Showing papers in "Astrophysics and Space Science in 2013"

Shadow of Kerr-Taub-NUT black hole

Abstract: The shadow of a rotating black hole with nonvanishing gravitomagnetic charge has been studied. It was shown that in addition to the angular momentum of black hole the gravitomagnetic charge term deforms the shape of the black hole shadow. From the numerical results we have obtained that for a given value of the rotation parameter, the presence of a gravitomagnetic charge enlarges the shadow and reduces its deformation with respect to the spacetime without gravitomagnetic charge. Finally we have studied the capture cross section for massive particles by black hole with the nonvanishing gravitomagnetic charge.

FRW bulk viscous cosmology with modified Chaplygin gas in flat space

Abstract: In this paper we study FRW bulk viscous cosmology in presence of modified Chaplygin gas. We write modified Friedmann equations due to bulk viscosity and Chaplygin gas and obtain time-dependent energy density for the special case of flat space.

FRW bulk viscous cosmology with modified cosmic Chaplygin gas

Abstract: In this paper we study FRW bulk viscous cosmology in presence of modified cosmic Chaplygin gas. We obtain generalized Friedmann equations due to bulk viscosity and modified cosmic Chaplygin gas. Then, we calculate time-dependent energy density and discuss Hubble expansion parameter.

Shadow of rotating Hořava-Lifshitz black hole

Abstract: The shadow of rotating Hořava-Lifshitz black hole has been studied and it was shown that in addition to the specific angular momentum a, parameters of Hořava-Lifshitz spacetime essentially deform the shape of the black hole shadow. For a given value of the black hole spin parameter a, the presence of a parameter Λ W and KS parameter ω enlarges the shadow and reduces its deformation with respect to the one in the Kerr spacetime. We have found a dependence of radius of the shadow R s and distortion parameter δ s from parameter Λ W and KS parameter ω both. Optical features of the rotating Hořava-Lifshitz black hole solutions are treated as emphasizing the rotation of the polarization vector along null congruences. A comparison of the obtained theoretical results on polarization angle with the observational data on Faraday rotation measurements provides the upper limit for the δ parameter as δ≤2.1⋅10−3.

Generalized second law of thermodynamics in f ( T ) gravity with entropy corrections

Abstract: We study the generalized second law (GSL) of thermodynamics in f(T) cosmology, where T is the torsion scalar in teleparallelism. We consider the universe as a closed bounded system filled with n component fluids in the thermal equilibrium with the cosmological boundary. We use two different cosmic horizons: the future event horizon and the apparent horizon. We show the conditions under which the GSL will be valid in specific scenarios of the quintessence and the phantom energy dominated eras. Further we associate two different entropies with the cosmological horizons: with a logarithmic correction term and a power-law correction term. We also find the conditions for the GSL to be satisfied or violated by imposing constraints on model parameters.

Gravitational lensing by a rotating massive object in a plasma

Abstract: We study gravitational lensing in the vicinity of a slowly rotating massive object surrounded by a plasma. We have studied two effects: (i) the influence of the frame drag- ging on the deflection angle of the light ray in the presence of plasma (ii) Faraday rotation of the polarization plane of the light. We derive the expression for the lensing angle in a non-diagonal space-time in the weak field regime in the presence of plasma and discuss it for the spacetime metric of the slowly rotating object. The obtained deflection angle depends on (i) the frequency of the electromagnetic wave, due to the dispersion properties of the plasma; (ii) the grav- itational mass M; and (iii) the angular momentum J of the gravitational lens. We studied the influence of rotation of the gravitational lens on the magnification of brightness of the source star in the case of microlensing and have shown that it is negligibly small. For the completeness of our study the effect of the Faraday rotation of the polarization plane is considered.

Compact models with regular charge distributions

Abstract: We model a compact relativistic body with anisotropic pressures in the presence of an electric field. The equation of state is barotropic, with a linear relationship between the radial pressure and the energy density. Simple exact models of the Einstein–Maxwell equations are generated. A graphical analysis indicates that the matter and electromagnetic variables are well behaved. In particular, the proper charge density is regular for certain parameter values at the stellar center unlike earlier anisotropic models in the presence of charge. We show that the electric field affects the mass of stellar objects and the observed mass for a particular binary pulsar is regained. Our models contain previous results of anisotropic charged matter with a linear equation of state for special parameter values.

A new class of Bianchi cosmological models in f(R,T) gravity

Abstract: The new class of cosmological model of the early Universe is considered with f(R,T) modified theories of gravity (Harko et al. in Phys. Rev. D 84:024020, 2011). The exact solutions to the corresponding field equations are obtained in quadrature form. The cosmological parameters have been discussed in detail. We have also discussed the well-known astrophysical phenomena, namely the Hubble parameter H(z), luminosity distance (d L ) and distance modulus μ(z) with redshift.

Equilibrium points in the photogravitational restricted four-body problem

Abstract: We study numerically the photogravitational version of the problem of four bodies, where an infinitesimal particle is moving under the Newtonian gravitational attraction of three bodies which are finite, moving in circles around their center of mass fixed at the origin of the coordinate system, according to the solution of Lagrange where they are always at the vertices of an equilateral triangle. The fourth body does not affect the motion of the three bodies (primaries). We consider that the primary body m 1 is dominant and is a source of radiation while the other two small primaries m 2 and m 3 are equal. In this case (photogravitational) we examine the linear stability of the Lagrange triangle solution. The allowed regions of motion as determined by the zero-velocity surface and corresponding equipotential curves, as well as the positions of the equilibrium points on the orbital plane are given. The existence and the number of the collinear and the non-collinear equilibrium points of the problem depends on the mass parameters of the primaries and the radiation factor q 1. Critical masses m 3 and radiation q 1 associated with the existence and the number of the equilibrium points are given. The stability of the relative equilibrium solutions in all cases are also studied. In the last section we investigate the existence and location of the out of orbital plane equilibrium points of the problem. We found that such critical points exist. These points lie in the (x,z) plane in symmetrical positions with respect to (x,y) plane. The stability of these points are also examined.

The motion around the libration points in the restricted three-body problem with the effect of radiation and oblateness

Abstract: This paper deals with the existence of libration points and their linear stability when the more massive primary is radiating and the smaller is an oblate spheroid. Our study includes the effects of oblateness of \(\bar{J}_{2i}\) (i=1,2) with respect to the smaller primary in the restricted three-body problem. Under combining the perturbed forces that were mentioned before, the collinear points remain unstable and the triangular points are stable for 0<μ<μc, and unstable in the range \(\mu_{c} \le\mu\le\frac{1}{2}\), where \(\mu_{c} \in(0,\frac{1}{2})\), it is also observed that for these points the range of stability will decrease. The relations for periodic orbits around five libration points with their semimajor, semiminor axes, eccentricities, the frequencies of orbits and periods are found, furthermore for the orbits around the triangular points the orientation and the coefficients of long and short periodic terms also are found in the range 0<μ<μc.

Charged fluid to anisotropic fluid distribution in general relativity

Abstract: A new class of well behaved anisotropic super-dense stars has been derived with the help of a given class of charged fluid distributions. The anisotropy parameter (or the electric intensity) is zero at the centre and monotonically increasing towards the pressure free interface. All the physical parameter such as energy density, radial pressure, tangential pressure and velocity of sound are monotonically decreasing towards the surface. The maximum mass measures 3.8593 solar mass and the corresponding radius is 21.2573 km for n=1 i.e. N tends to infinity.

Kaluza-Klein universe with cosmic strings and bulk viscosity in f ( R , T ) gravity

Abstract: A five dimensional Kaluza-Klein cosmological model is considered in the frame work of f(R,T) gravity proposed by Harko et al. (Phys. Rev. D 84:024020, 2011) when the source for energy momentum tensor is a bulk viscous fluid containing one dimensional cosmic strings. A barotropic equation of state is assumed to get a determinate solution of the field equations. Also, the bulk viscous pressure is assumed to be proportional to the energy density. The physical behavior of the model is also discussed.

Correspondence between f(G) gravity and holographic dark energy via power-law solution

Abstract: In this paper, we discuss cosmological application of holographic Dark Energy (HDE) in the framework of f(G) modified gravity. For this purpose, we construct f(G) model with the inclusion of HDE and a well-known power law form of the scale factor a(t). The reconstructed f(G) is found to satisfy a sufficient condition for a realistic modified gravity model. We find quintessence behavior of effective equation of state (EoS) parameter ω DE through energy conditions in this context. Moreover, we observe that the squared speed of sound $v_{s}^{2}$ remains negative, which indicates the instability of HDE f(G) model.

Some anisotropic cosmological models in a modified theory of gravitation

Abstract: Field equations in a modified theory of gravitation proposed by Harko et al. (Phys. Rev. D 84: 024020, 2011) are obtained with the aid of a spatially homogeneous and anisotropic LRS Bianchi type-II metric. Cosmological models corresponding to stiff fluid, disordered radiation, dust and false vacuum are obtained. Some physical and kinematical properties of each of the models are also studied.

Bianchi type-V bulk viscous string cosmological model in f ( R , T ) gravity

Abstract: In this paper, we investigate a spatially homogeneous and anisotropic Bianchi type-V cosmological model in a scalar-tensor theory of gravitation proposed by Harko et al. (Phys. Rev. D 84:024020, 2011) when the source for energy momentum tensor is a bulk viscous fluid containing one dimensional cosmic strings. To obtain a determinate solution, a special law of variation proposed by Berman (Nuovo Cimento B 74:182, 1983) is used. We have also used the barotropic equation of state for the pressure and density and bulk viscous pressure is assumed to be proportional to energy density. It is interesting to note that the strings in this model do not survive. Also the model does not remain anisotropic throughout the evolution of the universe. Some physical and kinematical properties of the model are also discussed.

Reconstruction of f ( T ) gravity from the Holographic Dark Energy

Abstract: Among different candidates to play the role of Dark Energy (DE), modified gravity has emerged as offering a possible unification of Dark Matter (DM) and DE. The purpose of this work is to develop a reconstruction scheme for the modified gravity with f(T) action using holographic energy density. In the framework of the said modified gravity we have considered the equation of state of the Holographic DE (HDE) density. Subsequently we have developed a reconstruction scheme for modified gravity with f(T) action. Finally we have obtained a modified gravity action consistent with the HDE scenario.

Dust-acoustic wave modulation in the presence of q-nonextensive electrons and/or ions in dusty plasma

Abstract: A study is presented of the nonlinear self-modulation of low-frequency electrostatic dust acoustic waves (DAWs) propagating in a dusty plasma, within the theoretical framework of the nonextensive statistics proposed by Tsallis. Using the reductive perturbation method (RPM), the nonlinear Schrodinger equation (NLSE) which governs the modulational instability (MI) of the DAWs is obtained. The presence of the nonextensive electron/ion distribution is shown to influence the MI of the waves. Furthermore it is observed that nonextensive distributed ions has more effect on the MI of the DAW than electrons.

Motion in the generalized restricted three-body problem

Abstract: This paper investigates the motion of an infinitesimal body in the generalized restricted three-body problem. It is generalized in the sense that both primaries are radiating, oblate bodies, together with the effect of gravitational potential from a belt. It derives equations of the motion, locates positions of the equilibrium points and examines their linear stability. It has been found that, in addition to the usual five equilibrium points, there appear two new collinear points L n1, L n2 due to the potential from the belt, and in the presence of all these perturbations, the equilibrium points L 1, L 3 come nearer to the primaries; while L 2, L 4, L 5, L n1 move towards the less massive primary and L n2 moves away from it. The collinear equilibrium points remain unstable, while the triangular points are stable for 0<μ<μ c and unstable for $\mu_{c} \le\mu\le\frac{1}{2}$ , where μ c is the critical mass ratio influenced by the oblateness and radiation of the primaries and potential from the belt, all of which have destabilizing tendency. A practical application of this model could be the study of the motion of a dust particle near the oblate, radiating binary stars systems surrounded by a belt.

From Big to Little Rip in modified F ( R , G ) gravity

Abstract: We discuss the cosmological reconstruction in modified Gauss-Bonnet (GB) gravity. It is demonstrated that the modified GB gravity may describe the most interesting features of late-time cosmology. We derive explicit form of effective phantom cosmological models ending by the finite-time future singularity (Big Rip) and without singularities in the future (Little Rip).

Dust-ion-acoustic solitary waves and their multi-dimensional instability in a magnetized nonthermal dusty electronegative plasma

Abstract: A rigorous theoretical investigation has been made on multi-dimensional instability of obliquely propagating electrostatic dust-ion-acoustic (DIA) solitary structures in a magnetized dusty electronegative plasma which consists of Boltzmann electrons, nonthermal negative ions, cold mobile positive ions, and arbitrarily charged stationary dust. The Zakharov-Kuznetsov (ZK) equation is derived by the reductive perturbation method, and its solitary wave solution is analyzed for the study of the DIA solitary structures, which are found to exist in such a dusty plasma. The multi-dimensional instability of these solitary structures is also studied by the small-k (long wave-length plane wave) perturbation expansion technique. The combined effects of the external magnetic field, obliqueness, and nonthermal distribution of negative ions, which are found to significantly modify the basic properties of small but finite-amplitude DIA solitary waves, are examined. The external magnetic field and the propagation directions of both the nonlinear waves and their perturbation modes are found to play a very important role in changing the instability criterion and the growth rate of the unstable DIA solitary waves. The basic features (viz. speed, amplitude, width, instability, etc.) and the underlying physics of the DIA solitary waves, which are relevant to many astrophysical situations (especially, auroral plasma, Saturn’s E-ring and F-ring, Halley’s comet, etc.) and laboratory dusty plasma situations, are briefly discussed.

Electron-acoustic solitary waves in a relativistically degenerate quantum plasma with two-temperature electrons

Abstract: Using the Quantum hydrodynamic (QHD) model Korteweg-de Vries (KdV) type solitary excitations of electron-acoustic waves (EAWs) have been examined in a two-electron-populated relativistically degenerate super dense plasma. It is shown that relativistic degeneracy parameter significantly influences the conditions of formation and properties of solitary structures.

Non-existence of Bianchi type-III bulk viscous string cosmological model in f ( R , T ) gravity

Abstract: A spatially homogeneous and anisotropic Bianchi type-III space-time is considered in the presence of bulk viscous fluid containing one dimensional cosmic strings in the frame work of f(R,T) gravity proposed by Harko et al (Phys Rev D 84:024020, 2011) To get a determinate solution of the field equations of this theory, we have used (i) a barotropic equation of state for the pressure and density and (ii) the bulk viscous pressure is proportional to the energy density It is interesting to observe that, in this case, Bianchi type-III bulk viscous string cosmological model does not exist and degenerates into vacuum model of general relativity

Effective recombination coefficient and solar zenith angle effects on low-latitude D-region ionosphere evaluated from VLF signal amplitude and its time delay during X-ray solar flares

Abstract: Excess solar X-ray radiation during solar flares causes an enhancement of ionization in the ionospheric D-region and hence affects sub-ionospherically propagating VLF signal amplitude and phase. VLF signal amplitude perturbation (ΔA) and amplitude time delay (Δt) (vis-a-vis corresponding X-ray light curve as measured by GOES-15) of NWC/19.8 kHz signal have been computed for solar flares which is detected by us during Jan–Sep 2011. The signal is recorded by SoftPAL facility of IERC/ICSP, Sitapur (22∘ 27′N, 87∘ 45′E), West Bengal, India. In first part of the work, using the well known LWPC technique, we simulated the flare induced excess lower ionospheric electron density by amplitude perturbation method. Unperturbed D-region electron density is also obtained from simulation and compared with IRI-model results. Using these simulation results and time delay as key parameters, we calculate the effective electron recombination coefficient (αeff) at solar flare peak region. Our results match with the same obtained by other established models. In the second part, we dealt with the solar zenith angle effect on D-region during flares. We relate this VLF data with the solar X-ray data. We find that the peak of the VLF amplitude occurs later than the time of the X-ray peak for each flare. We investigate this so-called time delay (Δt). For the C-class flares we find that there is a direct correspondence between Δt of a solar flare and the average solar zenith angle Z over the signal propagation path at flare occurrence time. Now for deeper analysis, we compute the Δt for different local diurnal time slots DT. We find that while the time delay is anti-correlated with the flare peak energy flux ϕmax independent of these time slots, the goodness of fit, as measured by reduced-χ2, actually worsens as the day progresses. The variation of the Z dependence of reduced-χ2 seems to follow the variation of standard deviation of Z along the Tx-Rx propagation path. In other words, for the flares having almost constant Z over the path a tighter anti-correlation between Δt and ϕmax was observed.

Periodic orbits in the restricted four-body problem with two equal masses

Abstract: The restricted (equilateral) four-body problem consists of three bodies of masses m1, m2 and m3 (called primaries) lying in a Lagrangian configuration of the three-body problem i.e., they remain fixed at the apices of an equilateral triangle in a rotating coordinate system. A massless fourth body moves under the Newtonian gravitation law due to the three primaries; as in the restricted three-body problem (R3BP), the fourth mass does not affect the motion of the three primaries. In this paper we explore symmetric periodic orbits of the restricted four-body problem (R4BP) for the case of two equal masses where they satisfy approximately the Routh’s critical value. We will classify them in nine families of periodic orbits. We offer an exhaustive study of each family and the stability of each of them.

Dust ion-acoustic shock waves in nonextensive dusty plasma

Abstract: A parametric survey on the propagation characteristics of the dust ion-acoustic (DIA) shock waves showing the effect of nonextesivity with nonextensive electrons in a dissipative dusty plasma system has been carried out using the reductive perturbation technique. We have considered continuity and momentum equations for inertial ions, q-distributed nonextensive electrons, and stationary charged dust grains, to derive the Burgers equation. It has been found that the basic features of DIA shock waves are significantly modified by the effects of electron nonextensivity and ion kinematic viscosity. Depending on the degree of nonextensivity of electrons, the dust ion-acoustic shock structures exhibit compression and rarefaction. The implications of our results would be useful to understand some astrophysical and cosmological scenarios like stellar polytropes, hadronic matter and quark-gluon plasma, protoneutron stars, dark-matter halos, etc., where effects of nonextensivity can play the significant roles.

Effects of nonthermal ions of distinct temperatures on dust acoustic shock waves in a dusty plasma

Abstract: The nonlinear propagation of dust acoustic (DA) waves in an unmagnetized dusty plasma system consisting of negatively charged mobile dust fluid, Boltzmann distributed electrons, and two-temperature nonthermally distributed ions, is rigorously investigated. The reductive perturbation method has been employed to derive the Burgers equation. The hydrodynamic equation for inertial dust grains has been used to derive the Burgers equation. The effects of two temperature nonthermally distributed ions and dust kinematic viscosity, which are found to significantly modify the basic features of DA shock waves, are briefly discussed. Our present investigation can be effectively utilized in many astrophysical situations (e.g. satellite or spacecraft observations, Saturn’s E ring, etc.), which are discussed briefly in this analysis.

Effects of electromagnetic field on shearfree spherical collapse

Abstract: This paper is devoted to study spherically symmetric shearfree charged gravitational collapse with radial heat flux and isotropic pressure. For the matching of the interior spacetime, we take Vaidya-Reissner-Nordstrom metric outside the spherical system. We solve the field equations numerically by taking ansatz on the metric functions and using Darmois junction conditions. The behavior of density, pressure, radial heat flux, luminosity and the mass function is analyzed. Finally, we check validity of the energy conditions through plots.

Ion acoustic solitons in a nonextensive plasma with multi-temperature electrons

Abstract: The properties of propagation of small amplitude ion acoustic solitary waves (IASWs) are studied in a plasma containing cold fluid ions and multi-temperature electrons (cool and hot electrons) with nonextensive distribution. Korteweg-de Vries (KdV) equation with finite amplitude is derived using a reductive perturbation method. From the solitary solutions of KdV equation, the combined effects of nonextensivity and density ratio are studied on characteristics of ion acoustic (IA) solitary waves. Positive as well as negative polarity solitons exist. Since singularity exists for A=0 so we have also derived modified Korteweg de Vries (mKdV) equation to study the solitonic solution for critical values of physical parameters (q,f,σ). The nonextensivity of electrons (via q) and density ratio of electrons and ions (via f) and temperature ratio (σ) significantly influence the characteristics of ion acoustic solitary structures.

Photons motion in charged anti-de sitter black holes

Abstract: In this work we study the null geodesics in the background of Reissner-Nordstrom Anti de Sitter black hole. We compute the exact trajectories in terms of Weierstrass elliptic functions, obtaining a detailed description of the orbits in terms of the charge, mass and the cosmological constant. The trajectories of the photon are classified using the impact parameter.

A Neuro-Fuzzy modeling for prediction of solar cycles 24 and 25

Abstract: The paper presents a Neuro-Fuzzy model to predict the features of the forthcoming sunspot cycles 24 and 25. The sunspot time series were analyzed with the proposed model. It is optimized based on Backpropagation scheme and applied to the yearly smoothed sunspot numbers. The appropriate number of network inputs for the sunspots data series is obtained based on sequential forward search for the Neuro-Fuzzy model. According to the model prediction the maximum amplitudes of the cycles 24 and 25 will occur in the year 2013 and year 2022 with peaks of 101±8 and 90.7±8, respectively. The correlation and error analysis are discussed to ensure the performance of the proposed Neuro-Fuzzy approach as a predictor for sunspot time series. The correlation coefficient between Neuro-Fuzzy model forecasted sunspot number values with the actual ones is 0.96.