Surface Redshift Research Articles

Analytical model of millisecond pulsar PSR J0514-4002A

We construct a relativistic model for the newly discovered millisecond pulsar PSR J0514 - 4002A located in the globular cluster NGC 1851 (A. Ridolfi, P.C.C. Freire, Y. Gupta, S.M. Ransom, MNRAS 490, 3860 (2019)) by using Tolman VII spacetime. We have obtained central density ($\rho_{0}$), central pressure ($p_{0}$), probable radius, compactness ($u$) and surface redshift ($Z_{s}$) of the above mentioned newly discovered millisecond pulsar, which is very much consistent with reported data. Equation of State (EoS) of the millisecond pulsar has comes out as stiff in nature which is physically acceptable. Not even that our proposed model can analyze most of the millisecond pulsars having masses up to $1.51 M_{\odot}$.

Hybrid compact stars model in Rastall gravity: a comparative study

The purpose of present work is to provide a new mathematical model for hybrid compact stars composed of strange quark matter ( $\mathit{SQM}$ ) and normal baryonic matter distribution in the modified Rastall theory of gravitation. The simplest form of phenomenological $\mathit{MIT}$ bag model equation of state ( $P_{q}=\frac{1}{3}(\rho _{q}-4B_{g})$ ) has been used to correlate the density and pressure of strange quark matter in the stellar interior, whereas radial pressure and density are associated by the simple linear equation of state ( $p_{r}=m\rho $ ) for normal matter distribution. In order to attain the solution of field equation, we used the static and spherically symmetric Krori and Barua ( $\mathit{KB}$ ) type geometry and then arising arbitrary constants are obtained from some physical conditions. It has been manifested that for reasonable choice of the Rastall coupling parameter (i.e., $\chi =0$ ), the original results in the counterpart General Relativity ( $\mathit{GR}$ ) can be reobtained. We examined the obtained model graphically and analytically in detail for physically plausible conditions and compare with the observational facts and counterpart $\mathit{GR}$ . For the investigation of obtained results, we incorporate three different compact stars namely, $\mathit{Her} X-1$ , $\mathit{SAX} J1808-3658 (\mathit{SSI})$ and $4U 1820-30$ with radii $7.7\mbox{ km}$ , $7.07\mbox{ km}$ , and $10\mbox{ km}$ , respectively as strange quark star candidates. In order to check the validity of present model, we perform various physical tests analytically and graphically, namely, stability test, energy conditions, compactification and Buchdahl condition, dynamical equilibrium of applied forces and surface redshift etc. We found that our model meets necessary physical requirements for realistic model and more consonant with the observational facts as compared to the counterpart $\mathit{GR}$ which deviates from essential physical requirements for $\mathit{Her} X-1$ . Therefore our model is more realistic and can be studied for strange quark stars.

Charged stellar structure in Tolman–Kuchowicz spacetime

In this paper, we have presented the Einstein–Maxwell equations which are described by the spherically symmetric spacetime in the presence of charge by exploiting the Tolman–Kuchowicz spacetime. The corresponding field equations are constructed and the form of charge distribution is chosen to be [Formula: see text], where [Formula: see text] is a constant quantity. We also find the values of unknown constants from junction conditions and discuss the behavior of effective energy density, effective radial and tangential pressure and anisotropic factor with two viable [Formula: see text] models. We examine the physical stability of charged stellar structure through energy conditions, causality and stability condition. We use modified form of TOV equation for anisotropic charged fluid sphere to analyze the equilibrium condition. In this work, we model the compact star candidate SAXJ 1808.4 – 3658 and study the compactness level and anisotropic behavior corresponding to the variation of physical parameters which are involved in [Formula: see text] models. Further, we evaluate some important properties such as mass-radius ratio compactness factor and surface redshift. It is depicted from this study that the obtained solutions provide strong evidences for more realistic and viable stellar model.

BPS Skyrme neutron stars in generalized gravity

We study the coupling of nuclear matter described by the BPS Skyrme model to generalized gravity. Concretely, we consider the Starobinsky model which provides the leading-order correction to the Einstein-Hilbert action. Static solutions describing neutron stars are found both for the full field theory and for the mean-field approximation. We always consider the full Starobinsky model in the nonperturbative approach, using appropriately generalized shooting methods for the numerical neutron star calculations. Many of our results are similar to previous investigations of neutron stars for the Starobinsky model using other models of nuclear matter, but there are some surprising discrepancies. The “Newtonian mass” relevant for the surface redshift, e.g., results larger than the ADM mass in our model, in contrast to other investigations. This difference is related to the particularly high stiffness of nuclear matter described by the BPS Skyrme model and offers an interesting possibility to distinguish different models of nuclear matter within generalized gravity.

Analytic radiation model for perfect fluid under homotopy perturbation method

An expression for mass of a spherically symmetric system is obtained by solving the Tolman–Oppenheimer–Volkoff equation, employing the homotopy perturbation method. With the help of this expression and the Einstein field equations, a set of interior solutions is arrived at. Thereafter, different aspects of the solution are explained as regards mass, density, pressure, energy, stability, mass–radius ratio, compactness factor and surface redshift. This analysis shows that all the physical properties, in connection to brown dwarf stars, are valid with the observed features.

Gravastar in the framework of braneworld gravity

Gravastars have been considered as a serious alternative to black holes in the past couple of decades. Stable models of gravastar have been constructed in many of the alternate gravity models besides standard General Relativity (GR). The Randall-Sundrum (RS) braneworld model has been a popular alternative to GR, specially in the cosmological and astrophysical context. Here we consider a gravastar model in RS brane gravity. The mathematical solutions in different regions have been obtained along with calculation of matching conditions. Various important physical parameters for the shell have been calculated and plotted to see their variation with radial distance. We also calculate and plot the surface redshift to check the stability of the gravastar within the purview of RS brane gravity.

Extended gravitational decoupled solutions in self-interacting Brans–Dicke theory

In this paper, we construct anisotropic spherical solutions from known isotropic solutions through extended gravitational decoupling method in the background of self-interacting Brans-Dicke theory. The field equations are decoupled into two sets by applying geometric deformations on radial as well as temporal metric components. The first array corresponds to isotropic fluid while influence of the anisotropic source is confined to the second set. The isotropic sector is determined through metric functions of isotropic solutions (Tolman IV/Krori-Barua) whereas two constraints on the anisotropic source are required to close the second system. The impact of the massive scalar field as well as the decoupling parameter on the physical characteristics of the anisotropic solutions is analyzed graphically. We also check the viability, compactness, surface redshift and stability of the obtained solutions. It is found that the resulting solutions follow accepted physical trend for some values of the decoupling parameter.

Compact stars with exotic matter

In this paper we employ Tolman VII solution with exotic matter that may be present in the extremely dense core of compact objects. The Tolman VII solution, an exact analytic solution to the spherically symmetric, static Einstein field equations with a perfect fluid source, has many characteristics that make it interesting for modeling high density stellar astronomical objects. For our purpose we use generalized non-linear equation of state which may incorporate exotic matter along with dust, radiation and dark energy. The equation of state (EoS) has a linear contribution comprising quark/radiation matters and a nonlinear contribution comprising dark energy/exotic matters. The amount of exotic matter contain can be modify by a parameter $n$ which can be linked to adiabatic index. As $n$ increases the exotic contribution increases which stiffens the EoS. The physical properties of the model such as pressure, density, mass function, surface redshift, gravitational redshift are examined and the stability of the stellar configuration is discussed in details. The model has promising features as it satisfies all energy conditions and is free from central singularities. The $M-R$ relation is constructed analytically and the maximum mass and its corresponding radius is determined using the exact solutions and is shown to satisfy various observational stellar compact stars.

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.

Satellite Alignment. III. Satellite Galaxies’ Spatial Distribution and Their Dependence on Redshift with a Novel Galaxy Finder

Abstract After being extensively explored, observations and theories have shown that satellites are preferentially aligned with major axes of their host centrals. There are still some unresolved issues on this topic. In this paper, we present studies on satellite spatial distribution. To fairly compare with observations, we develop a novel galaxy finder and reconstruction algorithm in hydrodynamical simulation, which is based on the projected mock image, taking into account the full consideration of the point-spread function, pixel size, surface brightness limit, resolution, and redshift dimming effects. With galaxy samples constructed using such an algorithm, the satellite alignment is examined by comparing with observational results. It is found that the observational alignment can be reproduced for red galaxies, which dominate the sample in this study, but not for blue galaxies. Satellites’ radial distribution is also investigated. It exhibits that outer satellites within host halos show stronger alignment signal than satellites in the inner regions, especially for red satellites, which is in contrast with previous studies. The disagreement is mainly due to extra galaxies identified by our new galaxy finder, which are mainly located in the inner region of host halos. Our study illustrates that at lower redshift, the alignment strength becomes stronger, while the radial distribution curve becomes flatter. This suggests differences in the evolution of the angular distribution between satellites residing in the inner and outer halos and implies that the post-infall evolution reduces the original alignment signal and that the impact decreases for satellites with later infall times.

Buchdahl compactness limit and gravitational field energy

The main aim of this paper is essentially to point out that the Buchdahl compactness limit of a static object is given by gravitational field energy being less than or equal to half of its non-gravitational matter energy. It is thus entirely determined without any reference to interior distribution by the unique exterior solutions, the Schwarzschild for neutral and the Reissner-Nordström for charged object. In terms of surface potential, it reads as Φ(R) = (M−Q2/2R)/R ⩽ 4/9 which translates to surface red-shift being less than or equal to 3. It also prescribes an upper bound on charge an object could have, Q2/M2 ⩽ 9/8 > 1.

Study of charged stellar models in f(𝒢) gravity with Tolman–Kuchowicz space–time

In the context of modified [Formula: see text] gravity model, the current study highlights the effect of electric charge for static spherically symmetric stellar models in presence of anisotropic matter distribution. For this purpose, we specifically consider the metric potentials of Tolman–Kuchowicz space–time, which are singularity free and satisfy the stability criteria. The possible existence of charge and a strong electric field, inside the stars is due to the higher values of energy density of matter, pressure distribution and gravitational fields. For the solution of Einstein–Maxwell field equations, the simplified phenomenological MIT bag equation of state, i.e. [Formula: see text] and a specific form of electrical charge distribution [Formula: see text] are to be considered. Further, to derive the values of unknown parameters of the stellar objects, we match interior Tolman–Kuchowicz space–time to exterior Reissner–Nordström metric, at the surface of stellar system. In addition to this, for the physical validity and stability of our suggested model, we conduct several physical tests such as effective energy density, effective pressures, energy conditions, stability against equilibrium of the forces, mass-radius relation, surface redshift, Herrera’s cracking concept and electric charge for well-known compact stellar objects viz., SAX J 1808.4-3658 and PSR 1937+21. It is observed that all these tests follow the physically accepted pattern and the influence of charge leads to more stable and viable stellar structures of compact objects in [Formula: see text] gravity.

Studies an analytic model of a spherically symmetric compact object in Einsteinian gravity

We propose a new compact stellar object model existing in a space filled with a distribution of anisotropic fluid matter for stellar configuration exposed to the hydrostatic equilibrium. An analytical solution was obtained using dark-energy (DE), which is characterized by a equation of state (EoS) of the type p=gamma rho - rho corresponding to the external Schwarzschild vacuum solution through a thin envelope. We have imposed a collective function based on an adjustable coefficient to solve the Einstein field equations (EFEs). We investigate the general physical characteristics of high-density astrophysical objects based on the required solutions, with the inside structure of the stellar objects, such as the energy conditions, stability analysis, mass function, surface redshift function, velocity of sound and compactness of stellar objects through theoretical expression as well as graphic plots. In terms of our results, the physical behavior of this model can be used to model ultra-compact objects.

Charged anisotropic compact stars in Logarithmic-Corrected R2 gravity

We consider [Formula: see text] corrected model, i.e. [Formula: see text], where [Formula: see text] is the Ricci scalar and [Formula: see text], [Formula: see text] are arbitrary constant values, to investigate some of the interior configurations of static anisotropic spherical charged stellar structures. The existence of electric charge and a strong electric field confirms due to the higher values of pressure distribution and energy density of the matter inside the stars. Furthermore, for compact star configurations, we also consider the simplified MIT bag model equation of state (EoS) given by [Formula: see text], where [Formula: see text] is radial pressure, [Formula: see text] is energy density and [Formula: see text] is bag constant. This approach allows to find electric charge from the Einstein–Maxwell field equations. We have extensively discussed the behavior of the electric charge and anisotropic fluid distribution factor for five different values of [Formula: see text]. Interestingly, it is noticed during this study, for smaller values of [Formula: see text] we get intensity in electric charge. The Tolman–Oppenheimer–Volkoff equation (TOV), is modified in order to carry electric charge. In particular, we model the compact star candidates SAXJ 1808.4–3658 and Vela X-1 and give graphical representation of some important properties such as equilibrium condition, mass-radius ratio and surface redshift. In the end, our calculated solutions provide strong evidences for more realistic and viable charged stellar model.

Embedding class I spherically symmetric charged compact star model

The present article is an attempt to arrive at a new solution for an isotropic charged compact star model admitting Karmarkar’s condition in the context of general relativity. For this we chose a new metric potential $e^{\lambda }$ and then determine another gravitational potential $e^{\nu }$ by using embedding class one condition. Different physical viewpoints have been investigated logically to watch the behavior of compact stars such as TOV condition, vitality conditions, Buchdahl condition, stability investigation, compactness and surface redshift by considering a particularized metric potential and the electric field. Graphical examination of these physical parameters involved within the gotten arrangement is additionally displayed to watch their behavior more clearly. The main physical and geometrical properties of the present charged model are quite consistent with compact stellar observational objects like SAXJ1808.4-3658, LMC X-4, CenX-3, PSRJ1614-2230, and PSR J1903+327.

Anisotropic fluid spheres in the framework of [formula omitted] gravity theory

The main aim of this paper is to obtain analytic relativistic anisotropic spherical solutions in f(R,T) scenario. To do so we use modified Durgapal–Fuloria metric potential and the pressure anisotropy condition is imposed in order to obtain the effective anisotropic factor Δ̃. Besides, a notable and viable election on f(R,T) gravity formulation is taken. The choice of f(R,T) function modifies the matter sector only, including new ingredients to the physical parameters that characterize the model such as density, pressure, subliminal speeds of sound, surface redshift etc. We analyze all the physical and mathematical general requirements of the configuration taking M=1.04M⊙ and varying χ from −0.1 to 0.1. It is shown by the graphical procedure that χ<0 yields a more compact object in comparison when χ≥0 (where χ=0.0 corresponds to general relativity) and increases the value of the surface redshift. However, negative values of χ introduce in the system an attractive anisotropic force (inward) and the configuration is completely unstable (corroborated employing Abreu’s criterion). Furthermore, the model in Einstein gravity theory presents cracking while for χ>0 the system is fully stable. The relationship between effective radial pressure p̃r and effective density ρ̃ is discussed and obtained. This is achieved by establishing the corresponding equation of state.

Analytical model of dark energy stars

In this paper, 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. 1 The obtained solutions are relevant to the explanation of compact fluid sphere. Further, we have observed at the junction interface that 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 [Formula: see text], 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 [Formula: see text]. Finally, we demonstrate that the features so obtained are physically acceptable and consistent with the observed/reported data.[Formula: see text] Thus, the present dark energy equation of state appears talented regarding the presence of several exotic astrophysical matters.

Stellar structures in f(G) gravity with Tolman–Kuchowicz spacetime

This paper is devoted to explore some relativistic configurations of stellar objects for static spherically symmetric structures in the context of modified $f(\mathcal{G})$ gravity, by exploiting the Tolman-Kuchowicz spacetime [1,2]. We develop the equations of motion for spherically symmetric spacetime in the presence of anisotropic matter distribution by considering the physically valid expressions of the metric potentials, $\nu=Br^2+2lnC$ and $\lambda=ln(1 + ar^2+br^4)$. To attain the values of the unknown constants we consider the observational data of $Cen~ X-3$, $EXO ~1785-248$ and $LMC~ X-4$ star models. Further, by using evaluated form of the solutions we provide many aspects which are described by the physical status like effective energy density, components of radial and transverse pressure, energy conditions, stability against equilibrium of the forces, speed of sound, mass-radius relation, surface redshift, compactness parameter, adiabatic index and anisotropic measurement. It is observed that all these features follow physically accepted patterns and the resulting outcome is in the experimental range which depicts the viability of our presented $f(\mathcal{G})$ gravity models.

Influence of strong magnetic field on the structure properties of strange quark stars

We investigate the thermodynamic properties of strange quark matter under the strong magnetic field in the framework of the MIT bag model in two cases of bag constants. We consider two cases of the magnetic field, the uniform magnetic field and the density-dependent magnetic field to calculate the equation of state of strange quark matter. For the case of density-dependent magnetic field, we use a Gaussian equation with two free parameters $\beta$ and $\theta$ and use two different sets of the parameters for the magnetic field changes (a slow and a fast decrease of the magnetic field from the center to the surface). Our results show that the energy conditions based on the limitation of the energy-momentum tensor, are satisfied in the corresponding conditions. We also show that the equation of state of strange quark matter becomes stiffer by increasing the magnetic field. In this paper, we also calculate the structure parameters of a pure strange quark star using the equation of state. We investigate the compactification factor ($2M/R$) and the surface redshift of star in different conditions. The results show that the strange quark star is denser than the neutron star and it is more compact in the presence of the stronger magnetic field. As another result, the compactification factor increases when we use a slow increase of the magnetic field from the surface to the center. Eventually, we compare our results with the observational results for some strange star candidates, and we find that the structure of the strange star candidates is comparable to that of the star in our model.

Relativistic charged spheres: compact stars, compactness and stable configurations

This paper aims to explore a class of static stellar equilibrium configuration of relativistic charged spheres made of a charged perfect fluid. For solving the Einstein-Maxwell field equations, we consider a particularized metric potential, Buchdahl ansatz [1] and then by using a simple transformation. The study is developed by matching the interior region with Riessner-Nordström metric as an exterior solution. The matter content the charged sphere satisfies all the energy conditions and hydrostatic equilibrium equation, i.e. the modified Tolman-Oppenheimer-Volkoff (TOV) equation for the charged case is maintained. In addition to this, we also discuss some important properties of the charged sphere such as total electric charge, mass-radius relation, surface redshift, and the speed of sound. Obtained solutions are presented by the graphical representation that provides strong evidence for a more realistic and viable stellar structure. Obtained results are compared with analogue objects with similar mass and radii, such as SAX J1808.4-3658, 4U 1538-52, PSR J1903+327, Vela X-1, and 4U1608-52. It is also noted that the Buchdahl ansatz for a given transformation provides a physically viable solution only for the charged case when 0 < K < 1, where density and pressure are maximum at the center and monotonically decreasing towards the boundary. Obtained results are also quite important both from theoretical and astrophysical scale to analyze other compact objects such as white dwarfs, neutron stars, boson stars, and quark stars.