16,956 research outputs found
Magnetic reconnection at the termination shock in a striped pulsar wind
Most of the rotational luminosity of a pulsar is carried away by a
relativistic magnetised wind in which the matter energy flux is negligible
compared to the Poynting flux. Near the equatorial plane of an obliquely
rotating pulsar magnetosphere, the magnetic field reverses polarity with the
pulsar period, forming a wind with oppositely directed field lines. This
structure is called a striped wind; dissipation of alternating fields in the
striped wind is the object of our study.
The aim of this paper is to study the conditions required for magnetic energy
release at the termination shock of the striped pulsar wind. Magnetic
reconnection is considered via analytical methods and 1D relativistic PIC
simulations.
An analytical condition on the upstream parameters for partial and full
magnetic reconnection is derived from the conservation laws of energy, momentum
and particle number density across the relativistic shock. Furthermore, by
using a 1D relativistic PIC code, we study in detail the reconnection process
at the termination shock.
We found a very simple criterion for dissipation of alternating fields at the
termination shock, depending on the upstream parameters of the flow. 1D
relativistic PIC simulations are in agreement with our criterion.
Thus, alternating magnetic fields annihilate easily at relativistic highly
magnetised shocks.Comment: Accepted by A&
The `bare' strange stars might not be bare
It is proposed that the `bare' strange matter stars might not be bare, and
radio pulsars might be in fact `bare' strange stars. As strange matter stars
being intensely magnetized rotate, the induced unipolar electric fields would
be large enough to construct magnetospheres. This situation is very similar to
that discussed by many authors for rotating neutron stars. Also, the strange
stars with accretion crusts in binaries could act as X-ray pulsars or X-ray
bursters. There are some advantages if radio pulsars are `bare' strange stars.Comment: 11 pages, 1 Postscript figures, LaTeX, Chin. Phys. Lett. 1998,
Vol.15, Nov.12, p.93
Quasi-periodic Oscillations in the X-ray Light Curves from Relativistic Tori
We use a relativistic ray-tracing code to analyze the X-ray emission from a
pressure-supported oscillating relativistic torus around a black hole. We show
that a strong correlation exists between the {\it intrinsic} frequencies of the
torus normal modes and the {\it extrinsic} frequencies seen in the observed
light curve power spectrum. This correlation demonstrates the feasibility of
the oscillating-torus model to explain the multiple peaks seen in black hole
high-frequency quasi-periodic oscillations. Using an optically thin,
monochromatic emission model, we also determine how a relativistically
broadened emission line and the amplitude of the X-ray modulations are
dependent on the observer's inclination angle and on the torus oscillation
amplitudes. Observations of these features can provide important information
about the torus as well as the black hole.Comment: 4 pages, 3 figures, submitted to ApJ
A Methodology to Engineer and Validate Dynamic Multi-level Multi-agent Based Simulations
This article proposes a methodology to model and simulate complex systems,
based on IRM4MLS, a generic agent-based meta-model able to deal with
multi-level systems. This methodology permits the engineering of dynamic
multi-level agent-based models, to represent complex systems over several
scales and domains of interest. Its goal is to simulate a phenomenon using
dynamically the lightest representation to save computer resources without loss
of information. This methodology is based on two mechanisms: (1) the activation
or deactivation of agents representing different domain parts of the same
phenomenon and (2) the aggregation or disaggregation of agents representing the
same phenomenon at different scales.Comment: Presented at 3th International Workshop on Multi-Agent Based
Simulation, Valencia, Spain, 5th June 201
Mode stability in delta Scuti stars: linear analysis versus observations in open clusters
A comparison between linear stability analysis and observations of pulsation
modes in five delta Scuti stars, belonging to the same cluster, is presented.
The study is based on the work by Michel et al. (1999), in which such a
comparison was performed for a representative set of model solutions obtained
independently for each individual star considered. In this paper we revisit the
work by Michel et al. (1999) following, however, a new approach which consists
in the search for a single, complete, and coherent solution for all the
selected stars, in order to constrain and test the assumed physics describing
these objects. To do so, refined descriptions for the effects of rotation on
the determination of the global stellar parameters and on the adiabatic
oscillation frequency computations are used. In addition, a crude attempt is
made to study the role of rotation on the prediction of mode instabilities.The
present results are found to be comparable with those reported by Michel et al.
(1999). Within the temperature range log T_eff = 3.87-3.88 agreement between
observations and model computations of unstable modes is restricted to values
for the mixing-length parameter alpha_nl less or equal to 1.50. This indicates
that for these stars a smaller value for alpha_nl is required than suggested
from a calibrated solar model. We stress the point that the linear stability
analysis used in this work still assumes stellar models without rotation and
that further developments are required for a proper description of the
interaction between rotation and pulsation dynamics.Comment: 8 pages, 4 figures, 3 tables. (MNRAS, in press
Asteroseismology of delta Scuti stars in open clusters: Praesepe
The present paper provides a general overview of the asteroseismic potential
of delta Scuti stars in clusters, in particular focusing on convection
diagnostics. We give a summarise of the last results obtained by the authors
for the Praesepe cluster of which five delta Scuti stars are analysed. In that
work, linear analysis is confronted with observations, using refined
descriptions for the effects of rotation on the determination of the global
stellar parameters and on the adiabatic oscillation frequency computations. A
single, complete, and coherent solution for all the selected stars is found,
which lead the authors to find important restrictions to the convection
description for a certain range of effective temperatures. Furthermore, the
method used allowed to give an estimate of the global parameters of the
selected stars and constrain the cluster.Comment: 6 pages, 1 figure. Accepted for publication in Communications in
Asteroseismolog
Depolarization volume and correlation length in the homogenization of anisotropic dielectric composites
In conventional approaches to the homogenization of random particulate
composites, both the distribution and size of the component phase particles are
often inadequately taken into account. Commonly, the spatial distributions are
characterized by volume fraction alone, while the electromagnetic response of
each component particle is represented as a vanishingly small depolarization
volume. The strong-permittivity-fluctuation theory (SPFT) provides an
alternative approach to homogenization wherein a comprehensive description of
distributional statistics of the component phases is accommodated. The
bilocally-approximated SPFT is presented here for the anisotropic homogenized
composite which arises from component phases comprising ellipsoidal particles.
The distribution of the component phases is characterized by a two-point
correlation function and its associated correlation length. Each component
phase particle is represented as an ellipsoidal depolarization region of
nonzero volume. The effects of depolarization volume and correlation length are
investigated through considering representative numerical examples. It is
demonstrated that both the spatial extent of the component phase particles and
their spatial distributions are important factors in estimating coherent
scattering losses of the macroscopic field.Comment: Typographical error in eqn. 16 in WRM version is corrected in arxiv
versio
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