A New Class of Analytical Solutions Describing Anisotropic Neutron Stars in General Relativity Jay Solanki Sardar Vallabhbhai National Institute of Technology, Surat - 395007, Gujarat, India E-mail: [email protected] Bhashin Thakore Department of Physics, Ludwig Maximilian University, Munich - 80539, Germany E-mail: [email protected] Abstract A new class of solutions describing analytical solutions for compact stellar structures has been developed within the tenets of General Relativity. Con- sidering the inherent anisotropy in compact stars, a stable and causal model for realistic anisotropic neutron stars was obtained using the general theory of relativity. Assuming a physically acceptable non-singular form of one metric potential and radial pressure containing the curvature parameter ', the constant :, and the radius A, analytical solutions to Einstein’s field equa- arXiv:2108.00161v1 [gr-qc] 31 Jul 2021 tions for anisotropic matter distribution were obtained. Taking the value of : as -0.44, it was found that the proposed model obeys all necessary physical conditions, and it is potentially stable and realistic. The model also exhibits a linear equation of state, which can be applied to describe compact stars. Keywords: Anisotropic compact star; general relativity; EOS; interior solu- tion. 1 1 Introduction A star towards the end of its life has exhausted all of its fuel, and undergoes core collapse[1, 2]. When the collapsing core is approximately between 1.4 to 3 " , it becomes a neutron star. To decipher the nature of such exotic compact bod- ies is made possible through careful perusal of Einstein’s field equations and the Schwarzschild metric. Ruderman[3] and Canuto [4] proposed that highly compact stars are anisotrop- ically composed. Therefore, it is important to include this anisotropy to model realistic neutron stars. Spearheaded by the work of Bowers and Liang in 1974 [5] on the anisotropic distribution of stellar matter, this concept became a hotbed of research activity and intensive investigation. While an isotropic distribution can also be considered too stringent a condition while looking at modelling compact stellar structures, anisotropy can be exposed through phenomena such as the exis- tence of solid stellar cores, phase transitions in the Type III-A superfluids present inside such stars [6], or pion condensations leading to uneven matter distribution [7]. This anisotropy is also made more significantly visible through stellar rota- tion and electromagnetic fields generated by the star. Chan et al. [8] described the quick onset of dynamical instability in compact stars through radiation and pressure anisotropy. Through Herrera and Santos [9], the effect of anisotropy described through pressure differences has been studied in detail. Maharaja and Marteens [10] modelled anisotropic compact stars by assuming a uniform energy density, described through general relativity. Heintzmann and Hillebrandt [11] showed through a model of relativistic neutron stars at high densities that while the maximum mass for compact stars is limited to 3-4 " , there is no limited mass for arbitrarily large anisotropy measurements.In recent years, many physicists have studied the effect of anisotropy of matter in compact stars, taking the radial com- ponent of pressure is different from tangential component of pressure in compact stars[12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31]. Attempts have also been made to model both charged and uncharged compact star models using the modified theories of gravity [32, 33, 34, 35]. In the study of anisotropic stars, there are two components of pressure, namely, radial pressure ?A and tangential pressure ?C. The magnitude of the anisotropy of ¹?A −?C º the star is given by S = p . If ?A ¡?C, the anisotropic force will be impulsive, 3 supporting the star against gravitational collapse. In our model ?A ¡?C so that it is potentially stable model. We have used procedure to find various parame- ters of model and space-time metric in the interior of the star in which particular form of one of the metric potential and radial pressure is assumed. Sharma and Ratanpal [36] devised a quadratic equation of state using similar methods to depict the anisotropy in compact stars. Some authors have studied compact stars using 2 metric potentials become non-singular at some point[37, 38]. To avoid physical loopholes within the model, we develop an analytical model such that every pa- rameter remains non-singular at every point of space-time. In this paper, we generate a new class of exact solutions to Einstein field equa- tions considering the physically reasonable form of one of the metric potential and radial pressure. We find an exact solution to Einstein’s field equations so that the model’s physical interpretation is simplified. Often solutions of Einstein field equations are presented in terms of special functions are required[12, 14]. However, we obtain exact solutions that satisfy the linear equation of state. We have organized the paper as follows: in Section (2), we have discussed basic field equations of the anisotropic model in the context of general relativity. In section (3), we choose physically reasonable metric potential and radial pressure to solve Einstein field equations in the star’s interior and obtain space-time metric. Now at the star’s boundary at A = ', the interior space-time metric must match the outer Schwarzschild space-time metric. By applying this condition, we obtain a physically acceptable space-time metric in the star’s interior in section (4). In sections (5), we show bounds on the model parameters such that the model satis- fies all necessary conditions to obtain the physically acceptable, stable, and causal model. The model of compact star is said to be causal if the radial and transverse velocity of sound do not exceed the speed of the light.In section (6), we consider the particular value of hypothetical model parameters to analyze the behavior of physical parameters such as energy density and two pressures at the star’s interior for the various cases (1) to (7). We also calculated energy conditions for different cases (1) to (7) in that section. In section (7), we calculated physical parameters for observed neutron stars showing the physical importance of the model to describe astronomical neutron stars with a radius ranging between 10 to 20 km. In section (8), we show that our model exhibits an approximately linear equation of state, followed by a discussion elaborating upon the physical connotations in Section (9). 2 Field Equations of the Anisotropic Compact Star Model For the interior of the static and spherically symmetric object, we consider the structure of space-time governed by the following metric. 3B2 = 4a¹Aº 3C2 − 4_¹Aº 3A2 − A2¹3\2 ¸ B8=2\3q2º (1) Where a and _ are functions of radial parameter r only for static and spheri- cally symmetric stellar configuration, which are needed to be determined. For 3 anisotropic star, we consider the energy-momentum tensor in the following form. )8 9 = 3806¹d, −?A , −?C, −?Cº (2) where d, ?A ,?C are energy-density, radial pressure and tangential pressure respec- tively. We write ?A and ?C in the following form 2( ?A = ? ¸ p (3) 3 ( ?C = ? − p (4) 3 Which gives, p ?A − ?C = 3( (5) Where p and S are isotropic pressure and magnitude of anisotropy respectively. For space-time metric (1) and energy-momentum (2) Einstein’s field equations becomes(considering G = c = 1), 1 − 4−_ 4−__0 8cd = ¸ (6) A2 A 1 a0 1 8c? = 4−_ ¸ − (7) A A2 A A2 4−_ 2 8c? = 2a00 ¸ ¹a0 − _0º a0 ¸ (8) C 4 A 3 Particular Model and its Interior Solution To solve equations (6) - (8), we choose metric potential 4_ in a particular form A2 1 ¸ 2 4_¹Aº = ' (9) − : A2 1 '2 Where k is a parameter of the model which is dimensionless and R is the outer radius of the neutron star which has unit of length in geometrical units. The motivation behind this particular metric form is that it is non-singular for 0 < r < R, for k < 1. We have made some modifications to the metric potential of the Tikekar star model, such that metric potential does not become singular at the 4 physical boundary of star r = R, and it becomes a physically acceptable model. By substituting equation (9) in equation (6), we obtain energy density in the form 2 1¸: ¸ A '2 3 '2 8cd = (10) 2 ¸ A2 1 '2 Now mass function is defined as 1 ¹ <¹Aº = ¹8cdºA23A (11) 2 From equation (6) mass function can be written as ¹1 − 4−_ºA <¹Aº = (12) 2 By substituting equation (9) in equation (12), we get mass function as ¹1¸:ºA3 2 <¹Aº = 2' (13) ¸ A2 1 '2 Now we write equation (7) in the following form 4_ − 1 a0 = 4_ ¹ c? ºA ¸ 8 A A (14) To solve equation (14) we choose 8c?A in the following form 2 2 ?0 − :A − A '2 1 '2 1 '2 8c? = (15) A 2 ¸ A2 1 '2 ? ?0 Where 0 is another dimensionless parameter of the model such that '2 denotes the central pressure of neutron star in our model(in geometrical units of 퐺 = 2 = 1). The motivation behind the particular form of radial pressure is that: 1. It is a monotonically decreasing function of r, 2. It satisfies the necessary condition that radial pressure must vanish at the surface of star r = R and 3. The particular form (15) makes equation (14) integrable. Substituting equation (15) in equation (14) gives, 2 ?0 1 − A A ¹1¸:ºA '2 '2 2 a0 = ¸ ' (16) A2 : A2 ¸ 1 − 2 1 '2 ' 5 By integrating equation (16), we get one of the metric tensor as follow ? 1¸: A2 0 :A2 2: ? A2 a¹Aº − 0 4 = 퐶 1 ¸ 1 − 4 2'2 (17) '2 '2 Where C is a Integration constant.