arXiv subjects

Saswati Roy

Publications and source records attributed to Saswati Roy.

10 recordsLinked to original sources

arXiv2026

Light bending around the Kerr-Bertotti-Robinson black hole using material medium approach

In this paper, we study the deflection of massless particles due to a rotating, axially symmetric Kerr-Bertotti-Robinson (KBR) black hole via; material medium approach. We explored the effect of spacetime geometry on the trajectory of light rays in the presence of a uniform magnetic field. To derive an analytical expression for the deflection of light rays due to the Kerr-Bertotti-Robinson black hole, the frame dragging effect and refractive index were also studied in greater detail. From the analysis it is evident that the magnetic field actively adds to the black hole's gravity, making the bending of light stronger and permanently changing the space far away from the black hole, preventing it to act as a normal flat vacuum. From thermodynamical investigation, it is clear that entropy monotonically decreases with magnetic field strength and rotation parameter; whereas the Hawking temperature increases with a uniform magnetic field but decreases with spin parameter.

gr-qc↗

arXiv2026

Gravitational lensing around a Kerr-Sen black hole in plasma background

We investigate the gravitational lensing of massless particles around a Kerr-Sen black hole immersed in a magnetized, cold, pressureless plasma medium. Both homogeneous and inhomogeneous plasma distributions are considered in this study to mimic realistic astrophysical environments. The light deflection angle is computed, and the effects of the black hole's rotation and charge on light bending are analyzed in detail. The conditions for the circular photon orbits are also examined in both plasma configurations. A comparison with the vacuum case (i.e. zero plasma frequency) highlights the role of plasma in modifying the light propagation, and the results obtained provide a deeper insight into plasma effects which improve our understanding of observational signatures of rotating charged black holes.

gr-qc↗

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.

gr-qc↗

arXiv2025

Non-Equatorial Deflection of Light due to Kerr-Newman Black Hole: A Material Medium Approach

We explored the effect of space-time geometry on the trajectory of light rays in the context of a charged, rotating black hole. We derived an analytical expression for the deflection of light rays in Kerr-Newman space-time geometry, using a material medium approach, on non-equatorial plane. From this deflection angle expression it is evident that the charge and rotation of the black hole can affect the light rays' paths. Additionally, we calculated the refractive index of light rays by treating space-time as a medium. We demonstrated how the rotation parameter and charge influence the refractive index and hence the deflection angle of light rays. For Kerr-Newman geometry, the deflection angle decreases with increasing charge when the rotation parameter is held constant. Conversely, for a constant charge, the deflection angle increases with the rotation parameter for prograde and decreases for retrograde trajectories. Applying both factors results in the deflection angle being lower than that of the Schwarzschild geometry. Non-equatorial study of the deflection angle reveals that it is maximum in the equatorial plane than in the pole. The frame-dragging effects in the Kerr-Newman field were taken into account to calculate the velocity of light rays, leading to the determination of the refractive index in this field geometry. This study concludes that depending on the values of the rotation parameter and charge parameter both prograde and retrograde trajectories coincide, resulting in the conclusion that at some point the frame dragging effect is the same for prograde and retrograde motion. Also, the frame dragging increases towards poles for retrograde trajectories while decreasing for prograde trajectories, and these nontrivial nature results because of the interplay between charge and rotation.

gr-qc↗

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.

gr-qc↗

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-Nordstr\"om, 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.

gr-qc↗

arXiv2017

Deflection of light ray due to a charged body using Material Medium Approach

The gravitational deflection of light ray is an important prediction of General Theory of Relativity. In this paper we develop analytical expression of the deflection of light ray without any weak field approximation due to a charged gravitational body represented by Reissner_Nordström (RN) and Janis-Newman-Winicour (JNW) space time geometry, using material medium approach. It is concluded that although both the geometries represent the charged, non-rotating, spherically symmetric gravitating body, but the effect of charge on the gravitational deflection is just opposite to each other. The gravitational deflection decreases with charge in the RN geometry and increases with charge in the JNW geometry. The calculations obtained here are compared with other methods done by different authors. The formalism is applied to an arbitrary selected pulsar PSRB1937+21 as a gravitating body, as a test case.

gr-qc↗

arXiv2015

Trajectory of a light ray in Kerr field: A material medium approach

The deflection of light ray as it passes around a gravitational mass can be calculated by different methods. Such calculations are generally done by using the null geodesics under both strong field and weak field approximation. However, several authors have studied the gravitational deflection of light ray using material medium approach. For a static, non-rotating spherical mass, one can determine the deflection in Schwarzschild field, by expressing the line element in an isotropic form and calculating the refractive index to determine the trajectory of the light ray. In this paper, we draw our attention to the refractive index of light ray in Kerr field using the material medium approach. The frame dragging effects in Kerr field was considered to calculate the velocity of light ray and finally the refractive index in Kerr field geometry was determined. Hence the deflection of light ray in Kerr field was calculated, assuming far field approximation and compared the results with those calculations done earlier using Null geodesics.

gr-qc↗

arXiv2013

Benchmarking the Immersed Finite Element Method for Fluid-Structure Interaction Problems

We present an implementation of a fully variational formulation of an immersed method for fluid-structure interaction problems based on the finite element method. While typical implementation of immersed methods are characterized by the use of approximate Dirac delta distributions, fully variational formulations of the method do not require the use of said distributions. In our implementation the immersed solid is general in the sense that it is not required to have the same mass density and the same viscous response as the surrounding fluid. We assume that the immersed solid can be either viscoelastic of differential type or hyperelastic. Here we focus on the validation of the method via various benchmarks for fluid-structure interaction numerical schemes. This is the first time that the interaction of purely elastic compressible solids and an incompressible fluid is approached via an immersed method allowing a direct comparison with established benchmarks.

math.NA↗

arXiv2012

A Fully Coupled Immersed Finite Element Method for Fluid Structure Interaction via the Deal.II Library

We present the implementation of a solution scheme for fluid-structure interaction problems via the finite element software library deal.II. The solution scheme is an immersed finite element method in which two independent discretizations are used for the fluid and immersed deformable body. In this type of formulation the support of the equations of motion of the fluid is extended to cover the union of the solid and fluid domains. The equations of motion over the extended solution domain govern the flow of a fluid under the action of a body force field. This body force field informs the fluid of the presence of the immersed solid. The velocity field of the immersed solid is the restriction over the immersed domain of the velocity field in the extended equations of motion. The focus of this paper is to show how the determination of the motion of the immersed domain is carried out in practice. We show that our implementation is general, that is, it is not dependent on a specific choice of the finite element spaces over the immersed solid and the extended fluid domains. We present some preliminary results concerning the accuracy of the proposed method.

math.NA↗