International Journal of Modern Physics D

Properties of rotating neutron stars with the use of relativistic mean-field theory are considered. The performed analysis of neutron star matter is based on the nonlinear Lgrangian density. The presence of nonlinear interaction of vector mesons modifies the density dependence of the ρ field and influences bulk parameters of neutron stars. The observed quasiperiodic X-ray oscillations of low mass X-ray binaries can be used in order to constrain the equation of state of neutron star matter. Having assumed that the maximum frequency of the quasiperiodic oscillations originates at the circular orbit it is possible to estimate masses and radii of neutron stars.

We ascribe the 15–60 Hz Quasi Periodic Oscillation (QPO) to the periastron precession frequency of the orbiting accreted matter at the boundary of magnetosphere-disk of X-ray neutron star (NS). Considering the relativistic motion mechanism for the kHz QPO, that the radii of the inner disk and magnetosphere-disk of NS are correlated with each other by a factor is assumed. The obtained conclusions include: all QPO frequencies increase with increasing the accretion rate. The theoretical relations between 15–60 Hz QPO (HBO) frequency and the twin kHz QPOs are similar to the measured empirical formula. Further, the better fitted NS mass by the proposed model is about 1.9 solar masses for the detected LMXBs.

The integral formulation of Maxwell's equations expressed in terms of an arbitrary observer family in a curved spacetime is developed and used to clarify the meaning of the lines of force associated with observer-dependent electric and magnetic fields.

We derive the "exact" Newtonian limit of general relativity with a positive cosmological constant Λ. We point out that in contrast to the case with Λ=0, the presence of a positive Λ in Einsteins's equations enforces, via the condition |Φ|≪1 on the potential Φ, a range ℛ_max(Λ)≫r≫ℛ_min(Λ), within which the Newtonian limit is valid. It also leads to the existence of a maximum mass, ℳ_max(Λ). As a consequence we cannot put the boundary condition for the solution of the Poisson equation at infinity. A boundary condition suitably chosen now at a finite range will then get reflected in the solution of Φ provided the mass distribution is not spherically symmetric.

The classic problem of the growth of density perturbations in an expanding Newtonian universe is revisited following the work of Bisnovatyi-Kogan and Zel'dovich. We propose a more general analytical approach: a system of free particles satisfying semidegenerate Fermi–Dirac statistics on the background of an exact expanding solution is examined in the linear approximation. This differs from the corresponding work of Bisnovatyi-Kogan and Zel'dovich where classical particles fulfilling Maxwell–Boltzmann statistics were considered. The solutions of the Boltzmann equation are obtained by the method of characteristics. An expression for the damping time of a decaying solution is discussed and a zone in which free streaming is hampered is found, corresponding to wavelengths less than the Jeans one. In the evolution of the system, due to the decrease of the Jeans length, those perturbations may lead to gravitational collapse. At variance with current opinions, we deduce that perturbations with λ≥λ_{J Max}/1.48 are able to generate structures and the lower limit for substructures mass is M=M_{J max}/(1.48)³≈M_{J max}/3, where M_{J max} is the maximum value of the Jeans mass.

In this paper, we study the consequences of the existence of timelike and spacelike Ricci collineation vectors (RCVs) for string cloud in the context of general relativity. Necessary and sufficient conditions are derived for a space-time with string cloud to admit a timelike RCV parallel to u^a and a spacelike RCV parallel to x^a. Also, some results are obtained.

We present new solutions to the Einstein–Maxwell equations representing a class of charged distorted black holes. These solutions are static-axisymmetric and are generalizations of the distorted black hole solutions studied by Geroch and Hartle. Physically, they represent a charged black hole distorted by external matter fields. We discuss the zeroth and first law for these black holes. The first law is proved in two different forms, one motivated by the isolated horizon framework and the other using normalizations at infinity.

The principal purpose of this paper is to study the effect of an impulsive light-like signal on neighbouring test particles. Such a signal can in general be unambiguously decomposed into a light-like shell of null matter and an impulsive gravitational wave. Our results are: (a) If there is anisotropic stress in the light-like shell then test particles initially moving in the signal front are displaced out of this 2-surface after encountering the signal; (b) For a light-like shell with no anisotropic stress accompanying a gravitational wave the effect of the signal on test particles moving in the signal front is to displace them relative to each other with the usual distortion due to the gravitational wave diminished by the presence of the light-like shell. An explicit example for a plane-fronted signal is worked out.

String Cosmology with magnetic field has been studied for Bianchi V space-time model for different directions of the magnetic field and string. For equation of state, we have examined geometric string (Nambu string), Takabayashi string (p-string), string with uniform energy density or barotropic equation of state. Exact analytic solution is possible only in few cases.

In this paper Kaluza–Klein cosmological model within the framework of Lyra geometry has been discussed. The physical behavior of the model is examined in vacuum and in the presence of perfect fluids.

In this paper we have evaluated solutions for domain wall in Lyra geometry. Solutions are obtained with separable form for the metric coefficients. Finally acceleration due to the domain wall has been analised by studying geodesic equation.

We consider a magnetic cylindrically symmetric universe with two degrees of freedom in the presence of an electrically neutral viscous fluid. The source of the magnetic field is an electric current along the x-axis. The coefficient of bulk viscosity is assumed to be a power function of the mass density. Without assuming any adhoc law, we obtain a cosmological constant as a decreasing function of time which is supported by results from recent supernovae observations.

Ricci collineations of the Bianchi types I and III, and Kantowski–Sachs spacetimes are classified according to their Ricci collineation vector (RCV) field of the form (i)–(iv) one component of ξ^a(x^b) is nonzero, (v)–(x) two components of ξ^a(x^b) are nonzero, and (xi)–(xiv) three components of ξ^a(x^b) are nonzero. Their relation with isometries of the spacetimes is established. In case (v), when det(R_ab)=0, some metrics are found under the time transformation, in which some of these metrics are known, and the other ones new. Finally, the family of contracted Ricci collineations (CRC) are presented.