9,649 research outputs found
Some basic properties of infinite dimensional Hamiltonian systems
We consider some fundamental properties of infinite dimensional Hamiltonian systems,
both linear and nonlinear. For exemple, in the case of linear systems, we prove a symplectic
version of the teorem of M. Stone. In the general case we establish conservation of energy
and the moment function for system with symmetry. (The moment function was introduced
by B. Kostant and J .M. Souriau). For infinite dimensional systems these conservation
laws are more delicate than those for finite dimensional systems because we are dealing with
partial as opposed to ordinary differential equations
Magnetic Field Limits on SGRs
We measure the period and spin-down rate for SGR 1900+14 during the
quiescient period two years before the recent interval of renewed burst
activity. We find that the spin-down rate doubled during the burst activity
which is inconsistent with both mangetic dipole driven spin down and a magnetic
field energy source for the bursts. We also show that SGRs 1900+14 and 1806-20
have braking indices of 1 which indicate that the spin-down is due to
wind torques and not magnetic dipole radiation. We further show that a
combination of dipole radiation, and wind luminosity, coupled with estimated
ages and present spin parameters, imply that the magnetic fields of SGRs
1900+14 and 1806-20 are less than the critical field of 4 G and
that the efficiency for conversion of wind luminosity to x-ray luminosity is
<2%.Comment: 5 pages, 2 figures submitted to 5th Huntsville GRB Symposium
proceeding
Is SGR 1900+14 a Magnetar?
We present RXTE observations of the soft gamma--ray repeater SGR 1900+14
taken September 4-18, 1996, nearly 2 years before the 1998 active period of the
source. The pulsar period (P) of 5.1558199 +/- 0.0000029 s and period
derivative (Pdot) of (6.0 +/- 1.0) X 10^-11 s/s measured during the 2-week
observation are consistent with the mean Pdot of (6.126 +/- 0.006) X 10^-11 s/s
over the time up to the commencement of the active period. This Pdot is less
than half that of (12.77 +/- 0.01) X 10^-11 s/s observed during and after the
active period. If magnetic dipole radiation were the primary cause of the
pulsar spindown, the implied pulsar magnetic field would exceed the critical
field of 4.4 X 10^13 G by more than an order of magnitude, and such field
estimates for this and other SGRs have been offered as evidence that the SGRs
are magnetars, in which the neutron star magnetic energy exceeds the rotational
energy. The observed doubling of Pdot, however, would suggest that the pulsar
magnetic field energy increased by more than 100% as the source entered an
active phase, which seems very hard to reconcile with models in which the SGR
bursts are powered by the release of magnetic energy. Because of this, we
suggest that the spindown of SGR pulsars is not driven by magnetic dipole
radiation, but by some other process, most likely a relativistic wind. The
Pdot, therefore, does not provide a measure of the pulsar magnetic field
strength, nor evidence for a magnetar.Comment: 14 pages, aasms4 latex, figures 1 & 2 changed, accepted by ApJ
letter
Reduction, Symmetry and Phases in Mechanics
Various holonomy phenomena are shown to be instances of the reconstruction procedure
for mechanical systems with symmetry. We systematically exploit this point of view for fixed
systems (for example with controls on the internal, or reduced, variables) and for slowly moving
systems in an adiabatic context. For the latter, we obtain the phases as the holonomy for a
connection which synthesizes the Cartan connection for moving mechanical systems with the
Hannay-Berry connection for integrable systems. This synthesis allows one to treat in a natural
way examples like the ball in the slowly rotating hoop and also non-integrable mechanical systems
Variational Multisymplectic Formulations of Nonsmooth Continuum Mechanics
This paper develops the foundations of the multisymplectic
formulation of nonsmooth continuum mechanics. It may be regarded as a PDE generalization of previous techniques that developed a variational approach to collision problems. These methods have already proved of value in
computational mechanics, particularly in the development of asynchronous integrators and efficient collision methods. The present formulation also includes solid-fluid interactions and material interfaces and, in addition, lays
the groundwork for a treatment of shocks
Astrometric observations of comets and asteroids and subsequent orbital investigations
Comets and minor planets were observed with a 155 cm reflector. Their orbital positions are presented in tabular form
Solar Orbiter: Exploring the Sun-heliosphere connection
The heliosphere represents a uniquely accessible domain of space, where
fundamental physical processes common to solar, astrophysical and laboratory
plasmas can be studied under conditions impossible to reproduce on Earth and
unfeasible to observe from astronomical distances. Solar Orbiter, the first
mission of ESA's Cosmic Vision 2015-2025 programme, will address the central
question of heliophysics: How does the Sun create and control the heliosphere?
In this paper, we present the scientific goals of the mission and provide an
overview of the mission implementation.Comment: 52 pages, 21 figures, 125 references; accepted for publication in
Solar Physic
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