5,360 research outputs found
Cerebral Amyloid and Hypertension are Independently Associated with White Matter Lesions in Elderly.
In cognitively normal (CN) elderly individuals, white matter hyperintensities (WMH) are commonly viewed as a marker of cerebral small vessel disease (SVD). SVD is due to exposure to systemic vascular injury processes associated with highly prevalent vascular risk factors (VRFs) such as hypertension, high cholesterol, and diabetes. However, cerebral amyloid accumulation is also prevalent in this population and is associated with WMH accrual. Therefore, we examined the independent associations of amyloid burden and VRFs with WMH burden in CN elderly individuals with low to moderate vascular risk. Participants (n = 150) in the Alzheimer's Disease Neuroimaging Initiative (ADNI) received fluid attenuated inversion recovery (FLAIR) MRI at study entry. Total WMH volume was calculated from FLAIR images co-registered with structural MRI. Amyloid burden was determined by cerebrospinal fluid Aβ1-42 levels. Clinical histories of VRFs, as well as current measurements of vascular status, were recorded during a baseline clinical evaluation. We tested ridge regression models for independent associations and interactions of elevated blood pressure (BP) and amyloid to total WMH volume. We found that greater amyloid burden and a clinical history of hypertension were independently associated with greater WMH volume. In addition, elevated BP modified the association between amyloid and WMH, such that those with either current or past evidence of elevated BP had greater WMH volumes at a given burden of amyloid. These findings are consistent with the hypothesis that cerebral amyloid accumulation and VRFs are independently associated with clinically latent white matter damage represented by WMHs. The potential contribution of amyloid to WMHs should be further explored, even among elderly individuals without cognitive impairment and with limited VRF exposure
Stacking Gravitational Wave Signals from Soft Gamma Repeater Bursts
Soft gamma repeaters (SGRs) have unique properties that make them intriguing
targets for gravitational wave (GW) searches. They are nearby, their burst
emission mechanism may involve neutron star crust fractures and excitation of
quasi-normal modes, and they burst repeatedly and sometimes spectacularly. A
recent LIGO search for transient GW from these sources placed upper limits on a
set of almost 200 individual SGR bursts. These limits were within the
theoretically predicted range of some models. We present a new search strategy
which builds upon the method used there by "stacking" potential GW signals from
multiple SGR bursts. We assume that variation in the time difference between
burst electromagnetic emission and burst GW emission is small relative to the
GW signal duration, and we time-align GW excess power time-frequency tilings
containing individual burst triggers to their corresponding electromagnetic
emissions. Using Monte Carlo simulations, we confirm that gains in GW energy
sensitivity of N^{1/2} are possible, where N is the number of stacked SGR
bursts. Estimated sensitivities for a mock search for gravitational waves from
the 2006 March 29 storm from SGR 1900+14 are also presented, for two GW
emission models, "fluence-weighted" and "flat" (unweighted).Comment: 17 pages, 16 figures, submitted to PR
Conference 2008 - Integrating Science and Mathematics Education Research into Teaching IV: Resources and Tool for Improved Learning
The Center for Science and Mathematics Education at the University of Maine continues its series of national conferences on providing professional development and resources for integrating mathematics and science education research into teaching. The first part of the conference consists of three days of parallel presentations and discussions by nationally recognized experts along with short workshops. The workshops provide first hand experience with either research-based STEM curricula or cutting edge STEM research projects that can serve as a basis for classroom instruction. The purposes of the conference include bringing together 150 participants in all aspects of STEM education (researchers, teachers, administrators, and preservice students) to exchange ideas about research, curriculum and assessment, to help teachers integrate research based instructional strategies in their teaching, and to build sustainable collaborations between participants. The second part of the conference is a two day summer academy in which about 60 participants have the option of (1) working on implementing a module of technology-rich curriculum in their classroom; (2) developing plans, curricular materials and assessments for involving teachers and students in a STEM research project; or (3) adapting and implementing a research-supported curriculum in their teaching. The academy continues throughput the year. A focus on research-based strategies that advance the successful participation of underrepresented groups is embedded in all activities
Maximum gravitational-wave energy emissible in magnetar flares
Recent searches of gravitational-wave (GW) data raise the question of what
maximum GW energies could be emitted during gamma-ray flares of highly
magnetized neutron stars (magnetars). The highest energies (\sim 10^{49} erg)
predicted so far come from a model [K. Ioka, Mon. Not. Roy. Astron. Soc. 327,
639 (2001)] in which the internal magnetic field of a magnetar experiences a
global reconfiguration, changing the hydromagnetic equilibrium structure of the
star and tapping the gravitational potential energy without changing the
magnetic potential energy. The largest energies in this model assume very
special conditions, including a large change in moment of inertia (which was
observed in at most one flare), a very high internal magnetic field, and a very
soft equation of state. Here we show that energies of 10^{48}-10^{49} erg are
possible under more generic conditions by tapping the magnetic energy, and we
note that similar energies may also be available through cracking of exotic
solid cores. Current observational limits on gravitational waves from magnetar
fundamental modes are just reaching these energies and will beat them in the
era of advanced interferometers.Comment: 16 pages, 5 figures, 1 tabl
An XUV-FEL amplifier seeded using high harmonic generation
A detailed design of a free electron laser ( FEL) amplifier operating in the extreme ultra violet ( XUV) and seeded directly by a high harmonic source is presented. The design is part of the 4th generation light source ( 4GLS) facility proposed for the Daresbury Laboratory in the UK which will offer users a suite of high brightness synchronised sources from THz frequencies into the XUV. The XUV-FEL will generate photons with tunable energies from 8 to 100 eV at giga-watt peak power levels in near Fourier-transform limited pulses of variable polarisation. The designs of the high harmonic generation ( HHG) seeding, FEL amplifier and synchronising systems are presented. Numerical simulations quantify the FEL output characteristics
An analysis of integrative outcomes in the Dayton peace negotiations
The nature of the negotiated outcomes of the eight issues of the Dayton Peace Agreement was studied in terms of their integrative and distributive aspects. in cases where integrative elements were Sound, further analysis was conducted by concentrating on Pruitt's five types of integrative solutions: expanding the pie, cost cutting, non-specific compensation, logrolling, and bridging. The results showed that real world international negotiations can arrive at integrative agreements even when they involve redistribution of resources tin this case the redistribution of former Yugoslavia). Another conclusion was that an agreement can consist of several distributive outcomes and several integrative outcomes produced by different kinds of mechanisms. Similarly, in single issues more than one mechanism can be used simultaneously. Some distributive bargaining was needed in order to determine how much compensation was required. Finally, each integrative formula had some distributive aspects as well
Variability in X-ray induced effects in [Rh(COD)Cl]₂ with changing experimental parameters
X-ray characterisation methods have undoubtedly enabled cutting-edge advances in all aspects of materials research. Despite the enormous breadth of information that can be extracted from these techniques, the challenge of radiation-induced sample change and damage remains prevalent. This is largely due to the emergence of modern, high-intensity X-ray source technologies and the growing potential to carry out more complex, longer duration in situ or in operando studies. The tunability of synchrotron beamlines enables the routine application of photon energy-dependent experiments. This work explores the structural stability of [Rh(COD)Cl]2, a widely used catalyst and precursor in the chemical industry, across a range of beamline parameters that target X-ray energies of 8 keV, 15 keV, 18 keV and 25 keV, on a powder X-ray diffraction synchrotron beamline at room temperature. Structural changes are discussed with respect to absorbed X-ray dose at each experimental setting associated with the respective photon energy. In addition, the X-ray radiation hardness of the catalyst is discussed, by utilising the diffraction data collected at the different energies to determine a dose limit, which is often considered in protein crystallography and typically overlooked in small molecule crystallography. This work not only gives fundamental insight into how damage manifests in this organometallic catalyst, but will encourage careful consideration of experimental X-ray parameters before conducting diffraction on similar radiation-sensitive organometallic materials
Swift observations of IBL and LBL objects
BL Lacs are an enigmatic class of active galactic nuclei (AGNs),
characterized by the non-thermal continuum typically attributed to synchrotron
and inverse Compton emission. Depending on the frequency location of the maxima
of these components, they are subdivided into three subclasses LBLs, IBLs, and
HBLs. We present the results of a set of observations of eight BL Lac objects
of LBL and IBL type performed by the XRT and UVOT detectors onboard the Swift
satellite between January 2005 and November 2006. We are mainly interested in
measuring the spectral parameters, and particularly the steepness between the
UV and the X-ray band, useful for determining the classification of these
sources. We compare the behavior of these sources with previous XMM-Newton,
BeppoSAX obser- vations and with historical data in the X-ray and in the
optical band. We are also interested in classifying the sources in our sample
on the basis of the observations and comparing them with their classification
presented in literature. We performed X-ray spectral analysis of observed BL
Lac objects using a simple powerlaw and in a few cases the log-parabolic model.
We also combined the UV emission with the low energy X-ray data to We used
observational data to classify sources in our sample and derived parameters of
their spectral energy distribution. We found that for the IBLs X-rays low
states show features of the high energy component, usually interpreted as due
to inverse Compton emission. Sources in our sample exhibit a range of temporal
UV and X-ray behaviors, some objects having clear and neat correlated UV and
X-ray variations (e.g. ON231) and other objects showing no clear (e.g. AO
0235+164) UV and X-ray correlation. Finally, we also note that our estimates of
spectral curvature are in the range of that measured for the HBLs.Comment: 10 pages, 7 figures, published in A&
How inert, perturbing, or interacting are cryogenic matrices? A combined spectroscopic (infrared, electronic, and x-ray absorption) and DFT investigation of matrix-isolated Iron, Cobalt, Nickel, and Zinc Dibromides
The interactions of FeBr2, CoBr2, NiBr2 and ZnBr2 with Ne, Ar, Kr, Xe, CH4 and N2 matrices have been investigated using IR, electronic absorption and X-ray absorption spectroscopies, as well as DFT calculations. ZnBr2 is linear in all the matrices. NiBr2 is linear in all but N2 matrices where it is severely bent. For FeBr2 and CoBr2 there is a more gradual change, with evidence of non-linearity in Xe and CH4 matrices as well as N2. In the N2 matrices the presence of νNN modes blue shifted from the “free” N2 values indicates the presence of physisorbed species, and the magnitude of the blue-shift correlates with the shift in the ν3 mode of the metal dibromide. In the case of NiCl2 and NiBr2 chemisorbed species are formed after photolysis, but only if deposition takes place below 10 K. There was no evidence for chemisorbed species for NiF2 and FeBr2 and in the case of CoBr2 the evidence was not strong
- …