1,437 research outputs found
Analysis and Simulation of the Pi of the Sky Detector Response
The Pi of the Sky project observes optical flashes of astronomical origin and other light sources variable on short timescales, down to tens of seconds. We search mainly for optical emissions of Gamma Ray Bursts, but also for variable stars, blazars, etc. Precise photometry with a very large field of view (20?×20?) requires a careful study and modelling of a point spread function (PSF), as presented in this paper
Conceptual design of elliptical cavities for intensity and position sensitive beam measurements in storage rings
Position sensitive beam monitors are indispensable for the beam diagnostics
in storage rings. Apart from their applications in the measurements of beam
parameters, they can be used in non-destructive in-ring decay studies of
radioactive ion beams as well as enhancing precision in the isochronous mass
measurement technique. In this work, we introduce a novel approach based on
cavities with elliptical cross-section, in order to compensate for existing
limitations in ion storage rings. The design is aimed primarily for future
heavy ion storage rings of the FAIR project. The conceptual design is discussed
together with simulation results.Comment: Added definition of Uv and Pdiss in the introduction section. Added
Mode numbering in table 1 and figure 1 for more clarity. Corrected one wrong
figure reference. Other minor typo correction
Advection and scavenging controls of Pa/Th in the northern NE Atlantic
Over the last 2 decades, significant advances have been made in reconstructing past rates of ocean circulation using sedimentary proxies for the dynamics of abyssal waters. In this study we combine the use of two rate proxies, sortable silt grain size, and sedimentary ²³¹Pa/²³⁰Th, measured on a depth transect of deep-sea sediment cores from the northern NE Atlantic, to investigate ocean circulation changes during the last deglacial. We find that at two deep sites, the core-top ²³¹Pa/²³⁰Th ratios reflect Holocene circulation rates, while during Heinrich Stadial 1, the deglacial ratios peaked as the sortable silt grain size decreased, reflecting a general circulation slowdown. However, the peak ²³¹Pa/²³⁰Th significantly exceeded the production ratio in both cores, indicating that ²³¹Pa/²³⁰Th was only partially controlled by ocean circulation at these sites. This is supported by a record of ²³¹Pa/²³⁰Th from an intermediate water depth site, where values also peaked during Heinrich Stadial 1, but were consistently above the production ratio over the last 24 ka, reflecting high scavenging below productive surface waters. At our study sites, we find that preserved sediment component fluxes cannot be used to distinguish between a scavenging or circulation control, although they are consistent with a circulation influence, since the core at intermediate depth with the highest ²³¹Pa/²³⁰Th recorded the lowest particle fluxes. Reconstruction of advection rate using ²³¹Pa/²³⁰Th in this region is complicated by high productivity, but the data nevertheless contain important information on past deep ocean circulation
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Synchronous Deglacial Overturning and Water Mass Source Changes
Understanding changes in ocean circulation during the last deglaciation is crucial to unraveling the dynamics of glacial-interglacial and millennial climate shifts. We used neodymium isotope measurements on postdepositional iron-manganese oxide coatings precipitated on planktonic foraminifera to reconstruct changes in the bottom water source of the deep western North Atlantic at the Bermuda Rise. Comparison of our deep water source record with overturning strength proxies shows that both the deep water mass source and the overturning rate shifted rapidly and synchronously during the last deglacial transition. In contrast, any freshwater perturbation caused by Heinrich event 1 could have only affected shallow overturning. These findings show how changes in upper-ocean overturning associated with millennial-scale events differ from those associated with whole-ocean deglacial climate events
Agent-based interoperability for e-government
The provision of valuable e-government services depends upon the capacity to integrate the disperse provision of services by the public administration and thus upon the availability of interoperability platforms. These platforms are commonly built according to the principles of service oriented architectures, which raise the question of how to dynamically orchestrate services while preserving information security. Recently, it was presented an e-government interoperability model that preserves privacy during the dynamic orchestration of services. In this paper we present a prototype that implements that model using software agents. The model and the prototype are briefly described; an illustrative use case is presented; and the advantages of using software agents to implement the model are discussed. © Springer International Publishing Switzerland 2013
Abyssal origin for the early Holocene pulse of unradiogenic neodymium isotopes in Atlantic seawater
The neodymium isotopic composition of authigenic phases of deep-sea sediment cores can be interpreted as reflecting past changes in water-mass mixing proportions if end-member water-mass compositions are constrained through time. Here we present three new records spanning 2480 to 4360 m depth in the North Atlantic Ocean that show seawater Nd isotope values in the early to mid-Holocene that are more radiogenic than values from the abyssal northwest Atlantic. This finding indicates that that the end-member composition of North Atlantic Deep Water was more stable within its core than it was at abyssal depths. The spatial distribution of the unradiogenic neodymium isotope values observed in the North Atlantic suggests a bottom source, and therefore that they were unlikely to have been due to the production of intermediate-depth Labrador Sea Water. We infer that the unradiogenic authigenic Nd isotope values were most likely derived from a pulse of poorly chemically weathered detrital material that was deposited into the Labrador Sea following Laurentide ice sheet retreat in the early Holocene. This unradiogenic sediment released neodymium into the bottom waters, yielding an unradiogenic seawater signal that was advected southward at abyssal depths and attenuated as it vertically mixed upward in the water column to shallower depths. The southward dispersion of these unradiogenic seawater values traces deep-water advection. However, the exact values observed at the most abyssal sites cannot be interpreted as proportionate to the strength of deep-water production without improved constraints on end-member changes
Interacting Binaries with Eccentric Orbits. Secular Orbital Evolution Due To Conservative Mass Transfer
We investigate the secular evolution of the orbital semi-major axis and
eccentricity due to mass transfer in eccentric binaries, assuming conservation
of total system mass and orbital angular momentum. Assuming a delta function
mass transfer rate centered at periastron, we find rates of secular change of
the orbital semi-major axis and eccentricity which are linearly proportional to
the magnitude of the mass transfer rate at periastron. The rates can be
positive as well as negative, so that the semi-major axis and eccentricity can
increase as well as decrease in time. Adopting a delta-function mass-transfer
rate of 10^{-9} M_\sun {\rm yr}^{-1} at periastron yields orbital evolution
timescales ranging from a few Myr to a Hubble time or more, depending on the
binary mass ratio and orbital eccentricity. Comparison with orbital evolution
timescales due to dissipative tides furthermore shows that tides cannot, in all
cases, circularize the orbit rapidly enough to justify the often adopted
assumption of instantaneous circularization at the onset of mass transfer. The
formalism presented can be incorporated in binary evolution and population
synthesis codes to create a self-consistent treatment of mass transfer in
eccentric binaries.Comment: 16 pages, 8 figures, Accepted by The Astrophysical Journa
A new methodology to simulate subglacial deformation of water-saturated granular material
The dynamics of glaciers are to a large degree governed by processes
operating at the ice–bed interface, and one of the primary
mechanisms of glacier flow over soft unconsolidated sediments is
subglacial deformation. However, it has proven difficult to
constrain the mechanical response of subglacial sediment to the
shear stress of an overriding glacier. In this study, we present
a new methodology designed to simulate subglacial deformation using
a coupled numerical model for computational experiments on
grain-fluid mixtures. The granular phase is simulated on a per-grain
basis by the discrete element method. The pore water is modeled as
a compressible Newtonian fluid without inertia. The numerical
approach allows close monitoring of the internal behavior under
a range of conditions.
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Our computational experiments support the findings of previous studies
where the rheology of a slowly deforming water-saturated granular bed in the
steady state generally conforms to the rate-independent plastic rheology.
Before this so-called critical state, deformation is in many cases accompanied
by volumetric changes as grain rearrangement in active shear zones changes the
local porosity. For previously consolidated beds porosity
increases can cause local pore-pressure decline, dependent on till
permeability and shear rate. We observe that the pore-water pressure reduction
strengthens inter-granular contacts, which results in increased shear strength
of the granular material. In contrast, weakening takes place when shear
deformation causes consolidation of dilated sediments or during rapid fabric
development. Both processes of strengthening and weakening depend inversely on
the sediment permeability and are transient phenomena tied to the porosity
changes during the early stages of shear.
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We find that the transient strengthening and weakening in turn influences the
distribution of shear strain in the granular bed. Dilatant strengthening has
the ability to distribute strain during early deformation to large depths, if
sediment dilatancy causes the water pressure at the ice–bed interface to
decline. Oppositely, if the ice–bed interface is hydrologically stable the
strengthening process is minimal and instead causes shallow deformation. The
depth of deformation in subglacial beds thus seems to be governed by not only
local grain and pore-water feedbacks but also larger-scale hydrological
properties at the ice base
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