arXiv subjects

Sohan Kumar Jha

Publications and source records attributed to Sohan Kumar Jha.

At least 19 recordsLinked to original sources

arXiv2026

Equatorial periodic orbits and gravitational waveforms in Bardeen black holes surrounded by perfect fluid dark matter

To probe the interplay between dark matter (DM) and non-linear electrodynamics (NED), we consider the Bardeen black hole (BH) surrounded by perfect fluid dark matter (PFDM). We first compute the effective potential governing the particle trajectory, and then, by imposing suitable conditions on the potential, examine the effects of DM and NED on the marginally bound orbit (MBO) and innermost stable circular orbit (ISCO). In this study, we confine the particle's trajectory to the equatorial plane. We then investigate periodic orbits around the Bardeen BH surrounded by PFDM (BPFDM BH), considering the rational number $q$ associated with each periodic orbit. We use the $(z,w,v)$ taxonomy, which is widely used to systematically organize periodic orbits. We examine the variation of $q$ with energy and angular momentum, and also the variation of the angular momentum and energy required for a specific $(z,w,v)$ configuration with the magnetic charge $g$ and DM parameter $\b$. Finally, with the help of the numerical "Kludge" method, we examine gravitational waveforms emitted from EMRIs where the central supermassive BH is modeled as a BPFDM BH. Our study reveals distinct signatures of NED and DM on orbital dynamics and gravitational waveforms.

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arXiv2026

Strong gravitational lensing and Quasiperiodic oscillations as a probe for an electrically charged Lorentz symmetry-violating black hole

This study examines the combined effect of electric charge and Lorentz symmetry breaking (LSB) on the observables of strong gravitational lensing (SGL) and the dynamics of quasiperiodic oscillations (QPOs) around an electrically charged, Lorentz symmetry-violating (LV) black hole (QKR BH). We first explore the SGL, which unravels an interesting effect that the two combined generate. We find cases where the competing effect of charge and LV cancels each other, leaving the underlying quantity unchanged from that of a \s BH. We find bounds on the LV parameter utilizing observations related to the shadow angular size of supermassive black holes (SMBHs) $M87^*$ and $SgrA^*$. No bound could be gleaned for the charge from these shadow observations. Observations of QPOs in microquasars provide an alternative method to probe our model and to extract bounds on its parameters. We use experimental data for the microquasars $GRO J1655-40$ and $XTE J1550-564$. Here we obtain bounds on both parameters. Our results provide deeper insights into the interplay between charge and LSB in the strong-gravity regime.

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arXiv2025

Probing a NED inspired Magnetically Charged Black Hole in the Hernquist Dark Matter Halo

With an intent to examine the combined effect of non-linear electrodynamics (NED) and dark matter (DM), we obtain a static and spherically symmetric solution with the black hole (BH) magnetically charged and immersed in the Hernquist DM halo (MHDM). The position of the event horizon $r_h$ and the critical impact parameter $b_m$ are then probed to gauge the extent of influence magnetic charge $g$ and halo parameters $\alpha$, $\beta$ have on them. A recurring outcome of our analysis with respect to different BH observables is the nullification of competing effects of charge and halo parameters, leading to observables obtaining values equal to those for a Schwarzschild BH. This is also observed for $r_h$ and $b_m$. We delve into unraveling the impact of NED and DM combined on the strong gravitational lensing (GL) and its related observables, such as the angular separation, relative magnification, and the angular position of the inner, closely packed bright ring. Interestingly, we find combinations of charge and halo parameters that leave the deflection angle unchanged from the Schwarzschild case, thereby leading to a situation where an MHDM BH and a Schwarzschild BH become indistinguishable. Similar results are also observed for lensing observables. Finally, utilizing observations related to the angular diameter of super-massive BHs (SMBHs) $M87^*$ and $SgrA^*$ and employing the $\chi^2$ test, we extract bounds on $g$, $\alpha$, and $\beta$ signifying the viability of our BH model as an SMBH.

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arXiv2025

Black hole surrounded by perfect fluid dark matter with a background Kalb-Ramond field

With an intent to explore the interplay between the Lorentz symmetry breaking (LSB) and the presence of dark matter (DM), we obtain a static and spherically symmetric black hole (BH) solution in the background of nonminimally coupled Kalb-Ramond (KR) field surrounded by perfect fluid dark matter (PFDM). The KR field is frozen to a non-zero vacuum expectation value (VEV) that breaks the particle Lorentz symmetry spontaneously. We explore scalar invariants, Ricci Scalar, Ricci squared, and Kretschmann Scalar, to probe the nature of singularities in the obtained solution. We then study strong gravitational lensing in the background of our BH, i.e., KRPFDM BH, revealing the adverse impact of LSB parameter $\alpha$ and PFDM parameter $\beta$ on the lensing coefficients. The significant effect of our model parameters is evident in strong lensing observables. Bounds on the deviation from Schwarzschild, $\delta$, for supermassive BHs (SMBHs) $M87^*$ and $SgrA^*$ from the EHT, Keck, and VLTI observatories are then utilized to put our BH model to the test and extract possible values of model parameters $\alpha$ and $\beta$ that generate theoretical predictions in line with experimental observations within $1\sigma$ confidence level. Our study sheds light on the combined effect of LSB and PFDM and may be helpful in finding their signature.

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arXiv2025

Non-Minimal RT Coupling and its Impact on Inflationary Evolution in f(R, T) Gravity

We examine inflationary models in the $f(R, T)$ gravity framework where we have a conformal constant and an $RT$-mixing term apart from an R term. The RT-mixing term introduces non-minimal coupling between gravity and matter. We consider the exponential SUSY potential $V(\p)=M^4 \lt(1-e^{-\l \p/\mp}\rt)$ and a novel potential $V(\p)=\l \mp^{4-2\a} \p^{2\a} \sin^2\lt(\frac{\b \mp^\a}{\p^\a}\rt)$. With the help of COBE normalization, we constrain values of different parameters and extract the field value at the time of Hubble crossing. The end of inflation is marked by $\tep(\p_i)=1$ where $\p_i$ is the field value at the end of inflation. Equipped with these values, we then move on to calculate values of spectral index $n_s$ and tensor-to-scalar ratio $r$. Our predicted values of $n_s$ and $r$ fall within their observed values from the Planck 2018 survey and BICEP/Keck array measurement for both potential, making them plausible candidates for the inflationary model. We also display the variation of the tensor-to-scalar ratio and spectral index with the coefficient of RT-mixing term for fixed values of e-fold number. There, we find the existence of two local maxima of $n_s$, which occur at a negative and a positive value of $\x$, the coefficient of $RT$-mixing term. Our analysis finds a significant impact of $\x$ on values of observables

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arXiv2025

Thermodynamics, Weak Gravitational Lensing, and Parameter Estimation of a Schwarzschild Black Hole Immersed in Hernquist Dark Matter Halo

In this article, we obtain a novel black hole (BH) solution of a Schwarzschild BH immersed in a Hernquist dark matter (SBHD) halo. The thermodynamic properties of the resultant spacetime are then studied to gauge the impact of dark matter (DM) on the local and global stability of the composite system of the BH-DM halo. With the intention of finding imprints of DM, we then studied weak gravitational lensing (GL) and shadow. Both display significant dependence on the DM parameters - core radius $r_s$ and core density $\rho_s$. Finally, we constrain DM parameters by utilizing bounds on the deviation parameter $\delta$ for super-massive BHs (SMBHs) $M87^*$ and $Sgr A^*$ reported by Event horizon telescope (EHT), Keck, and VLTI observatories. Our analysis finds SBHD congruent with experimental observations, thereby making it a feasible candidate for an SMBH.

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arXiv2024

Testing linear-quadratic GUP modified Kerr Black hole using EHT results

The linear-quadratic Generalized uncertainty principle (LQG) is consistent with predictions of a minimum measurable length and a maximum measurable momentum put forth by various theories of quantum gravity. The quantum gravity effect is incorporated into a black hole (BH) by modifying its ADM mass. In this article, we explore the impact of GUP on the optical properties of an LQG modified \k BH (LQKBH). We analyze the horizon structure of the BH, which reveals a critical spin value of $7M/8$. BHs with spin $(a)$ less than the critical value are possible for any real GUP parameter $\a$ value. However, as the spin increases beyond the critical value, a forbidden region in $\a$ values pops up that disallows the existence of BHs. This forbidden region widens as we increase the spin. We then examine the impact of $\a$ on the shape and size of the BH shadow for inclination angles $17^o$ and $90^o$, providing a deeper insight into the unified effect of spin and GUP on the shadow. The size of the shadow has a minimum at $\a=1.0M$, whereas, for the exact value of $\a$, the deviation of the shadow from circularity becomes maximum when the spin is less than the critical value. No extrema is observed for $a\,>\, 7M/8$. The shadow's size and deviation are adversely affected by a decrease in the inclination angle. Finally, we confront theoretical predictions with observational results for supermassive BHs $M87^*$ and $SgrA^*$ provided by the EHT collaboration to extract bounds on the spin $a$ and GUP parameter $\a$. We explore bounds on the angular diameter $\th_d$, axial ratio $D_x$, and the deviation from \s radius $\d$ for constructing constraints on $a$ and $\a$. Our work makes LQKBHs plausible candidates for astrophysical BHs.

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arXiv2024

Constrain from shadows of $M87^$ and $Sgr A^$ and quasiperiodic oscillations of galactic microquasars on a black hole arising from metric-affine bumblebee model

We examine a static spherically symmetric black hole metric that originates from the vacuum solution of the traceless metric-affine bumblebee model in which spontaneous Lorentz symmetry-breaking occurs when the bumblebee fields acquire a non-vanishing vacuum expectation value. A free Lorentz-violating parameter enters into the basic formulation of the metric-affine bumblebee model. In this study, we use observations from the Event Horizon Telescope (EHT) collaboration on $M87^*$ and $SgrA^*$ to analyse the shadow of the black hole and an attempt has been made to constrain that free Lorentz-violating parameter. We also investigate particle motion over time-like geodesics and compute the corresponding epicyclic frequencies. We further constrain the Lorentz-violating parameter by using the reported high-frequency quasi-periodic oscillations (QPOs) of microquasars, offering new insights into its possible impact on astrophysical phenomena.

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arXiv2024

Shadow, ISCO, Quasinormal modes, Hawking spectrum, Weak Gravitational lensing, and parameter estimation of a Schwarzschild Black Hole Surrounded by a Dehnen Type Dark Matter Halo

We consider \s black hole (BH) embedded in a Dehnen-$(1,4,0)$ type dark matter halo (DDM) with two additional parameters - core radius $r_s$ and core density $\rs$ apart from mass $M$. We analyze the event horizon, photon orbits, and ISCO around DDM BHs and emphasize the impact of DDM parameters on them. Our study reveals that the presence of dark matter (DM) favourably impacts the radii of photon orbits, the innermost stable circular orbit (ISCO), and the event horizon. We find the expressions for specific energy and angular momentum for massive particles in time-like geodesics around DDM BH and investigate their dependence on DDM parameters. We display BH shadows for various values of core density and radius that reveal larger shadows cast by a \s BH surrounded by DDM (SDDM) than a \s BH in vacuum (SV). We then move on to study quasinormal modes (QNMs) with the help of the $6th$ order WKB method, the greybody factor using the semi-analytic bounds method, and the Hawking spectrum for scalar and electromagnetic perturbations. Core density and radius are found to have a significant impact on QNMs. Since QNMs for scalar and electromagnetic perturbations differ significantly, we can differentiate the two based on QNM observation. The greybody factor increases with core density and radius, whereas, the power emitted as Hawking radiation is adversely impacted by the presence of DM. We then study the weak gravitational lensing using the Gauss-Bonnet theorem and obtain the deflection angle with higher-order correction terms. Here, we see the deflection angle gets enhanced due to DM. Finally, we use bounds on the deviation from \s, $\delta$, reported by EHT for $M87^*$, Keck, and VLTI observatories for $Sgr A^*$ to gauge the viability of our model. Our model is found to be concordant with observations. This leads to the possibility of our galactic center being surrounded by DDM.

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arXiv2024

GUP corrected black holes with cloud of string

We investigate shadows, deflection angle, quasinormal modes (QNMs), and sparsity of Hawking radiation of the Schwarzschild string cloud black hole's solution after applying quantum corrections required by the Generalised Uncertainty Principle (GUP). First, we explore the shadow's behaviour in the presence of a string cloud using three alternative GUP frameworks: linear quadratic GUP (LQGUP), quadratic GUP (QGUP), and linear GUP. We then used the weak field limit approach to determine the effect of the string cloud and GUP parameters on the light deflection angle, with computation based on the Gauss-Bonnet theorem. Next, to compute the quasinormal modes of Schwarzschild string clouds incorporating quantum correction with GUP, we determine the effective potentials generated by perturbing scalar, electromagnetic and fermionic fields, using the sixth-order WKB approach in conjunction with the appropriate numerical analysis. Our investigation indicates that string and linear GUP parameters have distinct and different effects on QNMs. We find that the greybody factor increases due to the presence of string cloud while the linear GUP parameter shows the opposite. We then examine the radiation spectrum and sparsity in the GUP corrected black hole with the cloud of string framework, which provides additional information about the thermal radiation released by black holes. Finally, our inquiries reveal that the influence of the string parameter and the quadratic GUP parameter on various astrophysical observables is comparable, however the impact of the linear GUP parameter is opposite.

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arXiv2024

Massless Dirac Perturbations of black holes in f(Q) gravity: quasinormal modes and weak deflection angle

This article considers a static and spherical black hole (BH) in f(Q) gravity. f(Q) gravity is the extension of symmetric teleparallel general relativity, where both curvature and torsion are vanishing, and gravity is described by nonmetricity. In this study, we investigate the possible implications of quasinormal modes (QNM) modified Hawking spectra, and deflection angles generated by the model. The WKB method is used to solve the equations of motion for massless Dirac perturbation fields and explore the impact of the nonmetricity parameter ($Q_{0}$). Based on the QNMs computation, we can ensure that the BH is stable against massless Dirac perturbations and as $Q_{0}$ increases the the oscillatory frequency of the mode decrease. We then discuss the weak deflection angle in the weak field limit approximation. We compute the deflection angle up to the fourth order of approximation and show how the nonmetricity parameter affects it. We find that the $Q_{0}$ parameter reduces the deflection angle.

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arXiv2024

Observational signature of Lorentz violation in Kalb-Ramond field model and Bumblebee model: A comprehensive comparative study

This article is devoted to the comparative study of the effects of the Lorentz symmetry violation (LV) arising in Kalb-Ramond (KR) and Bumblebee (BM) field models. We study optical appearance with accretion, Quasinormal modes, ringdown waveforms, Hawking radiation, and weak gravitational lensing. The horizon radius, photon radius, and the critical impact parameter for KR BHs decrease with the LV parameter $\a$. In contrast, they remain independent of the BM parameter $\b$ and have values the same as those for \s BH. We find that a KR BH is brighter than a \s or BM BH for static and infalling accretion. The BM BH, on the other hand, is brighter than a \s BH when the accretion is static but becomes darker for an infalling accretion. Our investigation into quasinormal modes (QNMs) and ringdown waveforms provides deeper insight into the difference in observational imprints of LV parameters. It reveals that GWs emitted by KR BHs have larger frequencies and decay faster than those emitted by \s or BM BHs for scalar and electromagnetic perturbations. We then study the greybody factor (GF) and power emitted for both BHs. The Hawking temperature is higher for a KR BM and lower for a BM BH than a \s BH. It also reveals that the transmission probability decreases with $\a$ and $\b$. A comparison of GFs for KR and BM BHs reveals that the transmission probability is higher for BM BH. We also study the effect of LV on the power emitted in the form of Hawking radiation. Power received by an asymptotic observer is larger for a KR BH. We obtain higher-order corrections in the deflection angle and graphically illustrate the impact of $\a$ and $\b$. We observe that a light ray gets deflected most from its path when passing by a \s BH, and the deflection is least when it passes by a KR BH. Our study conclusively shows that we can differentiate between KR and BM BHs based on astrophysical observations.

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arXiv2024

Hairy black hole, Fermionic greybody factors, Quasinormal modes, Hawking radiation, Power spectrum and sparsity

A hairy black hole (HBH) emerges due to matter surrounding the Schwarzschild metric when using the Extended Gravitational Decoupling (GD) approach. The fermionic greybody factors (GFs) and quasinormal modes (QNMs) as well as Hawking spectra and sparsity of HBH solutions are investigated. We consider massive and massless spin- 1/2 fermions, along with massless spin- 3/2 fermions. The equations of the effective potential for fermions with different spins are derived in HBH spacetime. Then, the rigorous bound method is used to calculate the fermionic spin- 1/2 and spin- 3/2 GFs. With the time domain integration method at our disposal, we illustrate the impact of additional parameters on the ringdown waveform of the massless fermionic spin -1/2 and spin -3/2 fields and, in turn, on their quasinormal modes. We then delve into investigating the Hawking spectra and sparsity of the radiation emitted by an HBH. Hairy parameters significantly affect the sparsity of Hawking radiation as well. We observe that the total power emitted by the BH increases both with $\alpha$ and $Q$ but decreases with $l_{0}$. Our study conclusively shows the significant impact of the additional parameters on important astrophysical phenomena such as quasinormal modes, Hawking spectra, and sparsity.

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arXiv2024

Accretion, greybody factor, quasinormal modes, power spectrum, sparsity of Hawking radiation, and weak gravitational lensing of a minimum measurable length inspired Schwarzchild black hole

In this manuscript, we delve into an analytic and numerical probe of shadow with different accretion models, quasinormal modes, Hawking radiation, and gravitational lensing to study observational impacts of quantum effect introduced throughh linear-quadratic GUP(LQG). Our investigation reveals that the shadows of LQG modified black holes are smaller and brighter than Schwarzschild black holes. To examine the impact of the quantum correction on the quasinormal mode, linear-quadratic GUP modified black holes are explored under scalar and electromagnetic field perturbation. Here, linear-quadratic GUP is used to capture quantum corrections. It is observed that the incorporation of quantum correction by linear-quadratic GUP alters the singularity structure of the black hole. To compute the quasinormal modes of this linear-quadratic GUP-inspired quantum-corrected black holes, we compute the effective potential generated under the perturbation of scalar and electromagnetic field, and then we use the sixth-order WKB approach in conjunction with the appropriate numerical analysis. We find that the greybody factor decreases with the GUP parameter $\alpha$ implying that the probability of transmission decreases with the GUP parameter. The total power emitted by LQG modified black hole is found to be greater than that emitted by Schwarzschild black hole. Finally, we study weak gravitational lensing and make a comparison with quadratic GUP and linear GUP modified black holes.

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arXiv2023

Superradiance and stability of rotating charged black holes in T-duality

We investigate the shadow images, the relation between Quasinormal Modes (QNMs) and the shadow radius, and the superradiance effect observed in the context of a rotating charged black hole under T-duality. Our investigation places particular emphasis on two key parameters: the electric charge denoted as $Q$ and the quantum deformed parameter represented by the zero-point length, $l_0$. Our findings reveal a distinct pattern: as the electric charge increases, the shadow radius experiences a consistent decrease. Intriguingly, when considering the quantum deformed parameter, we find a noteworthy phenomenona reflecting point. Specifically, we illustrate that the shadow radius initially increases with an increase in $l_0$ and subsequently decreases. Further analysis involves the computation of eikonal equatorial and polar QNMs, where a similar reflecting point emerges upon varying $l_0$. This establishes the inverse correlation between QNMs and shadow radius within our research framework. Our investigation into the effects of Q and $l_0$ on superradiance reveals that the amplification factor initially increases with $Q$ and $l_0$ and then starts decreasing. Moreover, the rotating black holes in T-duality allows superradiance scattering for a wider range of frequency than Kerr black holes, making black holes in T-duality brighter than the Kerr black holes. We also delve into the stability of the combined system of the rotating black hole and scalar field with the help of black hole bomb mechanism. It provides a window to observe the impact of parameters Q and $l_0$ on the stability. It shows that the combined system is stable for a wider regime for the Kerr black hole.

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arXiv2023

Study of quasinormal modes, greybody bounds, and sparsity of Hawking radiation within the metric-affine bumblebee gravity framework

We consider a static and spherically symmetric black hole metric that emerges from the vacuum solution of the traceless metric-affine bumblebee model. Our study focuses on the possible implications of the modifications induced by the model on various astrophysical observables that include quasinormal modes, ringdown waveforms, Hawking radiation spectrum, sparsity of that radiation, and the lifetime of a black hole. We explore the impact of the Lorentz symmetry-breaking parameter $\alpha$ on the quasinormal modes with the help of the $6th$-order WKB method. Our inquisition reveals that the emission frequency and decay rate initially decrease with $\alpha$ and then grow up. As a result, the LSB becomes critically important for maintaining the stability of the system after being exposed to perturbation. The convergence of the WKB method for various orders is also studied here. We then analyze the Hawking temperature, radiation spectrum, and sparsity in this modified gravity framework that provides valuable insights into the thermal radiation emitted by black holes. It points out that the Hawking temperature, the peak of the power spectrum, and the total power emitted initially decreases and then increases with $\alpha$. However, The variation of the sparsity with $\alpha$ follows a reverse trend. Finally, we obtain the analytical expression of the 'lifetime' of black holes and scrutinize the effect of $\alpha$ on it.

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arXiv2023

Shadow, quasinormal modes, greybody bounds, and Hawking sparsity of Loop Quantum Gravity motivated non-rotating black hole

We consider Loop Quantum Gravity(LQG) motivated $4D$ polymerized black hole and study shadow, quasinormal modes, and Hawking radiation. We obtain analytical expressions of photonsphere radius and shadow radius and study their qualitative and quantitative nature of variation with respect to the LQG parameter $α$. We also show shadows of the black hole for various values of $α$. Our study reveals that both radii increase with an increase in the parameter value. We, then, study quasinormal modes for scalar and electromagnetic perturbations using the $6th$ order WKB method. Our study reveals that the LQG parameter impacts quasinormal modes. We observe that the oscillation of gravitational wave(GW) and decay rate decrease as $α$ increases. At the same time, the error associated with the $6th$ order WKB method increases with an increase in $α$. The ringdown waveform for electromagnetic and scalar perturbations is shown. We also study greybody bounds, power spectrum, and sparsity of Hawking radiation. Greybody bounds for electromagnetic perturbations do not depend on $α$. For scalar perturbation, greybody bounds increase as the LQG parameter increases, but the variation with $α$ is very small. The peak of the power spectrum as well as total power emitted decrease as we increase the value of $α$. Also, the sparsity of Hawking radiation gets significantly impacted by quantum correction. Finally, we obtain the area spectrum of the black hole. It is found to be significantly different than that for the Schwarzschild black hole.

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arXiv2023

Photonsphere, shadow, quasinormal modes, and greybody bounds of non-rotating Simpson-Visser black hole

In this manuscript, we study photonsphere, shadow, quasinormal modes, Hawking temperature, and greybody bounds of a non-rotating Simpson-Visser black hole which is a regular black hole. We observe that though the radius of the photonsphere does depend on the Simpson-Visser parameter $α$, the shadow radius is independent of it. The shadow radius is found to be equal to that for Schwarzschild black hole. We, then, study quasinormal frequencies of the Simpson-Visser black hole for scalar and electromagnetic perturbations with the help of $6$th order WKB method. We tabulate values of quasinormal frequencies for various values of $α$, angular momentum $\ell$, and overtone number $n$. We also graphically show the dependence of real and imaginary parts of quasinormal frequency on $α$ and $\ell$. Additionally, We study the convergence of the WKB method for various values of pair $(n,\ell)$. Finally, we shed light on the dependence of the Hawking temperature on the parameter $α$ and the dependence of greybody bounds on $α$ and $\ell$.

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