39,583 research outputs found
X-rays from the Eclipsing Millisecond Pulsar PSR J1740-5340 in the Globular Cluster NGC 6397
The millisecond pulsar PSR J1740-5340 in the globular cluster NGC 6397 shows
radio eclipses over ~40% of its binary orbit. A first Chandra observation
revealed indications for the X-ray flux being orbit dependent as well. In this
work we analysed five data sets of archival Chandra data taken between 2000 and
2007 in order to investigate the emission across the pulsar's binary orbit.
Utilizing archival Chandra observations of PSR J1740-5340, we have performed a
systematic timing and spectral analysis of this binary system. Using a
chi-square-test the significance for intra-binary orbital modulation is found
to be between 88.5% and 99.6%, depending on the number of phase bins used to
construct the light curve. Applying the unbiased statistical Kolmogorov-Smirnov
(KS) test did not indicate any significant intra-binary orbital modulation,
though. However, comparing the counting rates observed at different epochs a
flux variability on times scales of days to years is indicated. The possible
origin of the X-ray emission is discussed in a number of different scenarios.Comment: 5 pages, 4 figures. Accepted for publication in Astronomy and
Astrophysic
Variational analysis of self-focusing of intense ultrashort pulses in gases
By using perturbation theory we derive an expression for the electrical field
of a Gaussian laser pulse propagating in a gas medium. This expression is used
as a trial solution in a variational method to get quasianalytical solutions
for the width, intensity and self-focusing distance of ultrashort pulse. The
approximation gives an improved agreement with results of numerical simulations
for a broad range of values of the input power of the pulse than previous
analytical results available in the literature.Comment: 19 pages, 8 figure
Designing behaviourally informed policies for land stewardship: A new paradigm
This paper argues the case for a new approach to the stewardship of land resources that uses behavioural science theory to support the design and application of policies that facilitate changes in behaviour by those who develop policy and the farmers who implement it. Current approaches have: focused on legally-based expert system; and have been devised by national and international bureaucracies with little or no knowledge of how land owners and managers are motivated, and how they think, behave and operate as stewards of their natural resources. A review of current approaches from the social scientific literature is provided, with a particular focus on principles from social psychology. This is followed by an examination of how these principles can be applied to influence behaviour related to land restoration and soil conservation. Examples of the problems with traditional approaches and the evolution of new approaches with full engagement of farmers as the delivery agents are provided from within the European Union, Iceland and Scotland. In the light of these examples and emerging thinking in other parts of the world, the paper sets out the basis for a new approach based on behavioural science theory and application, reinforcing the arguments already made in the literature for a social license for farming
Modelling Structural Change in Money Demand Using a Fourier-Series Approximation
The paper develops a simple method that can be used to test for a time-varying intercept and to approximate its form. The test is solidly grounded in asymptotic theory and has good small-sample properties. The methodology is based on the fact that a Fourier approximation can capture the variation in any absolutely integrable function of time. As such, it is possible to use successive applications of the test to "back-out" the form of the time-varying intercept. We illustrate the methodology using an extended example concerning the demand for money.structural break; fourier approximations; money demand
Cumulant approach to weakly doped antiferromagnets
We present a new approach to static and dynamical properties of holes and
spins in weakly doped antiferromagnets in two dimensions. The calculations are
based on a recently introduced cumulant approach to ground--state properties of
correlated electronic systems. The present method allows to evaluate hole and
spin--wave dispersion relations by considering hole or spin excitations of the
ground state. Usually, these dispersions are found from time--dependent
correlation functions. To demonstrate the ability of the approach we first
derive the dispersion relation for the lowest single hole excitation at
half--filling. However, the main purpose of this paper is to focus on the
mutual influence of mobile holes and spin waves in the weakly doped system. It
is shown that low-energy spin excitations strongly admix to the ground--state.
The coupling of spin waves and holes leads to a strong suppression of the
staggered magnetization which can not be explained by a simple rigid--band
picture for the hole quasiparticles. Also the experimentally observed doping
dependence of the spin--wave excitation energies can be understood within our
formalism.Comment: REVTEX, 25 pages, 7 figures (EPS), to be published in Phys. Rev.
Charge correlations and optical conductivity in weakly doped antiferromagnets
We investigate the dynamical charge-charge correlation function and the
optical conductivity in weakly doped antiferromagnets using Mori-Zwanzig
projection technique. The system is described by the two-dimensional t-J model.
The arising matrix elements are evaluated within a cumulant formalism which was
recently applied to investigate magnetic properties of weakly doped
antiferromagnets. Within the present approach the ground state consists of
non-interacting hole quasiparticles. Our spectra agree well with numerical
results calculated via exact diagonalization techniques. The method we employ
enables us to explain the features present in the correlation functions. We
conclude that the charge dynamics at weak doping is governed by transitions
between excited states of spin-bag quasiparticles.Comment: 5 pages, 2 figures, to appear in Europhys. Letter
All-at-once preconditioning in PDE-constrained optimization
The optimization of functions subject to partial differential equations (PDE) plays an important role in many areas of science and industry. In this paper we introduce the basic concepts of PDE-constrained optimization and show how the all-at-once approach will lead to linear systems in saddle point form. We will discuss implementation details and different boundary conditions. We then show how these system can be solved efficiently and discuss methods and preconditioners also in the case when bound constraints for the control are introduced. Numerical results will illustrate the competitiveness of our techniques
HST Proper Motion confirms the optical identification of the nearby pulsar PSR 1929+10
We report on the proper motion measurement of the proposed optical
counterpart of the X-ray/radio pulsar PSR 1929+10. Using images obtained with
the HST/STIS (average epoch 2001.73) we computed a yearly displacement of +97
+/- 1 mas yr^{-1} in RA and +46 +/- 1 mas yr^{-1} in Dec since the epoch
(1994.52) of the original HST/FOC detection. Both the magnitude and direction
of the optical proper motion components are found to be fully consistent with
the most recent VLBA radio measurements. This result provides an unambiguous
confirmation of the pulsar optical identification. In addition, we have used
the combined STIS/FOC datasets to derive information on the pulsar spectrum,
which seems characterized by a power law component, apparently unrelated to the
X-ray emission.Comment: 12 pages, 3 figures, submitted to ApJ Letter
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