Arunava Bhadra

Gravitational lensing by a charged black hole of string theory

We study gravitational lensing by the Gibbons-Maeda-Garfinkle-Horowitz-Strominger (GMGHS) charged black hole of heterotic string theory and obtain the angular position and magnification of the relativistic images. Modeling the supermassive central object of the galaxy as a GMGHS black hole, we estimate the numerical values of different strong-lensing parameters. We find that there is no significant string effect present in the lensing observables in the strong-gravity scenario.

On static spherically symmetric solutions of the vacuum Brans-Dicke theory

It is shown that among the four classes of the static spherically symmetric solutions of the vacuum Brans-Dicke theory of gravity only two are really independent. Further, by matching exterior and interior (due to physically reasonable spherically symmetric matter source) scalar fields it is found that only the Brans class I solution with a certain restriction on the solution parameters may represent an exterior metric for a nonsingular massive object. The physical viability of the black hole nature of the solution is investigated. It is concluded that no physical black hole solution different from the Schwarzschild black hole is available in the Brans-Dicke theory.

Strong field gravitational lensing in scalar?tensor theories

Strong field gravitational lensing in the Brans?Dicke scalar?tensor theory has been studied. The deflection angle for photons passing very close to the photon sphere is estimated for the static spherically symmetric spacetime of the theory and the position and magnification of the relativistic images are obtained. Modelling the super massive central object of the galaxy by the Brans?Dicke spacetime, numerical values of different strong lensing observables are estimated. It is found that against the expectation there is no significant scalar field effect on the strong field observable lensing parameters. This result raises question on the potentiality of strong field lensing to discriminate different gravitational theories.

BRANS DICKE THEORY: JORDAN VERSUS EINSTEIN FRAME

It is well known that, in contrast to general relativity, there are two conformally related frames, the Jordan frame and the Einstein frame, in which the Brans–Dicke theory, a prototype of generic scalar–tensor theory, can be formulated. There is a long standing debate on the physical equivalence of the formulations in these two different frames. It is shown here that gravitational deflection of light to second order accuracy may observationally distinguish the two versions of the Brans–Dicke theory.

Gravitational deflection of light in the Schwarzschild-de Sitter space-time

Recent studies suggest that the cosmological constant affects the gravitational bending of photons, although the orbital equation for light in Schwarzschild-de Sitter space-time is free from a cosmological constant. Here we argue that the very notion of a cosmological constant independent of the photon orbit in the Schwarzschild-de Sitter space-time is not proper. Consequently, the cosmological constant has some clear contributions to the deflection angle of light rays. We stress the importance of the study of photon trajectories from the reference objects in bending calculations, particularly for asymptotically nonflat space-time. When such an aspect is taken into consideration, the contribution of a cosmological constant to the effective bending is found to depend on the distances of the source and the reference objects.

Perfect fluid dark matter

Abstract Taking the flat rotation curve as input and treating the matter content in the galactic halo region as perfect fluid we obtain a space–time metric at the galactic halo region in the framework of general relativity. We find that the resultant space–time metric is a non-relativistic dark matter induced space–time embedded in a static Friedmann–Lemaitre–Robertson–Walker universe i.e. the flat rotation curve not only leads to the existence of dark matter but also suggests about the background geometry of the universe. Within its range of validity the flat rotation curve and the demand that the dark matter to be non-exotic together indicate for a (nearly) flat universe as favored by the modern cosmological observations. We obtain the expressions for energy density and pressure of dark matter there and consequently the equation of state of dark matter. Various other aspects of the solutions are also analyzed.

Wormholes in vacuum Brans-Dicke theory

It is shown that among the different classes of claimed static wormhole solutions of the vacuum Brans–Dicke theory only Brans Class I solution with coupling constant ω less than -1.5 (excluding the point ω = 2) gives rise to physically viable traversable wormhole geometry. Usability of this wormhole geometry for interstellar travel has been examined.

Contribution of a nearby pulsar to cosmic rays observed at Earth

Abstract Contribution of nearby pulsars to the cosmic rays observed at Earth has been studied. It is found that the experimental bound on amplitude of cosmic ray anisotropy may produce significant constraint on the efficiency of converting pulsar rotational energy to emitted particles kinetic energy. Cosmic ray fluxes from two well known nearby gamma ray pulsars, namely the Vela and Geminga pulsars, are estimated. The analysis suggests that observed bound on cosmic ray anisotropy restricts the contributions of the Vela and the Geminga pulsars to at most 1% of the observed cosmic rays below the knee.

Scaling behaviour of lateral distribution of electrons in EAS

From a Monte Carlo simulation study of cosmic ray air showers around the knee of the primary energy spectrum it is shown that, despite a strong radial dependence of the lateral shower age parameter, the lateral density distribution of electrons in cosmic ray EAS displays universality when expressed in terms of local age parameters. The nature of the radial variation of local age is found to depend on the choice of the effective Moliere radius, particularly for radial distances below about 400?m. The possible use of shower age parameters in a multi-parameter study of EAS for extracting information about the nature of the shower initiating particles, has been re-examined.

The vacuole model: new terms in the second order deflection of light

The present paper is an extension of a recent work (Bhattacharya et al. 2010) to the Einstein-Strauss vacuole model with a cosmological constant, where we work out the light deflection by considering perturbations up to order M3 and confirm the light bending obtained previously in their vacuole model by Ishak et al. (2008). We also obtain another local coupling term −5πM2Λ/8 related to Λ, in addition to the one obtained by Sereno (2008, 2009). We argue that the vacuole method for light deflection is exclusively suited to cases where the cosmological constant Λ disappears from the path equation. However, the original Rindler-Ishak method (2007) still applies even if a certain parameter γ of Weyl gravity does not disappear. Here, using an alternative prescription, we obtain the known term −γR/2, as well as another new local term 3πγM/2 between M and γ. Physical implications are compared, where we argue that the repulsive term −γR/2 can be masked by the Schwarzschild term 2M/R in the halo regime supporting attractive property of the dark matter.