arXiv subjects

Shubham Kala

Publications and source records attributed to Shubham Kala.

30 records · Page 2Linked to original sources

arXiv2025

Gravitational lensing and shadow around a non-minimally coupled Horndeski black hole in plasma medium

We investigate the light deflection and the shadow characteristics of a non-minimally coupled Horndeski black hole surrounded by a magnetized, cold, pressureless plasma medium, while considering both homogeneous and non-homogeneous plasma distributions. We consider an analytical expression for the deflection angle of light and analyze how it is influenced by the plasma properties and the Horndeski coupling constant. The circular light orbits, which define the photon sphere, are also analyzed for both types of plasma media, highlighting their impact on the shadow boundary. The shadow properties of the black hole are examined in detail, and constraints on the model parameters are derived by comparing the theoretical shadow radius with observational measurements of Sgr A* and M87* obtained by the Event Horizon Telescope Collaboration. We also study the black hole shadow images along with the corresponding intensity profiles produced by a radially infalling accretion flow in the plasma environment. The results are particularly interesting, as they reveal how the modified black hole geometry affects both the plasma distribution and the black hole parameters in a realistic astrophysical context.

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arXiv2025

Equatorial light bending around a Hairy Kiselev Black Hole

We investigate the deflection angle of light rays confined to the equatorial plane of a Hairy Kiselev black hole. The analysis includes a thorough study of the horizon structure and critical parameters, leading to an analytic expression for the deflection angle in terms of elliptic integrals. Our results confirm that the deflection angle decreases with increasing impact parameter, in agreement with classical predictions of gravitational lensing. The influence of the scalar field, characterized by the coupling constant, shows a nontrivial effect: while moderate values of the coupling constant initially enhance light bending, further increases lead to a suppression of the deflection in the strong-field regime due to modifications in spacetime geometry. Comparative analysis among the Schwarzschild, Kiselev, and Hairy Kiselev black holes indicates that the presence of a quintessential field tends to enhance the deflection, whereas the scalar hair component reduces it. These findings underscore the significant role of scalar fields and exotic matter distributions in shaping light propagation in a modified gravity scenario.

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arXiv2025

Testing black holes with cosmological constant in Einstein-bumblebee gravity through the black hole shadow using EHT data and deflection angle

This study probes spacetime solutions within Einstein-Bumblebee gravity, a modified gravitational framework incorporating spontaneous Lorentz symmetry violation through a vector field mechanism. By introducing a cosmological constant into this model, the research scrutinizes thermodynamic properties of black holes in both anti-de Sitter (AdS) and de Sitter (dS) geometries. The investigation demonstrates how Lorentz-violating parameters alter foundational thermodynamic principles, including revisions to the first law of black hole mechanics and shifts in critical phenomena during phase transitions. Notably, the bumblebee coupling parameter emerges as a critical factor governing horizon structure and thermal emission characteristics, with pronounced deviations from general relativity (GR) predictions observed as this parameter increases. The analysis extends to observational signatures by calculating shadow profiles of these modified black holes. Shadow morphology exhibits dual dependence on the cosmological constant and the bumblebee parameter, presenting measurable discrepancies from classical relativity that could be constrained through Event Horizon Telescope (EHT) observational data. Furthermore, using geometric formalisms, the study quantifies light deflection phenomena in weak and strong gravitational regimes. Results reveal that both the cosmological constant and Lorentz-violating parameter induce detectable modifications to lensing angles compared to Schwarzschild or Kerr benchmarks. These deviations, while subtle, underscore the necessity for next-generation astronomical instruments capable of resolving fine-scale spacetime curvature effects.

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arXiv2025

Deflection of Light due to Kerr Sen Black Hole in Heterotic String Theory using Material Medium Approach

The deflection of light in the gravitational field of a massive body can be analyzed through diverse theoretical approaches. The null geodesic approach is commonly employed to calculate light deflection within strong and weak field limits. Alternatively, several studies have explored the gravitational deflection of light using the material medium approach. For a static, non-rotating spherical mass, the deflection in a Schwarzschild field can be determined by expressing the metric in an isotropic form and evaluating the refractive index to trace the light ray's trajectory. In this study, we extend the above-mentioned approach to the Kerr-Sen black hole spacetime in heterotic string theory, a solution representing a rotating, charged solution in heterotic string theory. The frame-dragging effects inherent to the Kerr-Sen geometry are incorporated to compute the velocity of light rays, enabling the derivation of the refractive index in this field. Considering the far-field approximation, we calculate the deflection of light in the Kerr-Sen spacetime and compare our results with those obtained for the Kerr and Schwarzschild black hole solution in GR.

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arXiv2025

Quasinormal modes of a dyonic black hole in Einstein-Euler-Heisenberg theory

In this study, we investigate the quasinormal modes of a non-rotating dyonic black hole within the framework of Einstein-Euler-Heisenberg theory. We present a detailed analysis focuses on understanding the influence of dyonic charges on the oscillatory properties of these BHs. The quasinormal modes are calculated to explore the interplay between the dyonic charge and the characteristic frequencies of perturbations. The results are then systematically compared with those of black holes possessing purely electric or purely magnetic charges in the Einstein-Euler-Heisenberg framework. This comparison highlights the unique signatures and dynamic behavior introduced by the presence of dyonic charges, offering deeper insights into the properties of black holes in nonlinear electrodynamics theories.

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arXiv2025

Null Geodesics, Thermodynamics, Weak Gravitational Lensing, and Black Hole Shadow Characteristics of a Frolov Regular Black Hole with Constraints from EHT Observations

In this paper, we investigate the properties of null geodesics, thermodynamics, gravitational lensing, and black hole shadows in the vicinity of a static regular Frolov black hole. By analyzing the trajectories of null geodesics, we investigate the bending of light in weak field regimes. The black hole shadow is studied in detail, with constraints on its parameters derived from observational data of the EHT collaboration. Further, we examine shadow images under a spherically symmetric accretion flow and compute the energy emission rate to understand the black hole radiation characteristics. The obtained results demonstrate how the Frolov black hole differs from well-known black hole solutions, such as Schwarzschild, Reissner-Nordström, and Hayward black holes. This study provides new insights into the impact of modified non-rotating regular black hole metrics on observational signatures. The findings have significant implications for future astrophysical observations and the testing of alternative gravity theories.

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arXiv2025

QNMs of charged black holes in AdS spacetime: a geometrical optics perspective

We investigate the quasinormal modes of the Reissner$-$Nordström anti$-$de Sitter black hole using the Penrose limit, motivated by the geometrical optics approximation. This approach offers a novel framework for approximating quasinormal modes with large real frequencies by associating a plane wave to spacetime regions near null geodesics, providing a geometric interpretation of the geometrical optics approximation. Applying this limit to bound null orbits around black holes allows us to explore the black hole response to perturbations. We analyze the effects of black hole charge and negative cosmological constant on the quasinormal spectrum, finding that increasing charge enhances both the real and imaginary parts of the frequencies, while a decreasing cosmological constant leads to higher real frequencies and longer lived perturbations, with the spectrum stabilizing at larger values of the cosmological constant.

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arXiv2025

Strong Gravitational Lensing by Bardeen Black Hole in Cloud of Strings

We investigate the gravitational lensing by Bardeen black hole in cloud of strings (CoS) in strong field limit. The effect of CoS parameter $b$ has been outlined in comparison with Bardeen black hole lens. The strong deflection limit coefficients are determined in terms of impact parameter for various values of CoS parameter. We obtain magnification of relativistic images and determine relativistic Einstein rings by using the parameters of two astrophysical black hole lenses $SgrA^{*}$ and $M87^{*}$. We constrain CoS parameter of Bardeen black hole using EHT observations for these black holes.

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arXiv2022

Shadow and weak gravitational lensing of a rotating regular black hole in a non-minimally coupled Einstein-Yang-Mills theory in the presence of plasma

The null geodesics of the regular and rotating magnetically charged black hole in a non-minimally coupled Einstein-Yang-Mills theory surrounded by a plasma medium is studied. The effect of magnetic charge and Yang-Mills parameter on the effective potential and radius of photon orbits has investigated. We then study the shadow of a regular and rotating magnetically charged black hole along with the observables in presence of the plasma medium. The presence of plasma medium affects the apparent size of the shadow of a regular rotating black hole in comparison to vacuum case. Variation of shadow radius and deformation parameter with Yang-Mills and plasma parameter has examined. Furthermore, the deflection angle of the massless test particles in weak field approximation around this black hole spacetime in presence of homogeneous plasma medium is also investigated. Finally, we have compared the obtained results with Kerr-Newman and Schwarzschild black hole solutions in general relativity (GR).

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arXiv2021

Geodesics and Bending of Light around a BTZ Black Hole Surrounded by Quintessential Matter

Various observations from cosmic microwave background radiation (CMBR), type Ia supernova and baryon acoustic oscillations (BAO) are strongly suggestive of an accelerated expansion of the universe which can be explained by the presence of mysterious energy known as dark energy. The quintessential matter coupled with gravity minimally is considered one of the possible candidates to represents the presence of such dark energy in our universe. In view of this scenario, we study the geodesic of massless particles as well as massive particles around a (2+1) dimensional BTZ black hole (BH) spacetime surrounded by the quintessence. The effect of parameters involved in the deflection of light by such a BH spacetime is investigated in detail. The results obtained are then compared with a usual non-rotating BTZ BH spacetime.

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arXiv2020

Deflection of Light Around a Rotating BTZ Black Hole

We present a detailed study of gravitational lensing around a rotating BTZ black hole in (2+1) dimensional gravity. The study of orbits for massless test particle around this BH spacetime is performed to describe the nature of cosmological constant in lower dimensions. We study the effect of cosmological constant on the photon orbit in view of other critical parameters. The bending angle of light is studied in view of different values of cosmological constant for direct and retrograde motion of test particles. It is being observed that the bending angle slightly decreases as the value of cosmological constant increases in the negative region.

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arXiv2020

Deflection of Light and Shadow Cast by a Dual Charged Stringy Black Hole

Gravitational lensing and black hole shadows are one of the strongest observational evidences to prove the existence of black holes in the universe. The gravitational lensing arises due to the deflection of light by the gravitational field of a gravitating body such as a black hole. Investigation of the shadow cast by a compact object as well as deflection of light around it may provide the useful information about physical nature of the particular compact object and other related aspects. In this paper, we study the deflection of light by a dual charged stringy black hole spacetime derived in dilaton-Maxwell gravity. The variation of deflection angle with the impact parameter for different values of electric and magnetic charges is studied. We also study the shadow of this black hole spacetime to obtain the radius of shadow cast by it. We have considered an optically thin emission disk around it and observed that there are not significant changes in the shadow cast by this black hole compared to well-known Schwarzschild black hole spacetime in GR.

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