Advances in High Energy Physics

Volume 2015 (2015), Article ID 865086, 9 pages

http://dx.doi.org/10.1155/2015/865086

## Fate of Electromagnetic Field on the Cracking of PSR J1614-2230 in Quadratic Regime

^{1}Division of Science and Technology, University of Education, Township Campus, Lahore 54590, Pakistan^{2}Department of Mathematics, University of the Management and Technology, C-II, Johar Town, Lahore 54590, Pakistan

Received 28 July 2015; Accepted 7 September 2015

Academic Editor: Shi-Hai Dong

Copyright © 2015 M. Azam et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The publication of this article was funded by SCOAP^{3}.

#### Abstract

We study the cracking of compact object PSR J1614-2230 in quadratic regime with electromagnetic field. For this purpose, we develop a general formalism to determine the cracking of charged compact objects. We apply local density perturbations to hydrostatic equilibrium equation as well as physical variables involved in the model. We plot the force distribution function against radius of the star with different parametric values of model both with and without charge. It is found that PSR J1614-2230 remains stable (no cracking) corresponding to different values of parameters when charge is zero, while it exhibits cracking (unstable) when charge is introduced. We conclude that stability region increases as amount of charge increases.

#### 1. Introduction

Self-gravitating compact objects (CO), like neutron stars, white dwarfs, millisecond pulsars, and so forth, belong to a distinguished class of those celestial bodies whose study becomes very significant in novel astrophysical research. It is evident that when a star or system of stars burns out all its nuclear fuel, its remnants can have one of three possibilities: white dwarfs, neutron stars, and black holes. The stability of stellar remnants plays a key role in general relativity (GR) as well as modified relativistic theories [1]. The occurrence of gravitational collapse may be a result of cooling of gaseous material, change in anisotropy, fluctuation of gravitational waves, and variation of electromagnetic field of CO [2]. Therefore, such phenomena stimulate our interest to study the stability regions of these self-gravitating CO.

Astronomical objects are not physically viable, if they are unstable towards perturbations. Therefore, it is important to check the stability of these objects. In this context, Bondi [3] initially developed hydrostatic equilibrium equation to examine the stability of self-gravitating spheres. Chandrasekhar [4] calculated the principle value, that is, , to determine the dynamical instability of sphere filled with perfect fluid in GR. Herrera [5] presented the technique of cracking to discuss gravitational collapse of self-gravitating spherical CO. This technique interprets the behavior of inner fluid distribution of CO just after equilibrium state is disturbed. Cracking takes place in CO when radial forces changes its sign from positive to negative and vice versa [6]. Several authors [7–10] studied nonlocal effects of cracking through radial sound speed velocities and Raychaudhuri equation for spherically symmetric CO. Gonzalez [11, 12] presented the idea of local density perturbation (DP) to discuss the idea of cracking for relativistic spheres.

To study the effect of charge on the physical properties of stars is an important subject in GR. In this scenario, Bonnor [13, 14] explored the effect of charge on spherically symmetric CO and found that electric repulsion can halt the gravitational collapse. Bondi [15] used local Minkowski coordinates to describe the contraction of radiating isotropic spherical symmetry. The main hindrance in astrophysics and GR is to develop stable mathematical models which describes the characteristic of charged spherical CO. Bekenstein [16] presented the idea of gravitational collapse in charged CO. Ray et al. [17] found the maximum amount of charge (i.e., approximately coulomb), needed for CO to be in equilibrium configuration. Some authors [18, 19] studied the impact of charge on gravitational collapse of celestial objects and analyzed the tendency of self-gravitating systems to produce charged black holes or naked singularities. Sharif and Azam [20, 21] studied the stability of spherical and cylindrical symmetric objects under the influence of electromagnetic field.

Demorest et al. [22] used the Green Bank Telescope at the National Radio Astronomy Observatory to analyze the system of stars by means of Shapiro delay (SD) and presented the observed values of different physical parameters for PSR J1614-2230. These physical parameters like ecliptic longitude, ecliptic latitude, parallax pulsar spin, pulsar spin period, orbital period, companion mass, radius, and so forth are recorded with very high precision by SD for PSR J1614-2230. The availability of very accurate parametric values made PSR J1614-2230 extremely important for modern research in GR. Neutron stars are made of the most dense material existing in this universe. Tauris et al. [23] developed mathematical model of PSR J1614-2230 and provided the possible variation of masses to show that PSR J1614-2230 was born more massive as compared to any discovered neutron star. Lin et al. [24] used stellar evaluation code “MESA” to describe the relationship between PSR J1614-2230 and its stellar companion. This discovery of high massive neutron star has extensive consequences on the equation of state (EoS) of matter with high densities. The relationship between physical parameters becomes more complicated as linear EoS is replaced by nonlinear EoS. In this work, we apply the concept of cracking to self-gravitating CO in the presence of electromagnetic field in quadratic regime. Here, we take local density perturbation (DP) which is different from constant DP presented by Herrera [5]. We applied this technique to the model of charged compact objects with quadratic EoS presented by Takisa et al. [25] and determine the cracking of newly discovered PSR J1614-2230 with electromagnetic field. Recently, we have investigated the cracking of some compact objects with and without electromagnetic field in linear regime [26, 27].

This paper is arranged as follows. Section 2 deals with Einstein-Maxwell field and Tolman-Oppenheimer-Volkoff (TOV) equations corresponding to an isotropic fluid. We present the general formalism to determine the cracking of charged CO with local DP in the quadratic regime in Section 3. Section 4 investigates stable and unstable regions of compact star PSR J1614-2230. In the last section, we conclude our results.

#### 2. Einstein-Maxwell Field and Tolman-Oppenheimer-Volkoff Equations

We consider the line element for a static spherically symmetric space time in curvature coordinates given bywhere , , and , are gravitational potentials. The Maxwell’s equations are defined aswhere is the electromagnetic field tensor, is the four current densities, and is the electromagnetic energy-momentum tensor [28]. The skew-symmetric electromagnetic field tensor can be decomposed aswhere is the electric field and is the magnetic field. The electromagnetic field tensor and four current densities can be defined aswhere and are the four potential and proper charge densities and is four vector velocities of the fluid. The four potentials are defined asUsing this in the above equation, it yieldswhich can also be written aswhere we have used . The total energy-momentum tensor corresponding to charged anisotropic fluid sphere is defined by [28]The terms , , and and are electromagnetic field, energy density, and radial and tangential pressure, respectively.

The synergies of electromagnetic field and matter are governed by system of field equations. These synergies of spherically symmetric metric correspond to Einstein-Maxwell field equations given bywhere is the energy-momentum tensor for the fluid inside the star and is electromagnetic field tensor. The nonzero components of Einstein-Maxwell field equations corresponding to (1) and (8) are given as follows:where “” denotes the differentiation with respect to .

It is clear that the choice of EoS of fluid inside the star plays a key role for its physical significance. Thus, a star is physically acceptable, if it satisfies the barotropic EoS . In this work, we have used the quadratic EoS to explore the stability of PSR J1614-2230. The quadratic EoS is given by [25]where , , and are constants and are constrained by and , where = 0.5 × 10^{15} g/cm^{3} gives the density at the boundary surface of sphere. It is interesting to note that this equation reduces to linear EoS, when [25].

Solving (10)–(12) simultaneously, we obtain hydrostatic equilibrium equation (TOV) for anisotropic charged fluid:which shows that gradient of pressure is effected by charge and anisotropy of fluid. Using the relation in the above equation [28], it yieldswhere the mass function with is defined as

#### 3. Effect of Local Density Perturbation

In this section, we perturb the equilibrium configuration of charged CO through local DP (). Equation (16) depicts that cracking takes place in interior of spherical CO when equilibrium state is interrupted due to change in sign of perturb force, that is, , and vice versa. We apply the local DP to (16) and all the physical variables like mass, radial and tangential pressure, electromagnetic field, and their derivatives are involved in (16), given byThe radial sound speed and tangential sound speed are defined asThe perturb form of (16) is given bywherewhich can also be written asThis is the fundamental equation used to determine the effects of local DP on the cracking of charged anisotropic fluid. We will plot the force distribution function against radius “” of the star for different values of the parameters involved in the model. Using (16), the derivatives involved in the above equation are given as follows:

#### 4. Cracking of PSR J1614-2230

Here, we apply the formalism developed in the above section to investigate the cracking of charged objects for the model given by Takisa et al. [25]. This model is consistent with the physical features of observed objects and its connection can be made with PSR J1614-2230 for particular values of parameters given in [25]. The analysis of Takisa seems to be consistent with observational objects such as Vela X-1, Cen X-3, SMC X-1, PSR J1903-327, and PSR J1614-2230. But our focus in this analysis is the particular object PSR J1614-2230 because its mass and radius have been measured with great accuracy. The model is defined by following equations:whereThe radial and tangential sound speed velocities can be obtained from (26) and (27) as

The constants , , and have dimensions of length and are chosen in such way that the given system satisfies the following conditions: (i)Energy density must remain positive before and after equilibrium state. (ii)Radial pressure should be vanished at the boundary of star. (iii)At the center of star, that is, , we have . (iv) is constant in the quadratic regime. (v)Across boundary of star, when , we haveBy considering above conditions, we have where = 43.245 km and the values given above are compatible with observational values given by Takisa et al. [25].

For the sake of regions (stable and unstable) of PSR J1614-2230, we have plotted force distribution function against radius of the star for different values of the parameters involved in the model shown in Figures 1–8. We summarize these results as follows: (i)Figure 1 depicts that all curves do not change their sign with different values of and corresponding to Table 1. Hence, we find that PSR J1614-2230 is stable in the absence of charge in quadratic regime and it is unstable in linear regime which is analogous to the results found in [26, 27]. From Table 2, it is clear that any variation in coefficients of quadratic EoS does not affect stability; even radius of PSR J1614-2230 changes approximately to . (ii)In Figure 2, there are three curves corresponding to model parameters , and charge . It is noted that all three curves (red, blue, and green) change their sign for charge parameter (), respectively, in the linear regime. This shows that PSR J1614-2230 is unstable in linear regime, where the symbols “◊,” “,” and “” represent the cracking points (where curve changes its sign from negative to positive) corresponding to , respectively. The cracking values corresponding to (red, blue, and green) are given in (Tables 2, 3, and 4). In this case, our results are consistent with [26, 27] in linear regime. (iii)Figures 3–8 represent the cracking of PSR J1614-2230 star for fixed values of parameters , and charge in quadratic regime. We see that cracking takes place for charge parameters and , which are indicated by the cracking points “” and “” corresponding to blue and green curves, respectively. These cracking points () are given in Tables 3-4. However, in each case, the star remains stable; that is, no cracking takes place for in quadratic regime. Hence, PSR J1614-2230 exhibits cracking both in linear and quadratic regime when charge is present. From these illustrations, we conclude that as charge increases, cracking points are slightly shifted towards center, which indicates that binding forces of CO become stronger and more mass is directed towards origin.