26,150 research outputs found
Graphite/polyimide laminates with near-zero thermal expansion
Composite structures can be laminated to have very low coefficients of thermal expansion. Such structures are light and strong and have many uses where expansion or contraction with temperature change is undesirable. One application is with instruments that measure thermal expansion
Remote Monitoring of Implantable Cardioverter Defibrillator
The rate of implantable cardioverter defibrillator (ICD) implantation has gone up as primary and secondary prevention trials have relatively consistently shown significant improvement in mortality and morbidity. Most patients with ICDs are followed routinely at intervals ranging from 3 to 6 months. Many patients require additional non-scheduled visits to investigate symptoms that may or may not relate to their cardiac disease or device. Appropriate and inappropriate therapies of implantable cardioverter defibrillators have a major impact on morbidity and quality of life in ICD recipients. Remote monitoring systems can substitute for routine follow-up visits and/ or deliver continuous diagnostic and device status information. Remote monitoring of ICDs can decrease the need for many patient visits and, thereby, probably reduce expense
A powerful and highly variable off-nuclear X-ray source in the composite starburst/Seyfert 2 galaxy NGC 4945
We report on a powerful and variable off-nuclear X-ray source in the nearby
spiral galaxy NGC 4945. Two ROSAT PSPC observations show the source to brighten
in 0.5--2.0 keV flux by a factor of about 9 on a time-scale of 11 months or
less. It is seen by ASCA about one month after the second PSPC pointing, and is
seen to have dimmed by a factor of > 7 in a ROSAT HRI pointing about one year
after the second PSPC pointing. Its maximum observed 0.8--2.5 keV luminosity is
about 8E38 erg/s, making it brighter than any known persistent X-ray binary in
the Milky Way. Its total X-ray luminosity is probably larger than 1.2E39 erg/s.
The observed variability argues against a superbubble interpretation, and the
off-nuclear position argues against a low-luminosity active galactic nucleus.
The source is therefore probably either an ultra-powerful X-ray binary or an
ultra-powerful supernova remnant. Optical monitoring has not identified any
supernovae in NGC 4945 during the time of the X-ray observations, and any
supernova would have had to have been either very highly absorbed or
intrinsically optically faint.Comment: 5 pages, uuencoded compressed tar file, MNRAS in pres
Large-eddy simulation of a turbulent mixing layer
The three dimensional, time dependent (incompressible) vorticity equations were used to simulate numerically the decay of isotropic box turbulence and time developing mixing layers. The vorticity equations were spatially filtered to define the large scale turbulence field, and the subgrid scale turbulence was modeled. A general method was developed to show numerical conservation of momentum, vorticity, and energy. The terms that arise from filtering the equations were treated (for both periodic boundary conditions and no stress boundary conditions) in a fast and accurate way by using fast Fourier transforms. Use of vorticity as the principal variable is shown to produce results equivalent to those obtained by use of the primitive variable equations
The X-ray flaring properties of Sgr A* during six years of monitoring with Swift
Starting in 2006, Swift has been targeting a region of ~21'X21' around
Sagittarius A* (Sgr A*) with the onboard X-ray telescope. The short,
quasi-daily observations offer an unique view of the long-term X-ray behavior
of the supermassive black hole. We report on the data obtained between 2006
February and 2011 October, which encompasses 715 observations with a total
accumulated exposure time of ~0.8 Ms. A total of six X-ray flares were detected
with Swift, which all had an average 2-10 keV luminosity of Lx (1-4)E35 erg/s
(assuming a distance of 8 kpc). This more than doubles the number of such
bright X-ray flares observed from Sgr A*. One of the Swift-detected flares may
have been softer than the other five, which would indicate that flares of
similar intensity can have different spectral properties. The Swift campaign
allows us to constrain the occurrence rate of bright (Lx > 1E35 erg/s) X-ray
flares to be ~0.1-0.2 per day, which is in line with previous estimates. This
analysis of the occurrence rate and properties of the X-ray flares seen with
Swift offers an important calibration point to asses whether the flaring
behavior of Sgr A* changes as a result of its interaction with the gas cloud
that is projected to make a close passage in 2013.Comment: 8 pages, 5 figures, 3 tables. Shortened, accepted to Ap
Advection-Dominated Accretion with Infall and Outflows
We present self-similar solutions for advection-dominated accretion flows
with radial viscous force in the presence of outflows from the accretion flow
or infall. The axisymmetric flow is treated in variables integrated over polar
sections and the effects of infall and outflows on the accretion flow are
parametrised for possible configurations compatible with the self-similar
solution. We investigate the resulting accretion flows for three different
viscosity laws and derive upper limits on the viscosity parameter alpha. In
addition, we find a natural connection to non-rotating and spherical accretion
with turbulent viscosity, which is assumed to persist even without differential
rotation. Positive Bernoulli numbers for advection-dominated accretion allow a
fraction of the gas to be expelled in an outflow and the upper limit on the
viscosity predicts that outflows are inevitable for equations of state close to
an ideal gas.Comment: 17 pages, 9 figures, accepted for publication in the Astrophysical
Journa
Jamming, two-fluid behaviour and 'self-filtration' in concentrated particulate suspensions
We study the flow of model experimental hard sphere colloidal suspensions at
high volume fraction driven through a constriction by a pressure
gradient. Above a particle-size dependent limit , direct microscopic
observations demonstrate jamming and unjamming--conversion of fluid to solid
and vice versa--during flow. We show that such a jamming flow produces a
reduction in colloid concentration downstream of the constriction.
We propose that this `self-filtration' effect is the consequence of a
combination of jamming of the particulate part of the system and continuing
flow of the liquid part, i.e. the solvent, through the pores of the jammed
solid. Thus we link the concept of jamming in colloidal and granular media with
a 'two-fluid'-like picture of the flow of concentrated suspensions. Results are
also discussed in the light of Osborne Reynolds' original experiments on
dilation in granular materials.Comment: 4 pages, 3 figure
The Swift X-ray monitoring campaign of the center of the Milky Way
In 2006 February, shortly after its launch, Swift began monitoring the center
of the Milky Way with the onboard X-Ray Telescope using short 1-ks exposures
performed every 1-4 days. Between 2006 and 2014, over 1200 observations have
been obtained, amounting to ~1.2 Ms of exposure time. This has yielded a wealth
of information about the long-term X-ray behavior of the supermassive black
hole Sgr A*, and numerous transient X-ray binaries that are located within the
25'x25' region covered by the campaign. In this review we highlight the
discoveries made during these first nine years, which includes 1) the detection
of seven bright X-ray flares from Sgr A*, 2) the discovery of the magnetar SGR
J1745-29, 3) the first systematic analysis of the outburst light curves and
energetics of the peculiar class of very-faint X-ray binaries, 4) the discovery
of three new transient X-ray sources, 5) exposing low-level accretion in
otherwise bright X-ray binaries, and 6) the identification of a candidate X-ray
binary/millisecond radio pulsar transitional object. We also reflect on future
science to be done by continuing this Swift's legacy campaign of the Galactic
center, which includes high-cadence monitoring of how the interaction between
the gaseous object `G2' and Sgr A* plays out in the future.Comment: 13 pages, 6 figures, 4 tables. Invited review to appear in Elsevier's
Journal of High Energy Astrophysics dedicated issue "Swift: 10 years of
discovery
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