15,044 research outputs found
Superburst oscillations: ocean and crustal modes excited by Carbon-triggered Type I X-ray bursts
Accreting neutron stars (NS) can exhibit high frequency modulations in their
lightcurves during thermonuclear X-ray bursts, known as burst oscillations. The
frequencies can be offset from the spin frequency of the NS by several Hz, and
can drift by 1-3 Hz. One possible explanation is a mode in the bursting ocean,
the frequency of which would decrease (in the rotating frame) as the burst
cools, hence explaining the drifts. Most burst oscillations have been observed
during H/He triggered bursts, however there has been one observation of
oscillations during a superburst; hours' long Type I X-ray bursts caused by
unstable carbon burning deeper in the ocean. This paper calculates the
frequency evolution of an oceanic r-mode during a superburst. The rotating
frame frequency varies during the burst from 4-14 Hz, and is sensitive to the
background parameters, in particular the temperature of the ocean and ignition
depth. This calculation is compared to the superburst oscillations observed on
4U-1636-536. The predicted mode frequencies ( 10 Hz) would require a spin
frequency of 592 Hz to match observations; 6 Hz higher than the spin
inferred from an oceanic r-mode model for the H/He triggered burst
oscillations. This model also over-predicts the frequency drift during the
superburst by 90 %.Comment: Accepted for publication in MNRA
Geographical effects on cascading breakdowns of scale-free networks
Cascading breakdowns of real networks are severe accidents in recent years,
such as the blackouts of the power transportation networks in North America. In
this paper, we study the effects of geographical structure on the cascading
phenomena of load-carried scale-free networks, find that more geographically
constrained networks tend to have larger cascading breakdowns. Explanations by
the effects of circles and large betweenness of small degree nodes are
provided
Racial and Ethnic Differences in Falls Among Older Adults: a Systematic Review and Meta-analysis
The aim of this systematic review and meta-analysis was to determine whether differences in reported fall rates exist between different ethnic groups. Searches were carried out on four databases: Medline, Cumulative Index to Nursing and Allied Health Literature (CINAHL), Scopus, and Web of Science. Only English language studies with community-dwelling participants aged 60 + years were included. Studies also needed to compare fall prevalence for at least two or more ethnic groups. Two reviewers independently screened all articles and evaluated study quality. Twenty-three articles were included for systematic review, and meta-analyses were carried out on the 16 retrospective studies that reported falls in the previous 12 months. The Asian group demonstrated significantly lower fall prevalence than all other ethnic groups at 13.89% (10.87, 16.91). The Hispanic group had a fall prevalence of 18.54% (12.95, 24.13), closely followed by the Black group at 18.60% (13.27, 23.93). The White group had the highest prevalence at 23.77% (18.66, 28.88). Some studies provided adjusted estimates of effect statistics for the odds/risk of falls, which showed that differences still existed between some ethnic groups even after adjusting for other risk factors. Overall, differences in fall prevalence do appear to exist between different ethnic groups, although the reasons for these differences currently remain undetermined and require further investigation. These findings highlight the need to provide more ethnically tailored responses to public health challenges, which could potentially increase the adherence to prevention interventions, and allow for a more targeted use of resources
A tool for filtering information in complex systems
We introduce a technique to filter out complex data-sets by extracting a
subgraph of representative links. Such a filtering can be tuned up to any
desired level by controlling the genus of the resulting graph. We show that
this technique is especially suitable for correlation based graphs giving
filtered graphs which preserve the hierarchical organization of the minimum
spanning tree but containing a larger amount of information in their internal
structure. In particular in the case of planar filtered graphs (genus equal to
0) triangular loops and 4 element cliques are formed. The application of this
filtering procedure to 100 stocks in the USA equity markets shows that such
loops and cliques have important and significant relations with the market
structure and properties.Comment: 8 pages, 3 figures, 4 table
Thermodynamics of spin systems on small-world hypergraphs
We study the thermodynamic properties of spin systems on small-world
hypergraphs, obtained by superimposing sparse Poisson random graphs with p-spin
interactions onto a one-dimensional Ising chain with nearest-neighbor
interactions. We use replica-symmetric transfer-matrix techniques to derive a
set of fixed-point equations describing the relevant order parameters and free
energy, and solve them employing population dynamics. In the special case where
the number of connections per site is of the order of the system size we are
able to solve the model analytically. In the more general case where the number
of connections is finite we determine the static and dynamic
ferromagnetic-paramagnetic transitions using population dynamics. The results
are tested against Monte-Carlo simulations.Comment: 14 pages, 7 figures; Added 2 figures. Extended result
Spectroscopic biomedical imaging with the Medipix2 detector
This study confirms that the Medipix2 x-ray detector enables spectroscopic bio-medical plain radiography. We show that the detector has the potential to provide new, useful information beyond the limited spectroscopic information of modern dual-energy computed tomography (CT) scanners. Full spectroscopic 3D-imaging is likely to be the next major technological advance in computed tomography, moving the modality towards molecular imaging applications. This paper focuses on the enabling technology which allows spectroscopic data collection and why this information is useful. In this preliminary study we acquired the first spectroscopic images of human tissue and other biological samples obtained using the Medipix2 detector. The images presented here include the clear resolution of the 1.4mm long distal phalanx of a 20-week-old miscarried foetus, showing clear energy-dependent variations. The opportunities for further research using the forthcoming Medipix3 detector are discussed and a prototype spectroscopic CT scanner (MARS, Medipix All Resolution System) is briefly describe
Instability of scale-free networks under node-breaking avalanches
The instability introduced in a large scale-free network by the triggering of
node-breaking avalanches is analyzed using the fiber-bundle model as conceptual
framework. We found, by measuring the size of the giant component, the
avalanche size distribution and other quantities, the existence of an abrupt
transition. This test of strength for complex networks like Internet is more
stringent than others recently considered like the random removal of nodes,
analyzed within the framework of percolation theory. Finally, we discuss the
possible implications of our results and their relevance in forecasting
cascading failures in scale-free networks.Comment: 4 pages, 4 figures, final version to be published in Europhys. Let
The onset of low Prandtl number thermal convection in thin spherical shells
This study considers the onset of stress-free Boussinesq thermal convection
in rotating spherical shells with aspect ratio ( and
being the inner and outer radius), Prandtl numbers , and Taylor numbers . We are
particularly interested in the form of the convective cell pattern that
develops, and in its time scales, since this may have observational
consequences. For a fixed and by decreasing from
0.1 to a transition between spiralling columnar (SC) and
equatorially-attached (EA) modes, and a transition between EA and equatorially
antisymmetric or symmetric polar (AP/SP) weakly multicellular modes are found.
The latter modes are preferred at very low . Surprisingly, for the unicellular polar modes become also preferred at
moderate because two new transition curves between EA and
AP/SP and between AP/SP and SC modes are born at a triple-point bifurcation.
The dependence on and of the transitions is studied to
estimate the type of modes, and their critical parameters, preferred at
different stellar regimes.Comment: Accepted for publication in Physical Review Fluids. Contains 17
pages, 8 figures and 3 tables. Added brief erratum correcting values used for
estimates of neutron star ocean viscosit
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