1,402 research outputs found
Acceleration and Cyclotron Radiation, Induced by Gravitational Waves
The equations which determine the response of a charged particle moving in a
magnetic field to an incident gravitational wave(GW) are derived in the
linearized approximation to general relativity. We briefly discuss several
astrophysical applications of the derived formulae taking into account the
resonance between the wave and the particle's motion which occurs at
, whenever the GW is parallel to the constant magnetic field.
In the case where the GW is perpendicular to the constant magnetic field,
magnetic resonances appear at and . Such
resonant mechanism may be useful to build models of GW driven cyclotron
emitters.Comment: 16 pages, 1 figure, accepted for publication in Astronomy &
Astrophysic
Pre-Inflation in the Presence of Conformal Coupling
We consider a massless scalar field, conformally coupled to the Ricci scalar
curvature, in the pre-inflation era of a closed FLRW Universe. The scalar field
potential can be of the form of the Coleman-Weinberg one-loop potential, which
is flat at the origin and drives the inflationary evolution. For positive
values of the conformal parameter \xi, less than the critical value xi_c=(1/6),
the model admits exact solutions with non-zero scale factor and zero initial
Hubble parameter. Thus these solutions can be matched smoothly to the so called
Pre-Big-Bang models. At the end of this pre-inflation era one can match
inflationary solutions by specifying the form of the potential and the whole
solution is of the class C^(1).Comment: 11 pages, 5 figures, LaTeX, Accepted for publication in MPL
The Force-Free Magnetosphere of a Rotating Black Hole
We revisit the Blandford-Znajek process and solve the fundamental equation that governs the structure of the steady-state force-free magnetosphere around a Kerr black hole. The solution depends on the distributions of the magnetic field angular velocity and the poloidal electric current. These are not arbitrary. They are determined self-consistently by requiring that magnetic field lines cross smoothly the two singular surfaces of the problem: the inner "light surface" located inside the ergosphere and the outer "light surface" which is the generalization of the pulsar light cylinder.We find the solution for the simplest possible magnetic field configuration, the split monopole, through a numerical iterative relaxation method analogous to the one that yields the structure of the steady-state axisymmetric force-free pulsar magnetosphere. We obtain the rate of electromagnetic extraction of energy and confirm the results of Blandford and Znajek and of previous time-dependent simulations. Furthermore, we discuss the physical applicability of magnetic field configurations that do not cross both "light surfaces.
Alfven modes driven non-linearly by metric perturbations in anisotropic magnetized cosmologies
We consider anisotropic magnetized cosmologies filled with conductive plasma
fluid and study the implications of metric perturbations that propagate
parallel to the ambient magnetic field. It is known that in the first order
(linear) approximation with respect to the amplitude of the perturbations no
electric field and density perturbations arise. However, when we consider the
non-linear coupling of the metric perturbations with their temporal
derivatives, certain classes of solutions can induce steeply increasing in time
electric field perturbations. This is verified both numerically and
analytically. The source of these perturbations can be either high-frequency
quantum vacuum fluctuations, driven by the cosmological pump field, in the
early stages of the evolution of the Universe or astrophysical processes or a
non-linear isotropization process of an initially anisotropic cosmological
spacetime.Comment: 7 pages, RevTex, 3 figures ps, accepted for publication to IJMP
Brane Cosmology from Heterotic String Theory
We consider brane cosmologies within the context of five-dimensional actions
with O(a') higher curvature corrections. The actions are compatible with bulk
string amplitude calculations from heterotic string theory. We find wrapped
solutions that satisfy the field equations in an approximate but acceptable
manner given their complexity, where the internal four-dimensional scale factor
is naturally inflating, having an exponential De-Sitter form. The temporal
dependence of the metric components is non-trivial so that this metric cannot
be factored as in a conformally flat case. The effective Planck mass is finite
and the brane solutions localize four-dimensional gravity, while the
four-dimensional gravitational constant varies with time. The Hubble constant
can be freely specified through the initial value of the scalar field, to
conform with recent data.Comment: 15 pages, 3 figures, LaTeX, Accepted for Publication in IJT
Gravito-magnetic instabilities in anisotropically expanding fluids
Gravitational instabilities in a magnetized Friedman - Robertson - Walker
(FRW) Universe, in which the magnetic field was assumed to be too weak to
destroy the isotropy of the model, are known and have been studied in the past.
Accordingly, it became evident that the external magnetic field disfavors the
perturbations' growth, suppressing the corresponding rate by an amount
proportional to its strength. However, the spatial isotropy of the FRW Universe
is not compatible with the presence of large-scale magnetic fields. Therefore,
in this article we use the general-relativistic (GR) version of the
(linearized) perturbed magnetohydrodynamic equations with and without
resistivity, to discuss a generalized Jeans criterion and the potential
formation of density condensations within a class of homogeneous and
anisotropically expanding, self-gravitating, magnetized fluids in curved
space-time. We find that, for a wide variety of anisotropic cosmological
models, gravito-magnetic instabilities can lead to sub-horizonal, magnetized
condensations. In the non-resistive case, the power spectrum of the unstable
cosmological perturbations suggests that most of the power is concentrated on
large scales (small k), very close to the horizon. On the other hand, in a
resistive medium, the critical wave-numbers so obtained, exhibit a delicate
dependence on resistivity, resulting in the reduction of the corresponding
Jeans lengths to smaller scales (well bellow the horizon) than the
non-resistive ones, while increasing the range of cosmological models which
admit such an instability.Comment: 10 pages RevTex, 4 figures, accepted for publication in the
International Journal of Modern Physics
Galaxy formation and cosmic-ray acceleration in a magnetized universe
We study the linear magneto-hydrodynamical behaviour of a Newtonian cosmology
with a viscous magnetized fluid of finite conductivity and generalise the Jeans
instability criterion. The presence of the field favors the anisotropic
collapse of the fluid, which in turn leads to further magnetic amplification
and to an enhanced current-sheet formation in the plane normal to the ambient
magnetic field. When the currents exceed a certain threshold, the resulting
electrostatic turbulence can dramatically amplify the resistivity of the medium
(anomalous resistivity). This could trigger strong electric fields and
subsequently the acceleration of ultra-high energy cosmic rays (UHECRs) during
the formation of protogalactic structures.Comment: 10 pages, ApJL in pres
Torsional Oscillations of Magnetized Relativistic Stars
Strong magnetic fields in relativistic stars can be a cause of crust
fracturing, resulting in the excitation of global torsional oscillations. Such
oscillations could become observable in gravitational waves or in high-energy
radiation, thus becoming a tool for probing the equation of state of
relativistic stars. As the eigenfrequency of torsional oscillation modes is
affected by the presence of a strong magnetic field, we study torsional modes
in magnetized relativistic stars. We derive the linearized perturbation
equations that govern torsional oscillations coupled to the oscillations of a
magnetic field, when variations in the metric are neglected (Cowling
approximation). The oscillations are described by a single two-dimensional wave
equation, which can be solved as a boundary value problem to obtain
eigenfrequencies. We find that in the non-magnetized case, typical oscillation
periods of the fundamental l=2 torsional modes can be nearly a factor of two
larger for relativistic stars than previously computed in the Newtonian limit.
For magnetized stars, we show that the influence of the magnetic field is
highly dependent on the assumed magnetic field configuration and simple
estimates obtained previously in the literature cannot be used for identifying
normal modes observationally.Comment: 9 pages, 1 figure, revised figure and equations, MNRAS in pres
Impulsive electron acceleration by Gravitational Waves
We investigate the non-linear interaction of a strong Gravitational Wave with
the plasma during the collapse of a massive magnetized star to form a black
hole, or during the merging of neutron star binaries (central engine). We found
that under certain conditions this coupling may result in an efficient energy
space diffusion of particles. We suggest that the atmosphere created around the
central engine is filled with 3-D magnetic neutral sheets (magnetic nulls). We
demonstrate that the passage of strong pulses of Gravitational Waves through
the magnetic neutral sheets accelerates electrons to very high energies.
Superposition of many such short lived accelerators, embedded inside a
turbulent plasma, may be the source for the observed impulsive short lived
bursts. We conclude that in several astrophysical events, gravitational pulses
may accelerate the tail of the ambient plasma to very high energies and become
the driver for many types of astrophysical bursts.Comment: 13 pages, 8 figures, accepted to The Astrophysical Journa
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