25,716 research outputs found
Evaluation of a spacecraft nitrogen generator
A research and development program was successfully completed towards the development of a method of generating nitrogen for cabin leakage makeup aboard space vehicles. The nitrogen generation concept used liquid hydrazine as the stored form of nitrogen. This reduced tankage and expendables weight associated with high pressure gaseous and cryogenic liquid nitrogen storage. The hydrazine was catalytically dissociated to yield a mixture of nitrogen and hydrogen. The latter was separated to provide the makeup nitrogen. The hydrogen will be used in the reduction of metabolic carbon dioxide
Discovery of a Spin-Down State Change in the LMC Pulsar B0540-69
We report the discovery of a large, sudden, and persistent increase in the
spin-down rate of B0540-69, a young pulsar in the Large Magellanic Cloud, using
observations from the Swift and RXTE satellites. The relative increase in the
spin-down rate of 36% is unprecedented for B0540-69. No accompanying change in
the spin rate is seen, and no change is seen in the pulsed X-ray emission from
B0540-69 following the change in the spin-down rate. Such large relative
changes in the spin-down rate are seen in the recently discovered class of
'intermittent pulsars', and we compare the properties of B0540-69 to such
pulsars. We consider possible changes in the magnetosphere of the pulsar that
could cause such a large change in the spin-down rate.Comment: 6 pages, 2 figures, accepted for publication in ApJ Letter
Gratings photowritten in ion-exchanged glass channel waveguides
Gratings are photowritten in ion-exchanged glass channel waveguides. The transmission of these waveguides shows a rejection dip of almost 20dB. The polarisation dependence of these waveguide gratings is measured and discussed
Numerical Studies of the Compressible Ising Spin Glass
We study a two-dimensional compressible Ising spin glass at constant volume.
The spin interactions are coupled to the distance between neighboring particles
in the Edwards-Anderson model with +/- J interactions. We find that the energy
of a given spin configuration is shifted from its incompressible value, E_0, by
an amount quadratic in E_0 and proportional to the coupling strength. We then
construct a simple model expressed only in terms of spin variables that
predicts the existence of a critical value of the coupling above which the
spin-glass transition disappears.Comment: REVTeX, 4 pages, 4 figures. Submitted to Phys. Rev. Let
Grating formation in BGG31 glass by UV exposure
A three-dimensional index variation grating in bulk BGG31 glass written using neither hydrogen loading nor germanium doping is demonstrated. This material is useful for fabricating ion-exchanged waveguides, and its photosensitivity to ultraviolet (UV) radiation at 248nm has not been previously explored. Intensity measurements of the Bragg diffracted spots indicated a maximum index variation (Delta n) of similar to 4 x 10(-5)
Andreev Probe of Persistent Current States in Superconducting Quantum Circuits
Using the extraordinary sensitivity of Andreev interferometers to the
superconducting phase difference associated with currents, we measure the
persistent current quantum states in superconducting loops interrupted by
Josephson junctions. Straightforward electrical resistance measurements of the
interferometers give continuous read-out of the states, allowing us to
construct the energy spectrum of the quantum circuit. The probe is estimated to
be more precise and faster than previous methods, and can measure the local
phase difference in a wide range of superconducting circuits.Comment: Changes made in light of referees comments; to appear in PR
Hypoxia and hyperglycaemia determine why some endometrial tumours fail to respond to metformin
High expression of Ki67, a proliferation marker, is associated with reduced endometrial cancer-specific survival. Pre-surgical metformin reduces tumour Ki-67 expression in some women with endometrial cancer. Metformin's anti-cancer activity may relate to effects on cellular energy metabolism. Since tumour hypoxia and glucose availability are major cellular redox determinants, we evaluated their role in endometrial cancer response to metformin. Endometrial cancer biopsies from women treated with pre-surgical metformin were tested for the hypoxia markers, HIF-1α and CA-9. Endometrial cancer cell lines were treated with metformin in variable glucose concentrations in normoxia or hypoxia and cell viability, mitochondrial biogenesis, function and energy metabolism were assessed. In women treated with metformin (nâ=â28), Ki-67 response was lower in hypoxic tumours. Metformin showed minimal cytostatic effects towards Ishikawa and HEC1A cells in conventional medium (25âmM glucose). In low glucose (5.5âmM), a dose-dependent cytostatic effect was observed in normoxia but attenuated in hypoxia. Tumours treated with metformin showed increased mitochondrial mass (nâ=â25), while in cultured cells metformin decreased mitochondrial function. Metformin targets mitochondrial respiration, however, in hypoxic, high glucose conditions, there was a switch to glycolytic metabolism and decreased metformin response. Understanding the metabolic adaptations of endometrial tumours may identify patients likely to derive clinical benefit from metformin
Short gamma-ray bursts within 200 Mpc
We present a systematic search for short-duration gamma-ray bursts (GRBs) in the local Universe based on 14âyr of observations with the Neil Gehrels Swift Observatory. We cross-correlate the GRB positions with the GLADE catalogue of nearby galaxies, and find no event at a distance âČ100 Mpc and four plausible candidates in the range 100âMpc âČ DââČ 200âMpc. Although affected by low statistics, this number is higher than the one expected for chance alignments to random galaxies, and possibly suggests a physical association between these bursts and nearby galaxies. By assuming a local origin, we use these events to constrain the range of properties for X-ray counterparts of neutron star mergers. Optical upper limits place tight constraints on the onset of a blue kilonova, and imply either low masses (â âČ10â3Mââ ) of lanthanide-poor ejecta or unfavorable orientations (Ξ_(obs) âł 30 deg). Finally, we derive that the all-sky rate of detectable short GRBs within 200 Mpc is 1.3^(+1.7)_(â0.8) yrâ»Âč (68 perâcent confidence interval), and discuss the implications for the GRB outflow structure. If these candidates are instead of cosmological origin, we set a upper limit of âČ2.0 yrâ»Âč (90 perâcent confidence interval) to the rate of nearby events detectable with operating gamma-ray observatories, such as Swift and Fermi
Structure formation in a colliding flow: The Herschel view of the Draco nebula
The Draco nebula is a high Galactic latitude interstellar cloud likely to
have been formed by the collision of a Galactic halo cloud entering the disk of
the Milky Way. Such conditions are ideal to study the formation of cold and
dense gas in colliding flows of warm gas. We present Herschel-SPIRE
observations that reveal the fragmented structure of the interface between the
infalling cloud and the Galactic layer. This front is characterized by a
Rayleigh-Taylor instability structure. From the determination of the typical
length of the periodic structure (2.2 pc) we estimated the gas kinematic
viscosity and the turbulence dissipation scale (0.1 pc) that is compatible with
that expected if ambipolar diffusion is the main mechanism of energy
dissipation in the WNM. The small-scale structures of the nebula are typical of
that seen in some molecular clouds. The gas density has a log-normal
distribution with an average value of cm. The size of the
structures is 0.1-0.2 pc but this estimate is limited by the resolution of the
observations. The mass ranges from 0.2 to 20 M and the distribution
of the more massive clumps follows a power law . We
identify a mass-size relation with the same exponent as that found in GMCs
() but only 15% of the mass of the cloud is in gravitationally
bound structures. We conclude that the increase of pressure in the collision is
strong enough to trigger the WNM-CNM transition caused by the interplay between
turbulence and thermal instability as self-gravity is not dominating the
dynamics.Comment: 16 pages, A&A, in pres
Quantum numbers for relative ground states of antiferromagnetic Heisenberg spin rings
We suggest a general rule for the shift quantum numbers k of the relative
ground states of antiferromagnetic Heisenberg spin rings. This rule generalizes
well-known results of Marshall, Peierls, Lieb, Schultz, and Mattis for even
rings. Our rule is confirmed by numerical investigations and rigorous proofs
for special cases, including systems with a Haldane gap. Implications for the
total spin quantum number S of relative ground states are discussed as well as
generalizations to the XXZ model.Comment: 8 pages, 2 figures, submitted to Phys. Rev. B. More information at
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