects in parameters of gravity theories. In this work, we study for the first time the dynamics of spinning particles that have magnetic dipole moments around Schwarzschild black holes immersed in an external asymptotically uniform magnetic field using the Mathisson-Papapetrou-Dixon (MPD) equation. There are two combined interactions: gravitational interaction between the spin of the particle and (electro)magnetic interaction between the external magnetic field and the magnetic dipole moment of the particle to be taken into account. First, we derive the effective potential of the test spinning magnetized particles in motion around the black hole. We also study the combined effects of spin and magnetic interactions on innermost stable circular orbits (ISCOs), the energy, and angular momentum of the particles at ISCO together with superluminal bounds. We investigated the collision of the particles and evaluated the center-of-mass energy in the collisions. Finally, we consider various cases in which neutron stars and rotating stellar mass black holes can be treated as spinning magnetized particles, evaluating the effects of the spin and magnetic moment of objects around supermassive and intermediate-mass black holes.

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arXiv2023

Quasiperiodic oscillations around hairy black holes in Horndeski gravity

Testing gravity theories and their parameters using observations is an important issue in relativistic astrophysics. In this context, we investigate the motion of test particles and their harmonic oscillations in the spacetime of non-rotating hairy black holes (BHs) in Hordeski gravity, together with astrophysical applications of quasiperiodic oscillations (QPOs). We show possible values of upper and lower frequencies of twin-peak QPOs which may occur in the orbits from innermost stable circular orbits to infinity for various values of the Horndeski parameter $q$ in relativistic precession, warped disk models, and three different sub-models of the epicyclic resonant model. We also study the behaviour of the QPO orbits and their position relative to innermost stable circular orbits (ISCOs) with respect to different values of the parameter $q$. {It is obtained that at a critical value of the Horndeski parameter ISCO radius takes $6M$ which has been in the pure Schwarzschild case.} Finally, we obtain mass constraints of the central BH of microquasars GRS 1915+105 and XTE 1550-564 at the GR limit and the possible value of the Horndeski parameter in the frame of the above-mentioned QPO models. The analysis of orbits of twin peak QPOs with the ratio of upper and lower frequencies 3:2, around the BHs in the frame of relativistic precession (RP) and epicyclic resonance (ER4) QPO models have shown that the orbits locate close to the ISCO. The distance between QPO orbits and ISCO is obtained to be less than the error of the observations.

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arXiv2023

$4D$ Einstein-Gauss-Bonnet Black Hole in Power-Yang-Mills Field: A Shadow Study

We consider a static black hole immersed in the Power-Yang-Mills field in four dimensional Einstein-Gauss-Bonnet gravity and investigate the effect of various parameters on the radius of the photon sphere. The modified form of the Newman-Janis algorithm is used for obtaining a rotating black hole solution in this gravity. Further, we try to explore the influence of the Yang-Mills magnetic charge $Q$ with power $q$, Gauss-Bonnet parameter $α$ and spin $a$ on the horizon radius. The geodesic equations are constructed by incorporating the Hamilton-Jacobi formalism. The radial component of the geodesic equations gives the effective potential which is further used in deriving the mathematical structure for the shadows by using Bardeen's procedure for a fixed observer at infinity. The shadows are calculated and plotted in terms of two celestial coordinates for an equatorial observer. It is observed that all the parameters have a very significant effect on the shadow and related physical observables. {We also obtain the constraint values for the spin, magnetic charge and Gauss-Bonnet parameters, using the shadow size of supermassive black holes Sagittarius A$^*$ and M$87$* from the EHT observations for the cases of $q=0.6$ and $0.9$. It is shown that there are upper and lower bounds for the charge and spin of M$87$* at $q=0.6$, while only the upper bounds for charge and spin of Sagittarius A$^*$. Finally, we investigate the energy emission rate in the Hawking radiation around the $4D$ Einstein-Gauss-Bonnet black hole in the Power-Yang-Mills field.}

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arXiv2023

Charged spinning and magnetized test particles orbiting quantum improved charged black holes

In the present work, we aimed to investigate the dynamics of spinning charged and magnetized test particles around both electrically and magnetically charged quantum-improved black holes. We derive the equations of motion for charged spinning test particles using the Mathisson-Papapetrou-Dixon equations with the Lorentz coupling term. The radius of innermost stable circular orbits (ISCOs), specific angular momentum, and energy for charged spinless, uncharged spinning, and charged spinning test particles around the charged and non-charged quantum-improved black holes are analyzed separately. We found that the quantum parameter increases the maximum spin value, $s_\text{max}$, which leads to the nonphysical motion (superluminal motion) of the charged spinning test particle, whereas the black hole charge decreases its value. We also found that, in contrast to the Reissner Nordström black hole, spinning charged test particles in the quantum-improved charged black hole have higher $s_\text{max}$; moreover, positively charged spinning particles can have higher values of $s_\text{max}$ near the extreme black hole cases when compared with uncharged spinning particles. Finally, we investigate the magnetized test particle's dynamics around a quantum-improved magnetically charged black hole in Quantum Einstein Gravity using the Hamilton-Jacobi equation. We show that the presence of $ω$ increases the maximum value of the effective potential and decreases the minimum energy and angular momentum of magnetized particles at their circular orbits. We found an upper constraint in the black hole charge at the ISCO.

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arXiv2023

Probing Geometric Proca in Metric-Palatini Gravity with Black Hole Shadow and Photon Motion

Extended metric-Palatini gravity, quadratic in the antisymmetric part of the affine curvature, is known to lead to the general relativity plus a geometric Proca field. The geometric Proca, equivalent of the non-metricity vector in the torsion-free affine connection, qualifies to be a distinctive signature of the affine curvature. In the present work, we explore how shadow and photon motion near black holes can be used to probe the geometric Proca field. To this end, we derive static spherically symmetric field equations of this Einstein-geometric Proca theory, and show that it admits black hole solutions in asymptotically AdS background. We perform a detailed study of the optical properties and shadow of this black hole and contrast them with the observational data by considering black hole environments with and without plasma. As a useful astrophysical application, we discuss constraints on the Proca field parameters using the observed angular size of the shadow of supermassive black holes M87$^*$ and Sgr A$^*$ in both vacuum and plasma cases. Overall, we find that the geometric Proca can be probed via the black hole observations.

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arXiv2022

Quasiperiodic oscillations, weak field lensing and shadow cast around black holes in Symmergent gravity

In this work, we perform a systematic study of the symmergent gravity in a black hole environment. The symmergent gravity, an emergent gravity model in which gravity emerges in a way restoring the gauge symmetries and stabilizing the Higgs boson mass, possesses a quadratic-curvature term with a loop-induced coupling proportional to the boson-fermion number difference. We investigated boson-fermion number difference and black hole parameters in symmergent gravity by utilizing the values of various observables. In this regard, we investigated particle dynamics and obtained Keplerian frequencies describing the harmonic oscillations. We determined quasiperiodic oscillations about such orbits by utilizing relativistic precession, warped disk, and epicyclic resonant models. We studied weak deflection angle and planetary perihelion shift and determined bounds on symmergent gravity parameters. We computed shadow radius for both static and co-moving observers near and far from the symmergent black hole and revealed the sensitivity of these observers to the model parameters. We conclude the work by giving an overall discussion of the bounds, and giving future prospects concerning other possible analyses of the symmergent gravity.

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arXiv2022

Spinning test particle motion around a rotating wormhole

In this work, we investigated the motion of spinning test particles around a rotating wormhole, extending, in this way, the previous work of Benavides-Gallego et al. in [Phys. Rev. D 101, no.12, 124024] to the general case. Using the Mathisson-Papapetrous-Dixon equations, we study the effective potential, circular orbits, and the innermost stable circular orbit (ISCO) of spinning test particles. We found that both the particle and wormhole spins affect the location of the ISCO significantly. On the other hand, Similar to the non-rotating case, we also found two possible configurations in the effective potential: plus and minus. Furthermore, the minimum value of the effective potential is not at the throat due to its spin, in contrast to the motion of the non-spinning test particles in a non-rotating wormhole, where the effective potential is symmetric, and its minimum value is at the throat. In the case of the ISCO, we found that it increases as the spin of the wormhole a increases, in contrast to black holes where the presence of spin decreases the value of the ISCO. Finally, since the dynamical four-momentum and kinematical four-velocity of the spinning particle are not always parallel, we consider the superluminal bound, finding that the allowed values of s change as the wormhole's spin a increases.

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arXiv2022

Circular motion around a regular rotating Hayward black hole

In this article, we explore the geodesics motion of neutral test particles and the process of energy extraction from a regular rotating Hayward black hole. We analyse the effect of spin, as well as deviation parameter $g$, on ergoregion, event horizon and static limit of the said black hole. By making use of geodesic equations on the equatorial plane, we determine the innermost stable circular and photon orbits. Moreover, we investigate the effective potentials and effective force to have information on motion and the stability of circular orbits. On studying the negative energy states, we figure out the energy limits of Penrose mechanism. Using Penrose mechanism, we found expression for the efficiency of energy extraction and observed that both spin and deviation parameters, contribute to the efficiency of energy extraction. Finally, the obtained results are compared with that acquired from Kerr and braneworld Kerr black holes.

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arXiv2022

Quasiperiodic Oscillations, Quasinormal Modes and Shadows of Bardeen-Kiselev Black Holes

In this article, we study the particle dynamics around a static and spherically symmetric Bardeen-Kiselev black hole (BK BH) which is a solution of the Einstein-non-linear Maxwell field equations along with a quintessential field. We discuss its essential geometrical properties such as scalar invariants and size of innermost stable circular orbits. Dynamics of test particles around the BH is also studied. Moreover, we also computed the fundamental frequencies of a test particle orbiting the BH in a slightly perturbed orbit. Further, the degeneracy relations between the spin of rotating Kerr BH and magnetic charge of the BK BH at the values of the quintessential parameter $ω_q=-1/3$ and in terms of the same values of innermost stable circular orbits (ISCOs) radius, energy efficiency, twin-peaks quasiperiodic oscillations (QPOs) frequencies and impact parameter for photon are also discussed in detail. Finally, we have investigated the relationship between the shadow radius and the real part of the quasinormal mode (QNM) frequency.

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arXiv2021

Dynamics of particles and epicyclic motions around Schwarzschild-de-Sitter black hole in perfect fluid dark matter

In this paper we investigate circular orbits for test particles around Schwarzschild-de Sitter (dS) black hole surrounded by perfect fluid dark matter. We determine the region of circular orbits bounded by innermost and outermost stable circular orbits. We show that the impact of the perfect fluid dark matter shrinks the region where circular orbits can exist as the values of both innermost and outermost stable circular orbits decrease. We find that for specific lower and upper values of dark matter parameter there exist double matching values for inner and outermost stable circular orbits. It turns out that the gravitational attraction due to the dark matter contribution dominates over cosmological repulsion. This gives rise to a remarkable result in the Schwarzschild-dS black hole surrounded by dark matter field in contrast to the Schwarzschild-dS metric. Finally, we study epicyclic motion and its frequencies with their applications to twin peak quasi-periodic oscillations (QPO) for various models. We find corresponding values of the black hole parameters which could best fit and explain the observed twin peak QPO object GRS 1915+109 from microquasars.

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arXiv2021

Dynamics of charged particles and magnetic dipoles around magnetized quasi-Schwarzschild black holes

In the present paper, we have investigated the motion of charged particles together with magnetic dipoles to determine how well the spacetime deviation parameter $ε$ and external uniform magnetic field can mimic the spin of a rotating Kerr black hole. Investigation of charged particle motion has shown that the deviation parameter $ε$ in the absence of external magnetic fields can mimic the rotation parameter of Kerr spacetime up to $a/M \approx0.5$. The combination of external magnetic field and deviation parameter can do even a better job mimicking the rotation parameter up to $a/M\simeq0.93$, which corresponds to the rapidly rotating case. Study of the dynamics of magnetic dipoles around quasi-Schwarzschild black holes in the external magnetic field has shown that there are degeneracy values of ISCO radius of test particles at $ε_{cr}>ε\geq 0.35$ which may lead to two different values of the innermost stable circular orbit (ISCO) radius. When the deviation parameter is in the range of $ε\in (-1,\ 1)$, it can mimic the spin of a rotating Kerr black hole in the range $a/M \in (0.0537, \ 0.3952)$ for magnetic dipoles with values of magnetic coupling parameter $β\in [-0.25,\ 0.25]$ in corotating orbits.

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