33,314 research outputs found
Emergence of Classical Orbits in Few-Cycle Above-Threshold Ionization
The time-dependent Schr\"odinger equation for atomic hydrogen in few-cycle
laser pulses is solved numerically. Introducing a positive definite quantum
distribution function in energy-position space, a straightforward comparison of
the numerical ab initio results with classical orbit theory is facilitated.
Integration over position space yields directly the photoelectron spectra so
that the various pathways contributing to a certain energy in the photoelectron
spectra can be established in an unprecedented direct and transparent way.Comment: 4 pages, 4 figures REVTeX (manuscript with higher resolution figures
available at http://www.dieterbauer.de/publist.html
ROSAT HRI Observations of the Crab Pulsar: An Improved Temperature upper limit for PSR 0531+21
ROSAT HRI observations have been used to determine an upper limit of the Crab
pulsar surface temperature from the off-pulse count rate. For a neutron star
mass of 1.4 \Mo and a radius of 10 km as well as the standard distance and
interstellar column density, the redshifted temperature upper limit is\/
K . This is the lowest temperature
upper limit obtained for the Crab pulsar so far. Slightly different values for
are computed for the various neutron star models available in the
literature, reflecting the difference in the equation of state.Comment: 5 pages, uuencoded postscript, to be published in the Proceedings of
the NATO Advanced Study Insitute on "Lives of the Neutron Stars", ed. A.
Alpar, U. Kiziloglu and J. van Paradijs ( Kluwer, Dordrecht, 1995 )
X-ray Observations of Neutron Stars and Pulsars: First Results from XMM-Newton
The X-ray Multi-Mirror Mission XMM-Newton is ESA's largest observatory so
far; it is dedicated to explore the Universe in the 0.2 - 15 keV X-ray band of
the electromagnetic spectrum. Because of its large collecting area very faint
sources not accessible before can be observed and it is therefore the long
awaited instrument to study young pulsars and neutron stars in supernova
remnants, cooling neutron stars and millisecond pulsars at X-ray energies. The
high throughput of the instruments, which all are operated simultaneously,
provide high resolution spectral, spatial and temporal information from a
source during a single observation and make XMM-Newton unique and best suited
for pulsar studies. In this article we briefly describe the instrument
capabilities useful for pulsar observations and provide information on the
timing accuracy on the relative and absolute scale. We further provide
scientific results from observations of the Crab-pulsar, PSR J1617-5055 near
RCW 103, of young neutron stars in the supernova remnants RX J0852-4622,
Puppis-A and RCW 103 including 1E161348-5055.1 which is identified to be the
second binary in a supernova remnant. In addition we report on observations of
the cooling neutron star PSR B1055-52 and on the millisecond pulsar PSR
J0030+0451 which all were observed by XMM-Newton during the first two years of
scientific operation.Comment: 23 pages, 22 figures, 4 tables. To appear in the Proceedings of the
270.WE-Heraeus Seminar on Neutron Stars, Pulsars and Supernova Remnants, Jan
21-25, Physikzentrum Bad Honnef, eds W. Becker, H. Lesch & J. Truemper.
Proceedings available as MPE-Report 278. The paper with high resolution
images can be obtained from
ftp://ftp.xray.mpe.mpg.de/people/web/Becker_Aschenbach.pd
Tropical Pacific spatial trend patterns in observed sea level: internal variability and/or anthropogenic signature?
In this study we focus on the sea level trend pattern observed by satellite altimetry in the tropical Pacific over the 1993–2009 time span (i.e. 17 yr). Our objective is to investigate whether this 17-yr-long trend pattern was different before the altimetry era, what was its spatio-temporal variability and what have been its main drivers. We try to discriminate the respective roles of the internal variability of the climate system and of external forcing factors, in particular anthropogenic emissions (greenhouse gases and aerosols). On the basis of a 2-D past sea level reconstruction over 1950–2009 (based on a combination of observations and ocean modelling) and multi-century control runs (i.e. with constant, preindustrial external forcing) from eight coupled climate models, we have investigated how the observed 17-yr sea level trend pattern evolved during the last decades and centuries, and try to estimate the characteristic time scales of its variability. For that purpose, we have computed sea level trend patterns over successive 17-yr windows (i.e. the length of the altimetry record), both for the 60-yr long reconstructed sea level and the model runs. We find that the 2-D sea level reconstruction shows spatial trend patterns similar to the one observed during the altimetry era. The pattern appears to have fluctuated with time with a characteristic time scale of the order of 25–30 yr. The same behaviour is found in multi-centennial control runs of the coupled climate models. A similar analysis is performed with 20th century coupled climate model runs with complete external forcing (i.e. solar plus volcanic variability and changes in anthropogenic forcing). Results suggest that in the tropical Pacific, sea level trend fluctuations are dominated by the internal variability of the ocean–atmosphere coupled system. While our analysis cannot rule out any influence of anthropogenic forcing, it concludes that the latter effect in that particular region is stillhardly detectable
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.
Is pulsar B0656+14 a very nearby RRAT source?
The recently discovered RRAT sources are characterized by very bright radio
bursts which, while being periodically related, occur infrequently. We find
bursts with the same characteristics for the known pulsar B0656+14. These
bursts represent pulses from the bright end of an extended smooth pulse-energy
distribution and are shown to be unlike giant pulses, giant micropulses or the
pulses of normal pulsars. The extreme peak-fluxes of the brightest of these
pulses indicates that PSR B0656+14, were it not so near, could only have been
discovered as an RRAT source. Longer observations of the RRATs may reveal that
they, like PSR B0656+14, emit weaker emission in addition to the bursts.Comment: 4 pages, 4 figures, accepted by ApJ
Time Variability of Quasars: the Structure Function Variance
Significant progress in the description of quasar variability has been
recently made by employing SDSS and POSS data. Common to most studies is a
fundamental assumption that photometric observations at two epochs for a large
number of quasars will reveal the same statistical properties as well-sampled
light curves for individual objects. We critically test this assumption using
light curves for a sample of 2,600 spectroscopically confirmed quasars
observed about 50 times on average over 8 years by the SDSS stripe 82 survey.
We find that the dependence of the mean structure function computed for
individual quasars on luminosity, rest-frame wavelength and time is
qualitatively and quantitatively similar to the behavior of the structure
function derived from two-epoch observations of a much larger sample. We also
reproduce the result that the variability properties of radio and X-ray
selected subsamples are different. However, the scatter of the variability
structure function for fixed values of luminosity, rest-frame wavelength and
time is similar to the scatter induced by the variance of these quantities in
the analyzed sample. Hence, our results suggest that, although the statistical
properties of quasar variability inferred using two-epoch data capture some
underlying physics, there is significant additional information that can be
extracted from well-sampled light curves for individual objects.Comment: Presented at the "Classification and Discovery in Large Astronomical
Surveys" meeting, Ringberg Castle, 14-17 October, 200
On Retardation Effects in Space Charge Calculations Of High Current Electron Beams
Laser-plasma accelerators are expected to deliver electron bunches with high
space charge fields. Several recent publications have addressed the impact of
space charge effects on such bunches after the extraction into vacuum.
Artifacts due to the approximation of retardation effects are addressed, which
are typically either neglected or approximated. We discuss a much more
appropriate calculation for the case of laser wakefield acceleration with
negligible retardation artifacts due to the calculation performed in the mean
rest frame. This presented calculation approach also aims at a validation of
other simulation approaches
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