485 research outputs found
Measuring the Reduced Shear
Neglecting the second order corrections in weak lensing measurements can lead
to a few percent uncertainties on cosmic shears, and becomes more important for
cluster lensing mass reconstructions. Existing methods which claim to measure
the reduced shears are not necessarily accurate to the second order when a
point spread function (PSF) is present. We show that the method of Zhang (2008)
exactly measures the reduced shears at the second order level in the presence
of PSF. A simple theorem is provided for further confirming our calculation,
and for judging the accuracy of any shear measurement method at the second
order based on its properties at the first order. The method of Zhang (2008) is
well defined mathematically. It does not require assumptions on the
morphologies of galaxies and the PSF. To reach a sub-percent level accuracy,
the CCD pixel size is required to be not larger than 1/3 of the Full Width at
Half Maximum (FWHM) of the PSF. Using a large ensemble (> 10^7) of mock
galaxies of unrestricted morphologies, we find that contaminations to the shear
signals from the noise of background photons can be removed in a well defined
way because they are not correlated with the source shapes. The residual shear
measurement errors due to background noise are consistent with zero at the
sub-percent level even when the amplitude of such noise reaches about 1/10 of
the source flux within the half-light radius of the source. This limit can in
principle be extended further with a larger galaxy ensemble in our simulations.
On the other hand, the source Poisson noise remains to be a cause of systematic
errors. For a sub-percent level accuracy, our method requires the amplitude of
the source Poisson noise to be less than 1/80 ~ 1/100 of the source flux within
the half-light radius of the source, corresponding to collecting roughly 10^4
source photons.Comment: 18 pages, 3 figures, 4 tables, minor changes from the previous
versio
Statistical Survey of Type III Radio Bursts at Long Wavelengths Observed by the Solar TErrestrial RElations Observatory (STEREO)/Waves Instruments: Radio Flux Density Variations with Frequency
We have performed a statistical study of Type III radio bursts observed
by Solar TErrestrial RElations Observatory (STEREO)/Waves between May 2007 and
February 2013. We have investigated the flux density between kHz and
MHz. Both high- and low-frequency cutoffs have been observed in of
events suggesting an important role of propagation. As already reported by
previous authors, we observed that the maximum flux density occurs at MHz on
both spacecraft. We have developed a simplified analytical model of the flux
density as a function of radial distance and compared it to the STEREO/Waves
data.Comment: published in Solar Physic
Complete next-to-leading order calculation for pion production in nucleon-nucleon collisions at threshold
Based on a counting scheme that explicitly takes into account the large
momentum sqrt(M m_pi) characteristic for pion production in nucleon-nucleon
collisions we calculate all diagrams for the reaction NN --> NN pi at threshold
up to next-to-leading order. At this order there are no free parameters and the
size of the next-to-leading order contributions is in line with the expectation
from power counting. The sum of loop corrections at that order vanishes for the
process pp --> pp pi^0 at threshold. The total contribution at next-to-leading
order from loop diagrams that include the delta degree of freedom vanishes at
threshold in both reaction channels pp --> pp pi^0, pn pi^+.Comment: 9 pages, 4 figure
Self-assembly of a sulphur-terminated graphene nanoribbon within a single-walled carbon nanotube
The ability to tune the properties of graphene nanoribbons (GNRs) through modification of the nanoribbon’s width and edge structure1,2,3 widens the potential applications of graphene in electronic devices4,5,6. Although assembly of GNRs has been recently possible, current methods suffer from limited control of their atomic structure7,8,9,10,11,12,13, or require the careful organization of precursors on atomically flat surfaces under ultra-high vacuum conditions14. Here we demonstrate that a GNR can self-assemble from a random mixture of molecular precursors within a single-walled carbon nanotube, which ensures propagation of the nanoribbon in one dimension and determines its width. The sulphur-terminated dangling bonds of the GNR make these otherwise unstable nanoribbons thermodynamically viable over other forms of carbon. Electron microscopy reveals elliptical distortion of the nanotube, as well as helical twist and screw-like motion of the nanoribbon. These effects suggest novel ways of controlling the properties of these nanomaterials, such as the electronic band gap and the concentration of charge carriers.This work was supported by by the DFG (German Research Foundation) and the
Ministry of Science, Research and the Arts (MWK) of Baden-Württemberg in the frame
of the SALVE (Sub Angstrom Low-Voltage Electron microscopy project) and by the DFG
within the research project SFB 569 (U.K. and J.B.); the EPSRC (Career Acceleration
Fellowship), NanoTP COST action and High Performance Computing (HPC) facility at
the University of Nottingham (E.B.); the EPSRC, ESF and the Royal Society (A.N.K. and
A.C.); the FP7 Marie Curie Fellowship (M.C.G-L.); and the Nottingham Nanoscience
and Nanotechnology Centre (access to Raman spectrometer).S
Charge-Symmetry Breaking and the Two-Pion-Exchange Two-Nucleon Interaction
Charge-symmetry breaking in the nucleon-nucleon force is investigated within
an effective field theory, using a classification of isospin-violating
interactions based on power-counting arguments. The relevant
charge-symmetry-breaking interactions corresponding to the first two orders in
the power counting are discussed, including their effects on the 3He-3H
binding-energy difference. The static charge-symmetry-breaking potential linear
in the nucleon-mass difference is constructed using chiral perturbation theory.
Explicit formulae in momentum and configuration spaces are presented. The
present work completes previously obtained results.Comment: 15 pages, 2 figure
Parity Mixed Doublets in A = 36 Nuclei
The -circular polarizations () and asymmetries
() of the parity forbidden M1 + E2 -decays: MeV) and MeV)
MeV) are investigated theoretically. We use the recently proposed
Warburton-Becker-Brown shell-model interaction. For the weak forces we discuss
comparatively different weak interaction models based on different assumptions
for evaluating the weak meson-hadron coupling constants. The results determine
a range of values from which we find the most probable values:
= for and = for .Comment: RevTeX, 17 pages; to appear in Phys. Rev.
Parity nonconserving cold neutron-parahydrogen interactions
Three pion dominated observables of the parity nonconserving interactions
between the cold neutrons and parahydrogen are calculated. The transversely
polarized neutron spin rotation, unpolarized neutron longitudinal polarization,
and photon-asymmetry of the radiative polarized neutron capture are considered.
For the numerical evaluation of the observables, the strong interactions are
taken into account by the Reid93 potential and the parity nonconserving
interactions by the DDH model along with the two-pion exchange.Comment: 17 pages, 2 figure
3D evolution of a filament disappearance event observed by STEREO
A filament disappearance event was observed on 22 May 2008 during our recent
campaign JOP 178. The filament, situated in the southern hemisphere, showed
sinistral chirality consistent with the hemispheric rule. The event was well
observed by several observatories in particular by THEMIS. One day before the
disappearance, H observations showed up and down flows in adjacent
locations along the filament, which suggest plasma motions along twisted flux
rope. THEMIS and GONG observations show shearing photospheric motions leading
to magnetic flux canceling around barbs. STEREO A, B spacecraft with separation
angle 52.4 degrees, showed quite different views of this untwisting flux rope
in He II 304 \AA\ images. Here, we reconstruct the 3D geometry of the filament
during its eruption phase using STEREO EUV He II 304 \AA\ images and find that
the filament was highly inclined to the solar normal. The He II 304 \AA\ movies
show individual threads, which oscillate and rise to an altitude of about 120
Mm with apparent velocities of about 100 km s, during the rapid
evolution phase. Finally, as the flux rope expands into the corona, the
filament disappears by becoming optically thin to undetectable levels. No CME
was detected by STEREO, only a faint CME was recorded by LASCO at the beginning
of the disappearance phase at 02:00 UT, which could be due to partial filament
eruption. Further, STEREO Fe XII 195 \AA\ images showed bright loops beneath
the filament prior to the disappearance phase, suggesting magnetic reconnection
below the flux rope
- …