A. R. Prasanna

Charged particle motion in an electromagnetic field on Kerr background geometry

In this paper we study the trajectories of charged particles in an electromagnetic field superimposed on the Kerr background. The electromagnetic fields considered are of two types: (i) a dipole magnetic field with an associated quadrupole electric field, (ii) a uniform magnetic field. The contribution of the background geometry to the electromagnetic field is taken through the solutions of Petterson and Wald respectively. The effective potential is studied in detail for ther-motion of the particles in the equatorial plane and the orbits are obtained. The most interesting aspect of the study is the illustration of the effect of inertial frame dragging due to the rotation of the central star. This appears through the existence of nongyrating bound orbits at and inside the ergo surface. The presence of the magnetic field seems to increase the range of stable orbits, as was found in a previous study involving the Schwarzschild background.

Charged particle trajectories in a magnetic field on a curved space-time

In this paper we have studied the motion of charged particles in a dipole magnetic field on the Schwarzscbild background geometry. A detailed analysis has been made in the equatorial plane through the study of the effective potential curves. In the case of positive canonical angular momentum the effective potential has two maxima and two minima giving rise to a well-defined potential well rear the event horizon. This feature of the effective potential categorises the particle orbits into four classes, depending on their energies. (i) Particles, coming from infinity with energy less than the absolute maximum ofVeff, would scatter away after being turned away by the magnetic field. (ii) Whereas those with energies higher than this would go into the central star seeing no barrier. (iii) Particles initially located within the potential well are naturally trapped, and they execute Larmor motion in bound gyrating orbits. (iv) and those with initial positions corresponding to the extrema ofVeff follow circular orbits which are stable for non-relativistic particles and unstable for relativistic ones. We have also considered the case of negative canonical angular momentum and found that no trapping in bound orbits occur for this case.In the case when particles are not confined to the equatorial plane we have found that the particles execute oscillatory motion between two mirror points if the magnetic field is sufficiently high, but would continuously fall towards the event horizon otherwise.

CONSTRAINTS ON BACKGROUND TORSION FROM BIREFRINGENCE OF CMB POLARIZATION

We show that a non-minimal coupling of electromagnetism with background torsion can produce birefringence of the electromagnetic waves. This birefringence gives rise to a B-mode polarization of the CMB. From the bounds on B-mode from WMAP and BOOMERanG data, one can put limits on the background torsion at

Centrifugal force and ellipticity behaviour of a slowly rotating ultra-compact object

\xi_{1}T_{1}=(-3.35 \pm 2.65) \times 10^{-22} GeV^{-1}

Centrifugal force in Kerr geometry

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Constraints on non-minimally coupled curved space electrodynamics from astrophysical observations

Using the optical reference geometry approach, we have derived a general expression for the ellipticity of a slowly rotating fluid configuration using a Newtonian force balance equation in the conformally projected absolute 3-space, in the realm of general relativity. Further, with the help of the Hartle - Thorne (H - T) metric for a slowly rotating compact object, we have evaluated the centrifugal force acting on a fluid element and also evaluated the ellipticity and found that the centrifugal reversal occurs at around , and the ellipticity maximum at around . The result has been compared with that of Chandrasekhar and Miller which was obtained in the full 4-spacetime formalism.

Behaviour of the centrifugal force and of ellipticity for a slowly rotating fluid configuration with different equations of state

Using the 3+1 splitting of optical reference geometry, the authors have obtained the correct expression for the centrifugal force acting on a particle at the equatorial circumference of a rotating body in the locally non-rotating frame of the Kerr geometry. In contrast to the analysis based on Boyer-Lindquist coordinates, this expression is valid everywhere outside the event horizon.

Charged particle orbits in Kerr geometry with electromagnetic fields as viewed from locally non-rotating frames

We study interactions of electromagnetic fields with the curvature tensor of the form λRμναβFμνFαβ. Such coupling terms, though invariant under general coordinate transformation and CPT, violate the Einstein equivalence principle. These couplings do not cause any energy-dependent dispersion of photons but they exhibit birefringence. We put constraints on the coupling constant λ using results from solar system radar ranging experiments and millisecond-pulsar observations. We find that the most stringent constraint comes from pulsar observations and is given by λ < 1011 cm2 obtained from the timing of binary pulsar PSR B1534+12.

Constraints on cosmological viscosity and self interacting dark matter from gravitational wave observations

We have evaluated the centrifugal force acting on a fluid element and the ellipticity of the fluid configuration, which is slowly rotating, using the Hartle - Thorne solution for different equations of state. The centrifugal force shows a maximum in every case, whereas the reversal in sign could be seen in only one case and the system becomes unstable in other cases. The ellipticity as calculated from the usual definition shows maxima, whereas the definition obtained from the equilibration of the inertial forces, shows a negative behaviour, indicating that the system is prolate and not oblate. This prolate shape of the configuration is similar to the one earlier found by Pfister and Braun for a rotating shell of matter, using the correct centrifugal force expression for the interior. The location of the centrifugal maxima gets further away from the Schwarzschild radius as the equation of state gets softer.

Role of general relativity in accretion disk dynamics

The charged particle orbits in electromagnetic fields on Kerr background as viewed from a locally non-rotating frame do not exhibit non-gyrating bound orbits, which was an essential feature in the earlier study of Prasanna and Vishveshwara, thus showing the non gyration to be due to the effect of dragging of inertial frames produced by the rotating black hole.