10,069 research outputs found
Development of Ground-testable Phase Fresnel Lenses in Silicon
Diffractive/refractive optics, such as Phase Fresnel Lenses (PFL's), offer
the potential to achieve excellent imaging performance in the x-ray and
gamma-ray photon regimes. In principle, the angular resolution obtained with
these devices can be diffraction limited. Furthermore, improvements in signal
sensitivity can be achieved as virtually the entire flux incident on a lens can
be concentrated onto a small detector area. In order to verify experimentally
the imaging performance, we have fabricated PFL's in silicon using gray-scale
lithography to produce the required Fresnel profile. These devices are to be
evaluated in the recently constructed 600-meter x-ray interferometry testbed at
NASA/GSFC. Profile measurements of the Fresnel structures in fabricated PFL's
have been performed and have been used to obtain initial characterization of
the expected PFL imaging efficiencies.Comment: Presented at GammaWave05: "Focusing Telescopes in Nuclear
Astrophysics", Bonifacio, Corsica, September 2005, to be published in
Experimental Astronomy, 8 pages, 3 figure
On the high coherence of kilo-Hz Quasi-Periodic Oscillations
We have carried out a systematic study of the properties of the kilo-Hertz
quasi-periodic oscillations (QPO) observed in the X-ray emission of the neutron
star low-mass X-ray binary 4U1608-52, using archival data obtained with the
Rossi X-ray Timing Explorer. We have investigated the quality factor, Q, of the
oscillations (defined as the ratio of the frequency of the QPO peak to its full
width at half maximum). In order to minimise the effect of long-term frequency
drifts, power spectra were computed over the shortest times permitted by the
data statistics. We show that the high Q of ~200 reported by Berger et al.
(1996) for the lower frequency kilo-Hz QPO in one of their observations is by
no means exceptional, as we observe a mean Q value in excess of 150 in 14 out
of the 21 observations analysed and Q can remain above 200 for thousands of
seconds. The frequency of the QPO varies over the wide range 560--890 Hz and we
find a systematic trend for the coherence time of the QPO, estimated as tau=Q
/(pi nu), to increase with the frequency, up to a maximum level at ~ 800 Hz,
beyond which it appears to decrease, at frequencies where the QPO weakens.
There is a more complex relationship between tau and the QPO root mean squared
amplitude (RMS), in which positive and negative correlations can be found. A
higher-frequency QPO, revealed by correcting for the frequency drift of the
560-890 Hz one, has a much lower Q (~10) which does not follow the same
pattern. We discuss these results in the framework of competing QPO models and
show that those involving clumps orbiting within or above the accretion disk
are ruled out.Comment: Accepted for publication in MNRAS, 8 pages, 6 figures, 3 Table
A model of large volumetric capacitance in graphene supercapacitors based on ion clustering
Electric double layer supercapacitors are promising devices for high-power
energy storage based on the reversible absorption of ions into porous,
conducting electrodes. Graphene is a particularly good candidate for the
electrode material in supercapacitors due to its high conductivity and large
surface area. In this paper we consider supercapacitor electrodes made from a
stack of graphene sheets with randomly-inserted "spacer" molecules. We show
that the large volumetric capacitances C > 100 F/cm^3 observed experimentally
can be understood as a result of collective intercalation of ions into the
graphene stack and the accompanying nonlinear screening by graphene electrons
that renormalizes the charge of the ion clusters.Comment: 13 pages, 5 figures; additional discussion and supporting
calculations adde
We Came, We Saw, We Conferenced: Capturing and Sharing Campus Events at Georgia Southern
Your campus may be regularly hosting conferences and other events; what happens to the valuable scholarship presented over the course of the event? And do the conference organizers at your institution have an efficient and simple way to manage the submission, review, and acceptance process? At Georgia Southern University, the Zach S. Henderson Library has partnered with the Division of Continuing Education and other offices on campus to not only host 19 conferences on Digital Commons@Georgia Southern but also help the conference organizers streamline their review workflows. These successful partnerships have led to some additional, unexpected benefits, such as the opportunity to publish five new peer-reviewed journals stemming from the conferences.
Over the course of this webinar, Ashley Lowery, Digital Collections Specialist, and Debra Skinner, Coordinator of Cataloging & Metadata / Interim Department Head of Collection & Resource Services at Georgia Southern will share how they formed these valuable partnerships, how they manage multiple conference sites, and the benefits that have resulted from this work
A Fruitful Collaboration: Offering More than Faculty Profiles
The institutional repository Digital Commons@Georgia Southern launched in Fall 2013. Faculty immediately began to utilize the services including journals/conferences sites and SelectedWorks faculty profiles. The repository also sparked an unexpected success: collaboration between the Zach S. Henderson Library and the Office of Research. The Office of Research created an Expertise Search and purchased Plum Analytics, two services that integrate with SelectedWorks profiles. The Expertise Search is a tool for users to find faculty members by expertise keywords. Plum Analytics provides faculty with traditional metrics (citations) as well as altmetrics (usage, captures, mentions, and social media). Through these tools, the Henderson Library can reuse metadata and offer more than just faculty profiles. In this presentation, learn about each service, their integration with one another, and the broader significance they have to the scholarly community. We will highlight our successes and struggles, our marketing strategies, and facultyâs feedback on the systems
Bone collecting by striped hyaenas, Hyaena hyaena, in Israel
Main articleDifferences in bone collecting behaviour of three species of hyaena and porcupines are discussed.
Observations on feeding behaviour of striped hyaenas are described as well as their
habit of carrying pieces away particularly if feeding cubs at maternity dens. At one maternity
den near Arad the floor of the main cavern was littered with bones which covered an area of
40 m2. Of this 2,0 m2 was sampled and found to contain 267 bones and bone fragments from
no fewer than 57 individuals, mainly of domestic species such as camel, donkey, caprovines
and dogs.Non
A Persistent High-Energy Flux from the Heart of the Milky Way : Integral's view of the Galactic Center
The Ibis/Isgri imager on Integral detected for the first time a hard X-ray
source, IGR J17456-2901, located within 1' of Sgr A* over the energy range
20-100 keV. Here we present the results of a detailed analysis of ~7 Ms of
Integral observations of the GC. With an effective exposure of 4.7 Ms we have
obtained more stringent positional constraints on this HE source and
constructed its spectrum in the range 20-400 keV. Furthermore, by combining the
Isgri spectrum with the total X-ray spectrum corresponding to the same physical
region around SgrA* from XMM data, and collected during part of the Integral
observations, we constructed and present the first accurate wide band HE
spectrum for the central arcmins of the Galaxy. Our complete analysis of the
emission properties of IGR shows that it is faint but persistent with no
variability above 3 sigma contrary to what was alluded to in our first paper.
This result, in conjunction with the spectral characteristics of the X-ray
emission from this region, suggests that the source is most likely not
point-like but, rather, that it is a compact, yet diffuse, non-thermal emission
region. The centroid of IGR is estimated to be R.A.=17h45m42.5,
decl.=-28deg59'28'', offset by 1' from the radio position of Sgr A* and with a
positional uncertainty of 1'. Its 20-400 keV luminosity at 8 kpc is L=5.4x10^35
erg/sec. Very recently, Hess detected of a source of ~TeV g-rays also located
within 1' of Sgr A*. We present arguments in favor of an interpretation
according to which the photons detected by Integral and Hess arise from the
same compact region of diffuse emission near the central BH and that the
supernova remnant Sgr A East could play an important role as a contributor of
very HE g-rays to the overall spectrum from this region.Comment: 14 pages, 11 figures, Accepted for publication in Ap
Provenance and geochemistry of exotic clasts in conglomerates of the Oligocene Torehina Formation, Coromandel Peninsula, New Zealand
Non-marine pebble to cobble conglomerates of the lower Torehina Formation (Oligocene) crop out along western Coromandel Peninsula and overlie, with strong angular discordance, continental-margin metasedimentary rocks (Manaia Hill Group) of Mesozoic (Late Jurassic to ?Early Cretaceous) age. The conglomerates contain provenance information that identifies a pre-Oligocene depositional history obscured by the unconformable juxtaposition of these Tertiary and Mesozoic strata. Most clasts in the lower Torehina Formation are visually similar to local bedrock lithologies, including metamorphosed sandstones and argillites, but are kaolinitic and contain more detrital and authigenic chert, quartz, and potash feldspar. Local derivation of these clasts seems unlikely. By comparing geochemical ratios with those defined for continental margin sandstones, and well characterised New Zealand tectonic terranes, we interpret the majority of clasts in the lower Torehina Formation to have been derived from a dissected orogen, with mixtures of felsic and volcanogenic-derived sediment. The most likely sources are the Waipapa and Torlesse Terranes. The remaining 20â30% of the clasts in the lower Torehina Formation were originally friable, are coarse grained, and appear to be lithologically exotic relative to known metamorphosed sandstones in basement terrane sources on North Island. Some clasts contain coal laminae and particles, and all contain detrital kaolinite as lithic fragments and matrix. Such characteristics imply a non-marine to marginal-marine source containing sediment derived from strongly weathered granite or granodiorite. Mechanical fragility implies a likely proximal, easily erodible source. We propose that this group of clasts was derived from an Upper Cretaceous sedimentary cover, either part of a locally developed basin fill or part of a once regionally extensive cover on North Island. Either case defines a more widely distributed Cretaceous source than found today
Spiral Waves in Chaotic Systems
Spiral waves are investigated in chemical systems whose underlying
spatially-homogeneous dynamics is governed by a deterministic chaotic
attractor. We show how the local periodic behavior in the vicinity of a spiral
defect is transformed to chaotic dynamics far from the defect. The
transformation occurs by a type of period doubling as the distance from the
defect increases. The change in character of the dynamics is described in terms
of the phase space flow on closed curves surrounding the defect.Comment: latex file with three postscript figures to appear in Physical review
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