2,434 research outputs found
Seismic Design of Tialings Dam An Overview of its Evolution and New Challenges
This paper first provides an overview of the evolution of tailings dam engineering over the years. It then highlights key factors affecting seismic performance of tailings dams: such as site seismicity, foundation and damfill geotechnical characteristics, dam design, construction, operation and decommissioning, seismic slope stability, and dam rehabilitation. Finally, it reviews some of the recent trends that lead to new challenges to the engineering profession. These challenges include: coping with heightened public concern about dam safety; balancing potential conflict between seismic and environmental requirements; and addressing issues introduced by globalisation of the mining industry. Institutional responses and technological tools available to meet these challenges are outlined
NuSTAR observations of X-ray bursts from the magnetar 1E 1048.1-5937
We report the detection of eight bright X-ray bursts from the 6.5-s magnetar
1E 1048.1-5937, during a 2013 July observation campaign with the Nuclear
Spectroscopic Telescope Array (NuSTAR). We study the morphological and spectral
properties of these bursts and their evolution with time. The bursts resulted
in count rate increases by orders of magnitude, sometimes limited by the
detector dead time, and showed blackbody spectra with kT=6-8 keV in the T90
duration of 1-4 s, similar to earlier bursts detected from the source. We find
that the spectra during the tail of the bursts can be modeled with an absorbed
blackbody with temperature decreasing with flux. The bursts flux decays
followed a power-law of index 0.8-0.9. In the burst tail spectra, we detect a
~13 keV emission feature, similar to those reported in previous bursts from
this source as well as from other magnetars observed with the Rossi X-ray
Timing Explorer (RXTE). We explore possible origins of the spectral feature
such as proton cyclotron emission, which implies a magnetic field strength of
B~2X10^15 G in the emission region. However, the consistency of the energy of
the feature in different objects requires further explanation.Comment: 10 pages, 6 figures, accepted for publication in Ap
NuSTAR Observations of the Magnetar 1E 2259+586
We report on new broad band spectral and temporal observations of the
magnetar 1E 2259+586, which is located in the supernova remnant CTB 109. Our
data were obtained simultaneously with the Nuclear Spectroscopic Telescope
Array (NuSTAR) and Swift, and cover the energy range from 0.5-79 keV. We
present pulse profiles in various energy bands and compare them to previous
RXTE results. The NuSTAR data show pulsations above 20 keV for the first time
and we report evidence that one of the pulses in the double-peaked pulse
profile shifts position with energy. The pulsed fraction of the magnetar is
shown to increase strongly with energy. Our spectral analysis reveals that the
soft X-ray spectrum is well characterized by an absorbed double-blackbody or
blackbody plus power-law model in agreement with previous reports. Our new hard
X-ray data, however, suggests that an additional component, such as a
power-law, is needed to describe the NuSTAR and Swift spectrum. We also fit the
data with the recently developed coronal outflow model by Beloborodov for hard
X-ray emission from magnetars. The outflow from a ring on the magnetar surface
is statistically preferred over outflow from a polar cap.Comment: 37 pages, 9 figures, corresponding author, [email protected]
NuSTAR Observations of X-Ray Bursts from the Magnetar 1E 1048.1-5937
We report the detection of eight bright X-ray bursts from the 6.5 s magnetar 1E 1048.15937, during a 2013 July observation campaign with the Nuclear Spectroscopic Telescope Array. We study the morphological and spectral properties of these bursts and their evolution with time. The bursts resulted in count rate increases by orders of magnitude, sometimes limited by the detector dead time, and showed blackbody spectra with kT is approx. 6-8 keV in the T90 duration of 1-4 s, similar to earlier bursts detected from the source. We find that the spectra during the tail of the bursts can be modeled with an absorbed blackbody with temperature decreasing with flux. The burst flux decays followed a power law of index 0.8-0.9. In the burst tail spectra, we detect a is approx. 13 keV emission feature, similar to those reported in previous bursts from this source as well as from other magnetars observed with the Rossi X-ray Timing Explorer.We explore possible origins of the spectral feature such as proton cyclotron emission, which implies a magnetic field strength of B is approx. 210(exp15) G in the emission region. However, the consistency of the energy of the feature in different objects requires further explanation
Improved Left Ventricular Mass Quantification with Partial Voxel Interpolation – In-Vivo and Necropsy Validation of a Novel Cardiac MRI Segmentation Algorithm
Background—CMR typically quantifies LV mass (LVM) via manual planimetry (MP), but this approach is time consuming and does not account for partial voxel components - myocardium admixed with blood in a single voxel. Automated segmentation (AS) can account for partial voxels, but this has not been used for LVM quantification. This study used automated CMR segmentation to test the influence of partial voxels on quantification of LVM. Methods and Results—LVM was quantified by AS and MP in 126 consecutive patients and 10 laboratory animals undergoing CMR. AS yielded both partial voxel (ASPV) and full voxel (ASFV) measurements. Methods were independently compared to LVM quantified on echocardiography (echo) and an ex-vivo standard of LVM at necropsy. AS quantified LVM in all patients, yielding a 12-fold decrease in processing time vs. MP (0:21±0:04 vs. 4:18±1:02 min; pFV mass (136±35gm) was slightly lower than MP (139±35; Δ=3±9gm, pPV yielded higher LVM (159±38gm) than MP (Δ=20±10gm) and ASFV (Δ=23±6gm, both pPV and ASFV correlated with larger voxel size (partial r=0.37, pPV yielded better agreement with echo (Δ=20±25gm) than did ASFV (Δ=43±24gm) or MP (Δ=40±22gm, both pPV and ex-vivo results were similar (Δ=1±3gm, p=0.3), whereas ASFV (6±3g, P\u3c0.001) and MP (4±5 g, P=0.02) yielded small but significant differences with LVM at necropsy
De Sitter Holography and the Cosmic Microwave Background
We interpret cosmological evolution holographically as a renormalisation
group flow in a dual Euclidean field theory, as suggested by the conjectured
dS/CFT correspondence. Inflation is described by perturbing around the
infra-red fixed point of the dual field theory. The spectrum of the cosmic
microwave background radiation is determined in terms of scaling violations in
the field theory. The dark energy allows similar, albeit less predictive,
considerations. We discuss the cosmological fine-tuning problems from the
holographic perspective.Comment: 17 pages, 2 figures, uses JHEP style files; corrected and added
reference
Fabrication and performance of Constellation-X hard x-ray telescope prototype optics using segmented glass
We report on the fabrication and performance of prototype optics for the Constellation-X hard X-ray telescope (HXT). The prototypes utilize segmented-glass optics. Multiple glass segments are combined to produce telescope shells. The shells are separated by and epoxied to graphite rods, and each layer of rods is precisely machined to match the required optical geometry of the corresponding glass shell. This error-compensating, monolithic assembly and alignment (EMAAL) procedure is novel. Two prototypes are described. The first used 10cm long thermally-slumped glass pieces produced by slumping into a concave mandrel with no subsequent replication. This prototype obtained 45" (2-bounce HPD). The second prototype was the first attempt to mount epoxy-replicated, thermally-slumped glass optics using EMAAL. The latter prototype demonstrated our ability to produce and mount glass shells whose figure and performance are faithful representations of the original replication mandrel. The average performance was 45", with the best replicated segment providing 33" (2-bounce HPD) performance, consistent with the ~30" measured with laser reflectometry and interferometry prior to mounting. Both these prototypes substantially exceeded the HXT requirement of 60"
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