arXiv subjects

Bobomurat Ahmedov

Publications and source records attributed to Bobomurat Ahmedov.

At least 55 records · Page 3Linked to original sources

arXiv2025

Efficient estimation of error bounds for quantum multiparametric imaging with constraints

Advanced super-resolution imaging techniques require specific approaches for accurate and consistent estimation of the achievable spatial resolution. Fisher information supplied to Cramer-Rao bound (CRB) has proved to be a powerful and efficient tool for resolution analysis and optical setups optimization. However, the standard CRB is not applicable to constrained problems violating the unbiasedness condition, while such models are frequently encountered in quantum imaging of complex objects. Complimentary to the existing approaches based on modifying CRB, we propose a practical algorithm for approximate construction of a modified Fisher information matrix, which takes the constraints into account and can be supplied to the standard CRB. We demonstrate the efficiency of the proposed technique by applying it to 1-, 2-, and multi-parameter model problems in quantum imaging. The approach provides quantitative explanation of previous results with successful experimental reconstruction of objects with the spatial scale smaller than the theoretical limit predicted by the standard CRB.

quant-ph↗

arXiv2025

Gauge invariant perturbations in teleparallel Horndeski gravity

We present in the form of a catalogue of the cosmological perturbations within the Bahamonde- Dialektopoulos-Levi Said (BDLS) theory, which serves as the teleparallel counterpart of Horndeski gravity. To understand structure formation in cosmological models, it is essential to study both the background and perturbative aspects of their cosmology. While extensive analysis of both Horndeski gravity and its teleparallel analog exists in the literature, a quantitative understanding requires a detailed examination of their cosmological perturbations. We review here all the different gauges for the scalar, vector and tensor perturbations of a cosmological background up to second order and we hope this will help people who work with observations, to incorporate it in existing codes.

gr-qc↗

arXiv2024

Astrophysical insights into magnetic Penrose process around parameterized Konoplya-Rezzolla-Zhidenko black hole

In this study, we investigate the parameterized Konoplya-Rezzolla-Zhidenko (KRZ) black hole (BH) spacetime in the presence of an external asymptotically uniform magnetic field. We first examine the innermost stable circular orbit (ISCO) radii for both neutral and charged test particles, demonstrating that the deformation parameters, $δ_1$ and $δ_2$, reduce the ISCO values. Subsequently, we assess the energy efficiency of the magnetic Penrose process (MPP) for an axially symmetric parameterized BH, analyzing the effects of the deformation parameters and the magnetic field on the energy extraction process. Our findings indicate that the rotational deformation parameter $δ_2$ is crucial for the efficiency of energy extraction from the BH. The synergy between the rotational deformation parameter and the magnetic field significantly boosts the energy extraction efficiency, with values exceeding $100\%$. Interestingly, for extremal BHs with negative $δ_2$ values, the energy efficiency increases, in contrast to Kerr BHs where the MPP effect diminishes. Additionally, we explore the astrophysical implications of the MPP by deriving the maximum energy of a proton escaping from the KRZ parameterized BH due to the beta decay of a free neutron near the horizon. Our results show that negative $δ_2$ values require stronger magnetic fields to achieve equivalent energy levels for high-energy protons, providing deeper insights into high-energy astrophysical phenomena around the parameterized BH.

gr-qc↗

arXiv2024

Long-lived quasinormal modes and asymptotic tails of regular Schwarzschild-like black holes in the presence of a magnetic field

We analyze the evolution of perturbations of a (charged) massive scalar field near a regular Simpson-Visser black hole, allowing for a non-zero external magnetic field. We show that the damping rate of the quasinormal frequencies is strongly suppressed by both the magnetic field and the mass term, with indications that arbitrarily long-lived modes, or quasi-resonances, may exist in the spectrum. In the time domain, the quasinormal ringing transitions into slowly decaying oscillatory tails, which are qualitatively distinct from the massive tails observed in the absence of a magnetic field. For nonzero multipole and azimuthal numbers, the power-law envelope characteristic of cases without a magnetic field transforms into an oscillatory envelope that cannot be easily fitted with a simple analytical formula.

gr-qc↗

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.

gr-qc↗

arXiv2024

Exploring a Novel Feature of Ellis Spacetime: Insights into Scalar Field Dynamics

We have studied neutral and charged massive particles dynamics in Ellis spacetime in the presence of the external scalar field. Focusing on the circular motion of massive particles, the impact of an external scalar field on the Innermost Stable Circular Orbit (ISCO) position is analyzed, revealing a non-linear relationship with the scalar field parameter. Perturbation techniques are employed to investigate oscillatory motion near stable orbits in the Ellis spacetime, yielding analytical expressions for radial and angular oscillations. The throat of the wormhole has been constrained by comparing theoretical and observational results for fundamental frequencies of particles from quasars. Finally, scalar and gravitational perturbations in the Ellis spacetime have been studied. It is shown that the equation for the scalar profile function is fully independent from the tensor functions, and the solution can be represented in terms of the confluent Heun function. However, it has been shown that equations for the tensor profile functions strongly depend on the scalar profile functions in the Ellis spacetime, and they are reduced to the Regge-Wheeler-Zerilli equation. Finally, numerical solutions to the Regge-Wheeler-Zerilli equation for the radial functions in the Ellis spacetime have been presented.

gr-qc↗

arXiv2024

Influence of scalar field in massive particle motion in JNW spacetime

In this paper, we investigated the motion of massive particles in the presence of scalar and gravitational fields, particularly focusing on the Janis-Newman-Winicour (JNW) naked singularity solution. It is shown that the innermost stable circular orbit (ISCO) radius strongly depends on scalar coupling parameter. Additionally, we explored the radiation reaction effects on particle dynamics, incorporating a reaction term into the motion equations. Numerical simulations indicated minimal impact on particle trajectories from radiation reaction. We also examined the oscillatory motion of particles around compact objects in the JNW spacetime, focusing on radial and vertical oscillations. Our analysis indicated that the scalar field's coupling parameter and the spacetime deformation parameter $n$ significantly alter the fundamental frequencies of these oscillations. Furthermore, we studied quasi-periodic oscillations (QPOs) in X-ray binaries, using the relativistic precession (RP) model to analyze upper and lower frequency relationships. Our results indicated that increasing parameters ($n$ and $g_s$) shifts the frequency ratio of 3:2 QPOs closer to the naked singularity, with $n$ decreasing and $g_s$ increasing both frequencies. Finally, we analyzed QPO data from selected four X-ray binary systems using Markov Chain Monte Carlo (MCMC) analysis to constrain JNW parameters. Our findings provided insights into the mass, coupling and deformation parameter for each system, enhancing our understanding of compact object dynamics in strong gravitational fields.

gr-qc↗

arXiv2024

Gravitational lensing of neutrinos in the Rezzolla-Zhidenko spacetime

We consider gravitational lensing of neutrinos in the Rezzolla-Zhidenko spacetime in the weak-field limit with plane-wave approximation. We apply the analysis to an hypothetical system with a central object with its mass of the order of solar mass and a detector located at an Earth-like distance from the source. We find that the deformation parameters of the Rezzolla-Zhidenko metric can have significant impact on the oscillation probabilities of the neutrinos. We also investigate the role of decoherence on flavor oscillations of the lensed neutrinos and show that the parameters of the Rezzolla-Zhidenko metric does not have significant effects on the decoherence length.

gr-qc↗

arXiv2024

Observed jet power and radiative efficiency of black hole candidates in Kerr + PFDM model

In this research, we explore the electromagnetic energy emitted by astrophysical black holes within the Kerr+PFDM spacetime, a model encompassing rotating black holes surrounded by dark matter. Our investigation focuses on black holes within X-ray binary systems, namely GRS 1915+105, GRO J1655-40, XTE J1550-564, A0620-00, H1743-322, and GRS 1124-683. Our findings indicate that the Kerr+PFDM spacetime can account for the radiative efficiency of these sources as determined through the continuum fitting method (CFM). Additionally, employing the Blandford-Znajeck mechanism, we demonstrate the ability to replicate the observed jet power. By combining the outcomes of both analyses for the selected objects, we establish more rigorous constraints on the spacetime parameters. Notably, our results reveal that similar to the Kerr spacetime, the Kerr+PFDM spacetime cannot simultaneously account for the observed jet power and radiative efficiency of GRS 1915+105.

gr-qc↗

arXiv2024

Shadows and weak gravitational lensing by the black hole in Einstein-Maxwell-scalar theory

In this paper, we investigate the optical properties of a charged black hole in Einstein-Maxwell-scalar (EMS) theory. We evaluate the shadow cast by the black hole and obtain analytical solutions for both the radius of the photon sphere and the shadow radius. We observe that the black hole parameters $γ$ and $β$ both influence the shadow of black hole. It is shown that the photon sphere and the shadow radius increase as a consequence of the presence of parameter $γ$. Interestingly, we show that shadow radius decreases first and then remains unchanged due to the impact of parameter $β$. Finally, we consider the weak gravitational lensing and the total magnification of lensed images around black hole. We find that the black hole charge and parameter $β$ both give rise to a significant effect, reducing the deflection angle. Similarly, the same behavior for the total magnification is observed due to the effect of black hole charge and parameter $β$.

gr-qc↗

arXiv2024

Kerr-Newman-modified-gravity black hole's impact on the magnetic reconnection

In this paper, we study the magnetic reconnection process of energy extraction from a rapidly rotating Kerr-Newman-modified-gravity (MOG) black hole by investigating the combined effect of black hole charge and the MOG parameter. We explore the energy efficiency of energy extraction and power by applying the new energy extraction mechanism proposed by Comisso and Asenjo. Based on an attractive gravitational charge of the MOG parameter $α$ that physically manifests to strengthen black hole gravity, we show that the combined effect of the MOG parameter and black hole charge can play an increasingly important role and accordingly lead to high energy efficiency and power for the energy extraction via the magnetic reconnection. Further, we study to estimate the rate of energy extraction under the fast magnetic reconnection by comparing the power of the magnetic reconnection and Blandford-Znajek (BZ) mechanisms. We show that the rate of energy extraction increases as a consequence of the combined effect of black hole charge and MOG parameter. It suggests that magnetic reconnection is significantly more efficient than the BZ mechanism. In fact, the magnetic reconnection is fueled by magnetic field energy due to the twisting of magnetic field lines around the black hole for the plasma acceleration, and thus the MOG parameter gives rise to even more fast spin that can strongly change the magnetic field reconfiguration due to the frame dragging effect. This is how energy extraction is strongly enhanced through the magnetic reconnection, thus making the energy extraction surprisingly more efficient for the Kerr-Newman-MOG black hole than the Kerr black hole under the combined effect of black hole charge and MOG parameter.

gr-qc↗

arXiv2024

Constraining study of charged gravastars solutions in symmetric teleparallel gravity

This study explores the effect of charge on a special astronomical object known as a gravastar, which is viewed as an alternative to a black hole. Based on the conjecture put out by Mazur and Mottola in general relativity, the study primarily focuses on the consequences of $f(Q)$ gravity. The internal domain, the intermediate shell, and the external domain are the three separate sections that make up a gravastar. Using a particular $f(Q)$ gravity model that includes conformal Killing vectors to analyze these areas, we discover that the inner domain shows a repulsive force on the spherical shell since it is assumed that pressure is equivalent to negative energy density. The intermediate shell is made up of ultrarelativistic plasma and pressure, which is proportional to energy density and balances the repulsive force from the interior domain. For exterior region, we use two appraoches as first we calculate the vacuum exact solution and secondly considered as the Reissner-Nordström metric. Then, we match these spacetimes through junction condition and explore the stability constraints for both cases. Our results show that charged gravastar solutions with non-singular physical parameters including length, energy, entropy, and equation of state parameter are physically realistic.

gr-qc↗

arXiv2024

Parameter Constraints on Traversable Wormholes within Beyond Horndeski Theories through Quasi-Periodic Oscillations

{\it Hunting} compact astrophysical objects such as black holes and wormholes, as well as testing gravity theories, are important issues in relativistic astrophysics. In this sense, theoretical and observational studies of quasiperiodic oscillations (QPOs) observed in (micro)quasars become helpful in exploring their central object, which can be a black hole or a wormhole. In the present work, we study the throat properties of traversable wormholes beyond Horndeski theory. Also, we investigate the circular motion of test particles orbiting the wormhole. We analyze the test particle's effective potential and angular momentum for circular orbits. Frequencies of radial and vertical oscillations of the particles around stable circular orbits have also been studied and applied in explaining the quasiperiodic oscillations mechanism in the relativistic precession (RP) model. Finally, we obtain constraint values for the parameters of Horndeski gravity and the mass of the wormhole candidates using QPOs observed in the microquasars GRO J1655-40, GRS 1915+105 \& XTE J1550-564 and at the center of Milky Way galaxy through Monte-Carlo-Markovian-Chain (MCMC) analyses.

gr-qc↗

arXiv2024

Probing black hole in Starobinsky-Bel-Robinson gravity with thermodynamical analysis, effective force and gravitational weak lensing

In this work, we investigate the effects of plasma and the coupling parameter $β>0$, on the thermodynamic properties and weak gravitational lensing by the Schwarzschild-like black hole in the Starobinsky-Bel-Robinson gravity (SBRG). We observe that the horizon radius and the corrected entropy of the Schwarzschild-like black hole in the SBRG are not much sensitive to the parameter $β$. On contrary the energy emission rate of the Schwarzschild-like black hole in the SBRG is sensitive to the parameter $β$ and decreases with increase in the values of the parameter $β$. We see that the Schwarzschild-like black hoe in the SBRG is stable as the thermodynamicl temperature is positive for different values of the parameter $β$. Moreover we observe that the deflection angle of photon beam by the black hole in uniform plasma, nonuniform self-interacting scalar plasma and non-singular isothermal gas sphere reduces with the parameter $β$, against the impact parameter $b$. We see that the deflection angle enhances with increase in the concentration of the plasma fields for all the three types of plasma media. Further we find that the magnification of the image due to lensing increases in a higher concentration of plasma field. It is interesting to notice that the image magnification in uniform plasma is much higher as compared to the one in nonuniform plasma field. We compare our results with those for the Schwarzschild black hole of General Relativity. Further, effective force is also calculated for the current analysis.

gr-qc↗

arXiv2024

Radiation properties of the accretion disk around a black hole in Einstein-Maxwell-scalar theory

In this study, we explore the properties of a non-rotating black hole in the Einstein-Maxwell-scalar (EMS) theory and investigate the luminosity of the accretion disk surrounding it. We determine all the orbital parameters of particles in the accretion disk, including the radius of the innermost stable circular orbit (ISCO) with angular velocity, angular momentum, and energy. Further, we study the radiative efficiency for different values of black hole parameters. Finally, we analyze the flux, differential luminosity, and temperature of the accretion disk.

gr-qc↗

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.

gr-qc↗

arXiv2023

Stable bouncing solutions in Teleparallel Horndeski gravity: violations of the no-go theorem

In order to have singularity-free solutions at the beginning of the Universe, we need to violate the null energy condition. In the general class of Horndeski gravity, there are healthy NEC-violating solutions, which however are plagued with instabilities or some kind of pathologies in the history of the Universe; this is known as the no-go theorem. In this paper, we study the possibility of stable bouncing solutions in the Teleparallel analog of Horndeski gravity and we show explicitly that there exist healthy violations of the no-go theorem.

gr-qc↗

arXiv2023

Kerr-Taub-NUT spacetime to explain the jet power and the radiative efficiency of astrophysical black holes

In this work, we investigate the electromagnetic energy released by astrophysical black holes within the Kerr-Taub-NUT solution, which describes rotating black holes with a nonvanishing gravitomagnetic charge. In our study, we consider the black holes in the X-ray binary systems GRS 1915+105, GRO J1655-40, XTE J1550-564, A0620-00, H1743-322, and GRS 1124-683. We show that the Kerr-Taub-NUT spacetime can explain the radiative efficiency of these sources inferred from the continuum fitting method (CFM). We also show that, in the framework of the Blandford-Znajeck mechanism, it is possible to reproduce the observed jet power. We unify the results of the two analyses for the selected objects to get more stringent constraints on the spacetime parameters. We show that, as in the case of the Kerr spacetime, the Kerr-Taub-NUT solution cannot simultaneously explain the observed jet power and radiative efficiency of GRS 1915+105.

gr-qc↗