2,024 research outputs found
Binding between endohedral Na atoms in Si clathrate I; a first principles study
We investigate the binding nature of the endohedral sodium atoms with the
ensity functional theory methods, presuming that the clathrate I consists of a
sheaf of one-dimensional connections of Na@Si cages interleaved in three
perpendicular directions. Each sodium atom loses 30% of the 3s charge to
the frame, forming an ionic bond with the cage atoms; the rest of the electron
contributes to the covalent bond between the nearest Na atoms. The presumption
is proved to be valid; the configuration of the two Na atoms in the nearest
Si cages is more stable by 0.189 eV than that in the Si and
Si cages. The energy of the beads of the two distorted Na atoms is more
stable by 0.104 eV than that of the two infinitely separated Na atoms. The
covalent bond explains both the preferential occupancies in the Si cages
and the low anisotropic displacement parameters of the endohedral atoms in the
Si cages in the [100] directions of the clathrate I.Comment: First page: Affiliation added to PDF and PS versio
Tensile Strain/Transverse Compressive Stress Effects in Bronze Processed Nb-Matrix Nb3Sn Wires
The Slow X-Ray Expansion of the Northwestern Rim of the Supernova Remnant RX J0852.0-4622
The detection of radioactive decay line of 44Ti provides a unique evidence
that the gamma-ray source is a young (< 1,000 yr) supernova remnant because of
its short lifetime of about 90 yr. Only two Galactic remnants, Cassiopeia A and
RX J0852.0-4622, are hitherto reported to be the 44Ti line emitter, although
the detection from the latter has been debated. Here we report on an expansion
measurement of the northwestern rim of RX J0852.0-4622 obtained with X-ray
observations separated by 6.5 yr. The expansion rate is derived to be
0.023+/-0.006% that is about five times lower than those of young historical
remnants. Such a slow expansion suggests that RX J0852.0-4622 is not a young
remnant as has been expected. We estimate the age of 1,700-4,300 yr of this
remnant depending on its evolutionary stage. Assuming a high shock speed of
about 3000 km/sec, which is suggested by the detection of non-thermal X-ray
radiation, the distance of about 750 pc to this remnant is also derived.Comment: 12 pages, 3 figures, Accepted for publication in the Astrophysical
Journal Letter
Infrared/optical - X-ray simultaneous observations of X-ray flares in GRB 071112C and GRB 080506
We investigate the origin of short X-ray flares which are occasionally
observed in early stages of afterglows of gamma-ray bursts (GRBs). We observed
two events, GRB 071112C and GRB 080506, before the start of X-ray flares in the
optical and near-infrared (NIR) bands with the 1.5-m Kanata telescope. In
conjunction with published X-ray and optical data, we analyzed densely sampled
light curves of the early afterglows and spectral energy distributions (SEDs)
in the NIR-X-ray ranges. We found that the SEDs had a break between the optical
and X-ray bands in the normal decay phases of both GRBs regardless of the model
for the correction of the interstellar extinction in host galaxies of GRBs. In
the X-ray flares, X-ray flux increased by 3 and 15 times in the case of GRB
071112C and 080506, respectively, and the X-ray spectra became harder than
those in the normal decay phases. No significant variation in the optical-NIR
range was detected together with the X-ray flares. These results suggest that
the X-ray flares were associated with either late internal shocks or external
shocks from two-component jets.Comment: 10 pages, 5 figures, accepted to Astronomy and Astrophysic
Mechanisms Underlying Robustness and Tunability in a Plant Immune Signaling Network
SummaryThe plant immune signaling network needs to be robust against attack from fast-evolving pathogens and tunable to optimize immune responses. We investigated the basis of robustness and tunability in the signaling network controlling pattern-triggered immunity (PTI) in Arabidopsis. A dynamic network model containing four major signaling sectors, the jasmonate, ethylene, phytoalexin-deficient 4, and salicylate sectors, which together govern up to 80% of the PTI levels, was built using data for dynamic sector activities and PTI levels under exhaustive combinatorial sector perturbations. Our regularized multiple regression model had a high level of predictive power and captured known and unexpected signal flows in the network. The sole inhibitory sector in the model, the ethylene sector, contributed centrally to network robustness via its inhibition of the jasmonate sector. The model’s multiple input sites linked specific signal input patterns varying in strength and timing to different network response patterns, indicating a mechanism enabling tunability
Grain Size and Texture of Cu2ZnSnS4 Thin Films Synthesized by Cosputtering Binary Sulfides and Annealing: Effects of Processing Conditions and Sodium
We investigate the synthesis of kesterite Cu2ZnSnS4 (CZTS) polycrystalline
thin films using cosputtering from binary sulfide targets followed by annealing
in sulfur vapor at 500 {\deg}C to 650 {\deg}C. The films are the kesterite CZTS
phase as indicated by x-ray diffraction, Raman scattering, and optical
absorption measurements. The films exhibit (112) fiber texture and preferred
low-angle and Sigma3 grain boundary populations which have been demonstrated to
reduce recombination in Cu(In,Ga)Se2 and CdTe films. The grain growth kinetics
are investigated as functions of temperature and the addition of Na.
Significantly, lateral grain sizes above 1 um are demonstrated for samples
grown on Na-free glass,demonstrating the feasibility for CZTS growth on
substrates other than soda lime glass
Simulating Cherenkov Telescope Array observation of RX J1713.7-3946
We perform simulations of Cherenkov Telescope Array (CTA) observations of a
young supernova remnant RX J1713.7-3946. This target is not only one of the
brightest sources ever discovered in very high-energy gamma rays but also well
observed in other wavebands. In X-rays, the emission is dominated by
synchrotron radiation, which links directly to the existence of high-energy
electrons. Radio observations of CO and HI gas have revealed a highly
inhomogeneous medium surrounding the SNR, such as clumpy molecular clouds.
Therefore gamma rays from hadronic interactions are naturally expected.
However, the spectrum in GeV energy range measured by Fermi/LAT indicates more
typical of leptonic emission from accelerated electrons. Despite lots of
multi-wavelength information, the competing interpretations have led to much
uncertainty in the quest of unraveling the true origin of the gamma-ray
emission from RX~J1713.7--3946. CTA will achieve highest performance ever in
sensitivity, angular resolution, and energy resolution. We estimate CTA
capability to examine the emission mechanisms of the gamma rays through
simulated spatial distribution, spectra, and their time variation.Comment: 8 pages, 3 figures. In Proceedings of the 34th International Cosmic
Ray Conference (ICRC2015), The Hague, The Netherlands. All CTA contributions
at arXiv:1508.0589
Design Study of CANGAROO-III, Stereoscopic Imaging Atmospheric Cherenkov Telescopes for sub-TeV Gamma-ray
CANGAROO-III is an Imaging Atmospheric Cherenkov Telescope (IACT) array of
four 10 m telescopes for very high energy (sub-TeV) gamma-ray astronomy. A
design study of the CANGAROO-III telescope system was carried out using the
Monte Carlo technique in order to optimize the pixel size and the telescope
spacing. Studies were also made of observations at low elevation angles.Comment: 17pages, 13 figures, elsart, to appear in Astro. Part. Phy
Gamma-ray spectrum of RX J1713.7-3946 in the Fermi era and future detection of neutrinos
The recently launched satellite, Fermi Gamma-ray Space Telescope, is expected
to find out if cosmic-ray (CR) protons are generated from supernova remnants
(SNRs), especially RX J1713.7-3946, by observing the GeV-to-TeV gamma-rays. The
GeV emission is thought to be bright if the TeV emission is hadronic, i.e., of
proton origin, while dim if leptonic. We reexamine the above view using a
simple theoretical model of nonlinear acceleration of particles to calculate
the gamma-ray spectrum of Galactic young SNRs. If the nonlinear effects of CR
acceleration are considered, it may be impossible to distinguish the evidence
of proton acceleration from leptonic in the gamma-ray spectrum of Galactic
young SNRs like RX J1713.7-3946. On the other hand, future km^3-class neutrino
observations will likely find a clear evidence of the proton acceleration
there.Comment: 6 pages, 5 figures, A&A in pres
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