International Journal of Modern Physics D

In this paper, timing data for anomalous X-ray pulsars and soft gamma repeaters are compiled. Timing properties of these objects are investigated. The effect of bursts of soft gamma repeaters on their period history is investigated. The diagram for pulsars, X-ray binaries, anomalous X-ray pulsars, soft gamma repeaters and dim radio quiet neutron stars is constructed. The possible evolutionary tracks for anomalous X-ray pulsars, soft gamma repeaters and dim radio quiet neutron stars are examined.

In this paper we look at the gravitational spin–spin interaction between macroscopic astronomical bodies. In particular, we calculate their post-Newtonian orbital effects of order on the trajectory of a spinning particle with proper angular momentum s moving in the external gravitomagnetic field generated by a central spinning mass with proper angular momentum J. It turns out that, at order in the orbiter's eccentricity, the eccentricity the pericenter and the mean anomaly rates of the moving particle are affected by long-term harmonic effects. If, on one hand, they are undetectable in the Solar System, on the other, maybe in an astrophysical context like that of the binary millisecond pulsars there will be some hopes of measuring them in the future, provided that substantial improvements in the accuracy of the detection of the periastron rate will occur.

After a decade of gravitational microlensing experiments, 13 to 17 events MACHO group (depending on quality) and two events by EROS have been detected. All of those have been observed in the direction of Large Magellanic Cloud. We use Evans spherically symmetric halo model to study the rate of microlensing events. The expected number of events in this models obtained using EROS and MACHO observational efficiencies. We compare our numbers with the observed events to obtain the fraction of halo that is made by compact objects. It is shown that results derived from the two experiments are in good agreement with each other and MACHOs comprise only a fraction (depending on the model) of Milky Way halo. The results are also compatible with the White Dwarfs population studies in the Hubble Deep Field.

This paper continues the study of the properties of an accretion disc rotating around a non-baryonic (assumed super-massive) compact object. This kind of objects, generically known as boson stars, were earlier proposed as a possible alternative scenario to the existence of super-masive black holes in the center of every galaxy. A dilute boson star has also been proposed as a large part of the non-baryonic dark matter, flattening galactic rotational velocities curves. In this contribution, we compute the profile of the emission lines of Iron; its shape has been for long known as a useful diagnosis of the space-time geometry. We compare with the case of a Schwarzschild black hole, concluding that the differences are observationally distinguishable.

We investigate relation between density contrast and peculiar velocity in the Newtonian gauge, in general relativistic framework of theory of large scale structure formation in the universe. According to the gauge-invariant property of the energy-momentum tensor in this gauge, the velocity perturbation behaves like the Newtonian peculiar velocity. In this framework, the relation between peculiar velocity and density contrast with respect to the Newtonian–Peebles formula has an extra correction term which is ignorable for the small scale structures. The relativistic correction of peculiar velocity for structures of the extent of hundred mega parsec is about few percents which is smaller than the accuracy of the recent peculiar velocity measurements. We also study CMB anisotropies due to the Doppler effect in the Newtonian gauge compared with those obtained using Newtonian gravity.

The Ricci collineation classifications of Kantowski–Sachs, Bianchi types I and III spacetimes are studied according to their degenerate and non-degenerate Ricci tensor. When the Ricci tensor is degenerate, the special cases are classified and it is shown that there are many cases of Ricci collineations (RCs) with infinite number of degrees of freedom, and the group of RCs is ten-dimensional in some spacial cases. Furthermore, it is found that when the Ricci tensor is non-degenerate, the group of RCs is finite-dimensional, and we have only either four which coincides with the isometries or six proper RCs in addition to the four isometries.

In this paper, observational constraints on the cosmic equation of state of dark energy (p = wρ) have been investigated using gravitational lensing statistics. A likelihood analysis of the lens survey has been carried out to constrain the cosmological parameters Ω_m and w. Constraints on Ω_m and w are obtained in three different models of galaxy evolution: no evolution model (comoving number density of galaxies remain constant), Volmerange and Guiderdoni Model and fast merging model. The last two models consider the number evolution of galaxies in addition to the luminosity evolution. The likelihood analysis shows that for the no-evolution case w ≤ -0.04 and Ω_m ≤ 0.90 at 1σ (68% confidence level (CL)). Similarly for the Volmerange & Guiderdoni Model the constraints are w ≤ -0.04 and Ω_m ≤ 0.91 at 1σ. In fast merging model the constraint become weaker and it allows almost the entire range of parameters. For the case of constant Λ (w = -1), all the models permit Ω_m = 0.3 with 68% CL, which is consistent with the value of Ω_m inferred from various other cosmological observations.

The mass-energy formula of black holes implies that up to 50% of the energy can be extracted from a static black hole. Such a result is reexamined using the recently established analytic formulas for the collapse of a shell and the expression for the irreducible mass of a static black hole. It is shown that the efficiency of energy extraction process during the formation of the black hole is linked in an essential way to the gravitational binding energy, the formation of the horizon and the reduction of the kinetic energy of implosion. Here a maximum efficiency of 50% in the extraction of the mass energy is shown to be generally attainable in the collapse of a spherically symmetric shell: surprisingly this result holds as well in the two limiting cases of the Schwarzschild and extreme Reissner–Nordström space–times. Moreover, the analytic expression recently found for the implosion of a spherical shell to an already formed black hole leads to a new exact analytic expression for the energy extraction which results in an efficiency strictly less than 100% for any physical implementable process. There appears to be no incompatibility between General Relativity and Thermodynamics at this classical level.

We have investigated perfect fluid model in Brans–Dicke theory for Bianchi VI₀ space–time and have obtained exact analytical solutions considering barotropic equation of state. These solutions have been analyzed for different values of the parameters involved and some of them have shown a period of exponential expansion.

Dirac equation is written in a non-Riemannian spacetime with torsion and non-metricity by lifting the connection from the tangent bundle to the spinor bundle over spacetime. Foldy–Wouthuysen transformation of the Dirac equation in a Schwarzschild background spacetime is considered and it is shown that both the torsion and non-metricity couples to the momentum and spin of a massive, spinning particle. However, the effects are small to be observationally significant.

Using the generalised set of fluid equations that include the "Coriolis force" along with the centrifugal and pressure gradient forces, we have reanalysed the class of self-similar solutions, with the pseudo-Newtonian potential. We find that the class of solutions is well behaved for almost the entire parameter space except for a few selected combinations of γ and α for the co-rotating flow. The analysis of the Bernoulli number shows that whereas it remains positive for co-rotating flow for f > 1/3, for the counter-rotating flow it does admit both positive and negative values, indicating the possibility of energy transfer in either direction.