arXiv subjects

M. K. Jasim

Publications and source records attributed to M. K. Jasim.

14 recordsLinked to original sources

arXiv2025

Topological Signatures and Geometrothermodynamics of Critical Phenomena in Regularized Maxwell Black Holes

We study the thermodynamic topology and microscopic interaction properties of charged black holes in RegMax gravity, focusing on the role of the coupling parameter $α$. Using the Duan topological current method together with Ruppeiner geometry, we show that $α$ controls a sharp change in phase structure. Above a certain critical threshold, we find that the Duan defect curve develops an intermediate branch and vertical tangency points, producing continuous (second-order) critical behaviour. Furthermore, the Ruppeiner curvature becomes negative at very small horizon radii before turning positive and progressively vanishing at larger radii. By contrast, below the critical value of the coupling, the intermediate black hole phase disappears, and the system shows a simpler small/large first-order/coexistence behaviour driven by free-energy competition. In this regime, the Ruppeiner curvature remains predominantly positive. Overall, increasing $α$ enriches the thermodynamic topology (allowing for second-order criticality) while simultaneously reducing the domain in which classical energy conditions (ECs) are satisfied, thus linking exotic thermodynamic behaviour to more severe violations of standard energy conditions.

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arXiv2024

Topological AdS black holes surrounded by Chaplygin dark fluid: from stability to geometrothermodynamic analysis

Implementing the concept of Dark Fluid with a Chaplygin-like equation of state within General Relativity, we construct a new higher-dimensional, static, and spherically symmetric anti-de Sitter (AdS) black hole solution. Energy conditions are explored alongside curvature singularity tools. The inspection at the level of the phase structure and $P-v$ critical behavior is carried out in the context of the extended phase space, where the cosmological constant appears as pressure. Our findings disclose non-trivial similarities between the small/large phase transition of AdS black holes surrounded by Chaplygin dark fluid and van der Waals systems' liquid/gas phase transition. This analysis offers insights into the physical interpretation of the $P-v$ diagram and identifies critical exponents that reveal the scaling behavior of thermodynamic quantities close to criticality in a universal manner. We finally deepen our understanding of the thermodynamic properties and microstructure of AdS black holes by leveraging the geometrothermodynamic formalism. Specifically, we employ tools, including Weinhold, Ruppeiner, Hendi-Panahiyan-Eslam-Momennia (HPEM) and Quevedo classes I and II. We show that each class of metrics predicts either the physical limitation point and/or the phase-transition critical points, with HPEM and Quevedo formulations providing richer information about the phase transitions. Altogether, this study contributes to advancing our knowledge of the role of Chaplygin gas in General Relativity and thoroughly examining the thermodynamic phase structure of high-dimensional AdS black holes under extreme conditions.

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arXiv2021

Structural properties of charged compact stars with color-flavour-locked quarks matter

The observations of pulsars with masses close to 2$M_{\odot}$ have put strong constraints on the equation of state (EoS) of neutron-rich matter at supranuclear densities. Moreover, the exact internal composition of those objects is largely unknown to us. Aiming to reach the 2$M_{\odot}$ limit, here we investigate the impact of electric charge on properties of compact stars assuming that the charge distribution is proportional to the mass density. The study is carried out by solving the Tolman-Oppenheimer-Volkoff (TOV) equation for a well-motivated exotic quark matter in the color-flavor-locked (CFL) phase of color superconductivity. The existence of the CFL phase may be the true ground state of hadronic matter with the possibility of the existence of a pure stable quark star (QS). Concerning the equation-of-state, we obtain structural properties of quark stars and compute the mass, the radius as well as the total electric charge of the star. We analyze the dependence of the physical properties of these QSs depending on the free parameters with special attention on mass-radius relation. We also briefly discuss the mass vs central mass density $(M-ε_c)$ relation for stability, the effect of electric charge and compactness. Finally, our results are compared with the recent observations data on mass-radius relationship.

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arXiv2021

Wormholes in $f(R,T)$ gravity satisfying the null energy condition with isotropic pressure

We consider $f(R, T)$ theory of gravity, in which the gravitational Lagrangian is given by an arbitrary function of the Ricci scalar and the trace of the energy-momentum tensor, to study static spherically symmetric wormhole geometries sustained by matter sources with isotropic pressure. According to restrictions on the wormhole geometries, we carefully adopt different strategies to construct solutions with the properties and characteristics of wormholes. Using a completely general procedure, we provide several examples of wormholes in which the matter threading the wormhole throat satisfies all of the energy conditions and discuss general mechanisms for finding them. Finally, we postulate a smooth transformation for simplifying the nonlinear field equations and have more consistent results than the other ones to conclude that the results can be viewed as specific exact wormhole solutions without the presence of exotic matter.

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arXiv2021

Anisotropic quark stars in $R^2$ gravity

In the present paper, we treat the problem of the existence of quark stars (QSs) for selected homogeneous and unpaired charge-neutral $3$-flavor interacting quark matter with $\mathcal{O}(m_s^4)$ corrections equations of state (EoS). Using the EoS combined with the Tolmann-Oppenheimer-Volkoff (TOV) structure equations, the properties of stars are explored by obtaining their mass-radius relations. All calculations are carried out within the framework of the $R$-squared gravity defined by $f(R) = R+a R^2$. Our main goal is to discuss the effect for a wide range of the $R$-squared gravity parameter, $a$, on the mass-radius and the mass-central mass density $(M-ε_c)$ relation of QSs. Furthermore, we investigate the dynamical stability condition for those stars, and we show that their dynamical stability depends on the anisotropic parameters $ B_{\perp}$ and $a_4^{\perp}$ coming from the respective EoS. In such a scenario our results provide circumstantial evidence in favor of super-massive pulsars.

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arXiv2020

Isotropic compact stars in 4D Einstein-Gauss-Bonnet gravity

Recently it has been proposed that the Gauss-Bonnet coupling parameter of Lovelock gravity may suitably be rescaled in order to admit physically viable models of celestial phenomena such that higher curvature effects are active in standard four dimensions as opposed to the usual higher dimensions. We investigate the consequences of this modification in the context of stellar modelling. The evolution of perfect fluid distributions is governed by the pressure isotropy condition and through stipulation of one of the metric potentials complete models emerge from solutions of the master differential equation. New classes of exact solution with this approach have been reported. One particular model is analysed in detail and shown to comport with elementary physical requirements demanded of realistic compact stars suggesting that the modified theory is not inconsistent with observations.

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arXiv2020

Conformally symmetric traversable wormholes in $f(R,T)$ gravity

To find more deliberate $f(R, T)$ astrophysical solutions, we proceed by studying wormhole geometries under the assumption of spherical symmetry and the existence of a conformal Killing symmetry to attain the more acceptable astrophysical results. To do this, we consider a more plausible and simple model $f(R,T)=R+2χT$, where $R$ is the Ricci scalar and $T= -ρ+p_r+2p_t$ denotes the trace of the energy-momentum tensor of the matter content. We explore and analyze two cases separately. In the first part, wormhole solutions are constructed for the matter sources with isotropic pressure. However, the obtained solution does not satisfy the required wormhole conditions. In the second part, we introduce an EoS relating with pressure (radial and lateral) and density. We constrain the models with phantom energy EoS i.e. $ω= p_r/ ρ< -1$, consequently violating the null energy condition. Next, we analyze the model via $p_t= n p_r $. Several physical properties and characteristics of these solutions are investigated which are consistent with previous references about wormholes. We mainly focus on energy conditions (NEC, WEC and SEC) and consequently for supporting the respective wormhole geometries in details. In both cases it is found that the energy density is positive as seen by any static observer. To support the theoretical results, we also plotted several figures for different parameter values of the model that helps us to confirm the predictions. Finally, the volume integral quantifier, which provides useful information about the total amount of exotic matter required to maintain a traversable wormhole is discussed briefly.

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arXiv2020

Anisotropic Compact stars in the Buchdahl model: A comprehensive study

In this article, we present a class of relativistic solutions describing spherically symmetric and static anisotropic stars in hydrostatic equilibrium. For this purpose, we consider a particularized matric potential, namely, Buchdahl ansatz [Phys. Rev. D \textbf{116}, 1027 (1959).] which encompasses almost all the known analytic solutions to the spherically symmetric, static Einstein field equations(EFEs) with a perfect fluid source, including in particular the Vaidya-Tikekar and Finch-Skea. We here developed the model by considering an anisotropic spherically symmetric static general relativistic configuration that plays a significant effect on the structure and properties of stellar objects. We have considered eight different cases for generalized Buchdahl dimensionless parameter $K$, and analyzed them in a uniform manner. As a result, it turns out that all the considered cases are valid at every point in the interior spacetime. In addition to this, we show that the model satisfies all the energy conditions and maintains the hydrostatic equilibrium equation. In the framework of the anisotropic hypothesis, we consider analogue objects with similar mass and radii such as LMC X-4, SMC X-1, EXO 1785-248 \emph{etc} to restrict the model parameter arbitrariness. Also, establishing a relation between pressure and density in the form of $P = P (ρ)$, we demonstrate that EoSs can be approximated to a linear function of density. Despite the simplicity of this model, the obtained results are satisfactory.

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arXiv2020

Charged strange stellar model with Tolman $V$ metric potential in the Einstein-Maxwell space-time

This paper deals with the existence of a compact stellar object, precisely strange (quark) star, in the framework of Einstein's General Theory of Relativity with Tolman $V$ metric potential, which is one of the simplest forms of potential among his proposals. The potential is given by $e^ν=Kr^{2n}$, where $K$ is the constant and $n$ is a parameter [R.C. Tolman, Phys. Rev. {\bf55}, 364 (1939)]. Considering charged, static, spherically symmetric, isotropic fluid sphere we have studied different physical features of some strange star candidates namely $EXO\ 1785-248$, $LMC\ X-4$, $SMC\ X-1$, $SAX\ J1808.4-3658$, $4U\ 1538-52$ and $Her\ X-1$. To represent the strange quark matter (SQM) distribution we have employed the simplest form of MIT bag equation of state (EOS), which provides a linear relationship between pressure and density of the matter through Bag constant $B$. We have done several tests for the stability criteria and the physical acceptability of the proposed model. The results show consistency with energy condition, TOV equation, adiabatic index, etc. We have calculated different physical parameters of our model for the three different consecutive values of Bag constant $B$ which are $83\ MeV/fm^3$, $90\ MeV/fm^3$ and $100\ MeV/fm^3$. Among them with $B=90\ MeV/fm^3$ we have analyzed different properties of the proposed strange star candidates.

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arXiv2020

Conformally symmetric traversable wormholes in modified teleparallel gravity

In this paper, we consider wormhole geometries in the context of teleparallel equivalent of general relativity (TEGR) as well as $f(T)$ gravity. The TEGR is an alternative geometrical formulation of Einstein's general relativity, where modified teleparallel gravity or $f(T)$ gravity has been invoked as an alternative approach for explaining an accelerated expansion of the universe. We present the analytical solutions under the assumption of spherical symmetry and the existence of a conformal Killing vectors to proceed a more systematic approach in searching for exact wormhole solutions. More preciously, the existence of a conformal symmetry places restrictions on the model. Considering the field equations with a diagonal tetrad and anisotropic distribution of the fluid, we study the properties of traversable wormholes in TEGR that violates the weak and the null energy conditions at the throat and its vicinity. In the second part, wormhole solutions are constructed in the framework of $f(T)$ gravity, where $T$ represents torsion scalar. As a consistency check, we also discuss the behavior of energy conditions with a viable power-law $f(T)$ model and the corresponding shape functions. In addition, a wide variety of solutions are deduced by considering a linear equation of state relating the density and pressure, for the isotropic and anisotropic pressure, independently of the shape functions, and various phantom wormhole geometries are explored.

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arXiv2019

Analytical model of dark energy stars

In this article we study the structure and stability of compact astrophysical objects which are ruled by the dark energy equation of state (EoS). The existence of dark energy is important for explaining the current accelerated expansion of the universe. Exact solutions to Einstein field equations (EFE) have been found by considering particularized metric potential, Finch and Skea ansatz \cite{Finch}. The obtained solutions are relevant to the explanation of compact fluid sphere. Further, we have observed at the junction interface, the interior solution is matched with the Schwarzschild's exterior vacuum solution. Based on that, we have noticed the obtained solutions are well in agreement with the observed maximum mass bound of $\approx$ 2$M_{\bigodot}$, namely, PSR J1416-2230, Vela X-1, 4U 1608-52, Her X-1 and PSR J1903+327, whose predictable masses and radii are not compatible with the standard neutron star models. Also, the stability of the stellar configuration has been discussed briefly, by considering the energy conditions, surface redshift, compactness, mass-radius relation in terms of the state parameter ($ω$). Finally, we demonstrate that the features so obtained are physically acceptable and consistent with the observed/reported data \citep{Gondek-Rosinska,Glendenning}. Thus, the present dark energy equation of state appears talented regarding the presence of several exotic astrophysical matters.

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arXiv2018

Anisotropic strange stars in Tolman-Kuchowicz spacetime

We attempt to study a singularity-free model for the spherically symmetric anisotropic strange stars under Einstein's general theory of relativity by exploiting the Tolman-Kuchowicz metric. Further, we have assumed that the cosmological constant $Λ$ is a scalar variable dependent on the spatial coordinate $r$. To describe the strange star candidates we have considered that they are made of strange quark matter (SQM) distribution, which is assumed to be governed by the MIT bag equation of state. To obtain unknown constants of the stellar system we match the interior Tolman-Kuchowicz metric to the exterior modified Schwarzschild metric with the cosmological constant, at the surface of the system. Following Deb et al. we have predicted the exact values of the radii for different strange star candidates based on the observed values of the masses of the stellar objects and the chosen parametric values of the $Λ$ as well as the bag constant $\mathcal{B}$. The set of solutions satisfies all the physical requirements to represent strange stars. Interestingly, our study reveals that as the values of the $Λ$ and $\mathcal{B}$ increase the anisotropic system becomes gradually smaller in size turning the whole system into a more compact ultra-dense stellar object.

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arXiv2015

Generalized relativistic anisotropic models for compact stars

We present new anisotropic generalization of Buchdahl [1] type perfect fluid solution by using the method of earlier work [2]. In similar approach we have constructed the new pressure anisotropy factor $Δ$ by the help both the metric potential $e^λ$ and $e^ν$. The metric potential $e^λ$ same as Buchdahl [1] and $e^ν$ is monotonic increasing function as suggested by Lake [3]. After that we obtain new well behaved general solution for anisotropic fluid distribution. We calculated the physical quantities like energy density, radial and tangential pressures, velocity of sound and red-shift etc. We observe that these quantities are positive and finite inside the compact star. Also note that mass and radius of our models can represent the structure of realistic astrophysical objects such as like Her X-1 and RXJ1856-37.

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arXiv2015

Relativistic modelling of stable anisotropic super-dense star

In the present article we have obtained new set of exact solutions of Einstein field equations for anisotropic fluid spheres by using the Herrera et al.[1] algorithm. The anisotropic fluid solution so obtained join continuously to Schwarzschild exterior solution across the pressure free boundary. It is observed that most of the new anisotropic solutions are well behaved and utilized to construct the super-dense star models such as neutron star and pulsars.

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