5,940 research outputs found
Strongly magnetized rotating dipole in general relativity
Electromagnetic waves arise in many area of physics. Solutions are difficult
to find in the general case. In this paper, we numerically integrate Maxwell
equations in a 3D spherical polar coordinate system. Straightforward finite
difference methods would lead to a coordinate singularity along the polar axis.
Spectral methods are better suited to deal with such artificial singularities
related to the choice of a coordinate system. When the radiating object is
rotating like for instance a star, special classes of solutions to Maxwell
equations are worthwhile to study such as quasi-stationary regimes. Moreover,
in high-energy astrophysics, strong gravitational and magnetic fields are
present especially around rotating neutron stars. In order to study such
systems, we designed an algorithm to solve the time-dependent Maxwell equations
in spherical polar coordinates including general relativity as well as quantum
electrodynamical corrections to leading order. As a diagnostic, we compute the
spindown luminosity expected from these stars and compare it to the classical
i.e. non relativistic and non quantum mechanical results. It is shown that
quantum electrodynamics leads to an irrelevant change in the spindown
luminosity even for magnetic field around the critical value of
\numprint{4.4e9}~\si{\tesla}. Therefore the braking index remains close to
its value for a point dipole in vacuum namely . The same conclusion holds
for a general-relativistic quantum electrodynamically corrected force-free
magnetosphere.Comment: Accepted for publication in A&
Effect of geodetic precession on the evolution of pulsar high-energy pulse profiles as derived with the striped-wind model
Geodetic precession has been observed directly in the double-pulsar system
PSR J0737-3039. Its rate has even been measured and agrees with predictions of
general relativity. Very recently, the double pulsar has been detected in
X-rays and gamma-rays. This fuels the hope observing geodetic precession in the
high-energy pulse profile of this system. Unfortunately, the geometric
configuration of the binary renders any detection of such an effect unlikely.
Nevertheless, this precession is probably present in other relativistic
binaries or double neutron star systems containing at least one X-ray or
gamma-ray pulsar.}{We compute the variation of the high-energy pulse profile
expected from this geodetic motion according to the striped-wind model. We
compare our results with two-pole caustic and outer gap emission patterns.}{For
a sufficient misalignment between the orbital angular momentum and the spin
angular momentum, a significant change in the pulse profile as a result of
geodetic precession is expected in the X-ray and gamma-ray energy band.}{The
essential features of the striped wind are indicated in several plots showing
the evolution of the maximum of the pulsed intensity, the separation of both
peaks, if present, and the variation in the width of each peak. We highlight
the main differences with other competing high-energy models.}{We make some
predictions about possible future detection of high-energy emission from double
neutron star systems with the highest spin precession rate. Such observations
will definitely favour some pulsed high-energy emission scenarios.Comment: Accepted for publication in A&A, typos correcte
Small-scale dynamos in simulations of stratified turbulent convection
Small-scale dynamo action is often held responsible for the generation of
quiet-Sun magnetic fields. We aim to determine the excitation conditions and
saturation level of small-scale dynamos in non-rotating turbulent convection at
low magnetic Prandtl numbers. We use high resolution direct numerical
simulations of weakly stratified turbulent convection. We find that the
critical magnetic Reynolds number for dynamo excitation increases as the
magnetic Prandtl number is decreased, which might suggest that small-scale
dynamo action is not automatically evident in bodies with small magnetic
Prandtl numbers as the Sun. As a function of the magnetic Reynolds number
(), the growth rate of the dynamo is consistent with an scaling. No evidence for a logarithmic increase of the growth rate
with is found.Comment: 6 pages, 5 figures, submitted to Astron. Nach
Helical coronal ejections and their role in the solar cycle
The standard theory of the solar cycle in terms of an alpha-Omega dynamo
hinges on a proper understanding of the nonlinear alpha effect. Boundary
conditions play a surprisingly important role in determining the magnitude of
alpha. For closed boundaries, the total magnetic helicity is conserved, and
since the alpha effect produces magnetic helicity of one sign in the large
scale field, it must simultaneously produce magnetic helicity of the opposite
sign. It is this secondary magnetic helicity that suppresses the dynamo in a
potentially catastrophic fashion. Open boundaries allow magnetic helicity to be
lost. Simulations are presented that allow an estimate of alpha in the presence
of open or closed boundaries, either with or without solar-like differential
rotation. In all cases the sign of the magnetic helicity agrees with that
observed at the solar surface (negative in the north, positive in the south),
where significant amounts of magnetic helicity can be ejected via coronal mass
ejections. It is shown that open boundaries tend to alleviate catastrophic
alpha quenching. The importance of looking at current helicity instead of
magnetic helicity is emphasized and the conceptual advantages are discussed.Comment: 8 pages, 7 figs, IAU Symp. 223, In: Multi-Wavelength Investigations
of Solar Activity. Eds: A.V. Stepanov, E.E. Benevolenskaya & A.G. Kosoviche
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