arXiv subjects

Mehedi Kalam

Publications and source records attributed to Mehedi Kalam.

34 records · Page 2Linked to original sources

arXiv2020

Does Dark Matter admixed pulsar exist ?

In this paper, we have considered a twofluid model assuming that the pulsars are made of ordinary matter admixed with dark matter.Contribution of dark matter comes from the fitting of the rotation curves of the SPARC sample of galaxies[95]. For this we have investigated the dark matter based on the Singular Isothermal Sphere (SIS) dark matter density profile in the galactic halo region. Considering this twofluid model, we have studied the physical features of the pulsars present in different galaxy in details. Here, we compute the probable radii, compactness (u) and surface red-shift (Zs) of the four pulsars namely : PSR J1748-2021B in NGC 6440B galaxy, PSR J1911-5958A in NGC 6752 galaxy, PSR B1802-07 in NGC 6539 galaxy and PSR J1750-37A in NGC 6441 galaxy.

gr-qc↗

arXiv2018

Analytical model of compact star in low-mass X-ray binary with de Sitter spacetime

In this article, we introduce a de-sitter model in favor of compact stars in low-mass X-ray binary. Here, we merge the presence of the cosmological constant on a small scale to discussion the stellar structure and conclude that this doping is very well suitable with the familiar physical mode of the low-mass X-ray binary compact stars. We calculate the probable radii, compactness (u) and surface red-shift (Zs) of six compact stars in low-mass X-ray binaries namely Cyg X-2, V395 Carinae/2S 0921-630, XTE J2123-058, X1822-371 (V691 CrA), 4U 1820-30 and GR Mus (XB 1254-690). We also offer possible equation of state (EOS) of the stellar object.

astro-ph.HE↗

arXiv2015

Neutron Stars : A Comparative Study

The inner structure of neutron star is considered from theoretical point of view and is compared with the observed data. We have proposed a form of an equation of state relating pressure with matter density which indicates the stiff equation of state of neutron stars. From our study we have calculated mass(M), compactness(u) and surface red-shift(Zs ) for the neutron stars namely PSR J1614-2230, PSR J1903+327, Cen X-3, SMC X-1, Vela X-1, Her X-1 and compared with the recent observational data. We have also indicated the possible radii of the different stars which needs further study. Finally we have examined the stability for such type of theoretical structure.

gr-qc↗

arXiv2014

Isotropic star in low-mass X-ray binaries and X-ray pulsars

We present a model for compact stars in the low mass X-ray binaries(LMXBs) and X-ray pulsars using a metric given by John J. Matese and Patrick G. Whitman \citep{Matese and Whitman1980}. Here the field equations are reduced to a system of two algebraic equations considering the isotropic pressure. Compact star candidates 4U 1820-30(radius=10km) in LMXBs, and Her X-1(radius=7.7km), SAX J 1808.4-3658(SS1)(radius=7.07km) and SAX J 1808.4-3658(SS2)(radius=6.35km) in X-ray pulsars satisfy all the energy conditions, TOV-equation and stability condition. From our model, we have derived mass($M$), central density($ρ_{0}$), suface density($ρ_{b}$), central pressure($p_{0}$), surface pressure($p_{b}$) and surface red-shift($Z_{s}$) of the above mentioned stars, which are very much consistant with the observed/reported datas\citep{N. K. Glendenning1997,Gondek2000}. We have also observe the adiabatic index($γ$>4/3) of the above steller objects.

gr-qc↗

arXiv2014

Compact Stars in low-mass X-ray binaries

We propose a model for compact stars in low-mass X-ray binaries(LMXBs) namely KS 1731-260, EXO 1745-248 and 4U 1608-52. Here we investigate the physical phenomena of a compact star in the LMXBs. Using our model, we have calculated central density, surface density, mass(M) and red-shift for the above mentioned compact stars, which is very much consistent with the reported data. We also obtain the possible equation of state(EOS) of the stars which is physically acceptable.

gr-qc↗

arXiv2014

Analytical model of strange star in the low-mass X-ray binary 4U 1820-30

In this article, we have proposed a model for a realistic strange star under Tolman VII metric\citep{Tolman1939}. Here the field equations are reduced to a system of three algebraic equations for anisotropic pressure. Mass, central density and surface density of strange star in the low-mass X-ray binary 4U 1820-30 has been matched with the observational data according to our model. Strange materials clearly satisfies the stability condition (i.e. sound velocities < 1) and TOV-equation. Here also surface red shift of the star has been found to be within reasonable limit.

gr-qc↗

arXiv2013

Anisotropic Quintessence stars

We propose a relativistic model for quintessence stars with the combination of an anisotropic pressure corresponding to normal matter and a quintessence dark energy having a characteristic parameter ωq such that -1 <ωq < - 1/3. We discuss various physical features of the model and show that the model satisfies all the regularity conditions and can provide stable equilibrium configurations.

gr-qc↗

arXiv2013

Galactic rotation curves and strange quark matter with observational constraints

We obtain the space time of the galactic core in the framework of general relativity by taking the flat rotational curve as input and considering the matter content in the galactic core region as strange quark matter. We also obtain the energy density, radial and tangential pressures of the core strange quark matter. Significantly we have shown that Bag Constant takes an important role to stabilize the circular orbit of the test particles. We also give a limit of the Bag Constant for the existence of quark matter in the galactic halo region.

gr-qc↗

arXiv2013

Geodesic study of a charged black hole

The behavior of the timelike and null geodesics of charged E. Ay$\acute{o}$n-Beato and A. Garcia (ABG) black hole are investigated. For circular and radial geodesics, we investigate all the possible motions by plotting the effective potentials for different parameters. In conclusion, we have shown that there is no phenomenon of \textit{superradiance} in this case.

gr-qc↗

arXiv2012

Central Density Dependent Anisotropic Compact Stars

Stars can be treated as self-gravitating fluid. In this connection, we propose a model for an anisotropic star under the relativistic framework of Krori-Barua (1975) spacetime. It is shown that the solutions are regular and singularity free. The uniqueness of the model is that interior physical properties of the star solely depend on the central density of the matter distribution.

physics.gen-ph↗

arXiv2012

Anisotropic Compact stars with variable cosmological constant

Recently the small value of the cosmological constant and its ability to accelerate the expansion of the Universe is of great interest. We discuss the possibility of forming of anisotropic compact stars from this cosmological constant as one of the competent candidates of dark energy. For this purpose we consider the analytical solution of Krori and Barua metric. We take the radial dependence of cosmological constant and check all the regularity conditions, TOV equations, stability and surface redshift of the compact stars. It has been shown as conclusion that this model is valid for any compact star and we have cited $4U 1820-30$ as a specific example of that kind of star.

gr-qc↗

arXiv2012

Anisotropic strange star with de Sitter spacetime

Stars can be treated as self-gravitating fluid. Krori and Barua (1975) gave an analytical solutions to that kind of fluids. In this connection, we propose a de-Sitter model for an anisotropic strange star with the Krori-Barua spacetime. We incorporate the existence of cosmological constant in a small scale to study the structure of anisotropic strange stars and come to conclusion that this doping is very much compatible with the well known physical features of strange stars.

gr-qc↗

arXiv2012

A relativistic model for Strange Quark Star

We propose a spherically symmetric and anisotropic model for strange quark stars within the framework of MIT Bag model. Though the model is found to comply with all the physical requirements of a realistic star satisfying a strange matter equation of state (EOS), the estimated values the Bag constant for different strange star candidates like Her X-1, SAX J 1808.4-3658 and 4U 1820-30, clearly indicate that the Bag constant need not necessarily lie within the range of 60-80 MeV fm$^{-3}$ as claimed in the literature.

gr-qc↗

arXiv2010

Charged anisotropic matter with linear or nonlinear equation of state

Ivanov pointed out substantial analytical difficulties associated with self-gravitating, static, isotropic fluid spheres when pressure explicitly depends on matter density. Simplification achieved with the introduction of electric charge were noticed as well. We deal with self-gravitating, charged, anisotropic fluids and get even more flexibility in solving the Einstein-Maxwell equations. In order to discuss analytical solutions we extend Krori and Barua's method to include pressure anisotropy and linear or non-linear equations of state. The field equations are reduced to a system of three algebraic equations for the anisotropic pressures as well as matter and electrostatic energy densities. Attention is paid to compact sources characterized by positive matter density and positive radial pressure. Arising solutions satisfy the energy conditions of general relativity. Spheres with vanishing net charge contain fluid elements with unbounded proper charge density located at the fluid-vacuum interface. Notably the electric force acting on these fluid elements is finite, although the acting electric field is zero. Net charges can be huge ($10^{19}\,C$) and maximum electric field intensities are very large ($10^{23}-10^{24}\,statvolt/cm$) even in the case of zero net charge. Inward-directed fluid forces caused by pressure anisotropy may allow equilibrium configurations with larger net charges and electric field intensities than those found in studies of charged isotropic fluids. Links of these results with charged strange quark stars as well as models of dark matter including massive charged particles are highlighted. The van der Waals equation of state leading to matter densities constrained by cubic polynomial equations is briefly considered. The fundamental question of stability is left open.

gr-qc↗

arXiv2009

On the role of pressure anisotropy for relativistic stars admitting conformal motion

We investigate the spacetime of anisotropic stars admitting conformal motion. The Einstein field equations are solved using different ansatz of the surface tension. In this investigation, we study two cases in details with the anisotropy as: [1] $p_t = n p_r$ [2] $p_t - p_r = \frac{1}{8 π}(\frac{c_1}{r^2} + c_2)$ where, n, $c_1$ and $c_2$ are arbitrary constants. The solutions yield expressions of the physical quantities like pressure gradients and the mass.

astro-ph↗

arXiv2009

Cosmic coincidence problem and variable constants of physics

The standard model of cosmology is investigated using time dependent cosmological constant $Λ$ and Newton's gravitational constant $G$. The total energy content is described by the modified Chaplygin gas equation of state. It is found that the time dependent constants coupled with the modified Chaplygin gas interpolate between the earlier matter to the later dark energy dominated phase of the universe. We also achieve a convergence of parameter $ω\to-1$, with minute fluctuations, showing an evolving $ω$. Thus our model fairly alleviates the cosmic coincidence problem which demands $ω=-1$ at present time.

gr-qc↗