2,784 research outputs found
Unpulsed UBV Optical Emission from the Crab Pulsar
Based on observations of the Crab pulsar using the TRIFFID high speed imaging
photometer in the UBV bands using the Special Astrophysical Observatory's 6m
telescope in the Russian Caucasus, we report the detection of pronounced
emission during the so-called `off' phase of emission. Following de-extinction,
this unpulsed component of emission is shown to be consistent with a power law
with an exponent of alpha = -0.60 +/- 0.37, the uncertainty being dominated by
the error associated with the independent CCD photometry used to reference the
TRIFFID data. This suggests a steeper power law form than that reported
elsewhere in the literature for the total integrated spectrum, which is
essentially flat with alpha ~ 0.1, although the difference in this case is only
significant at the ~ 2 sigma level. Deeper reference integrated and TRIFFID
phase-resolved photometry in these bands in conjunction with further
observations in the UV and R region would constrain this fit further.Comment: 26 pages, 2 figures, uses aasms4.sty, accepted for publication in the
Astrophysical Journa
Shear-driven size segregation of granular materials: modeling and experiment
Granular materials segregate by size under shear, and the ability to
quantitatively predict the time required to achieve complete segregation is a
key test of our understanding of the segregation process. In this paper, we
apply the Gray-Thornton model of segregation (developed for linear shear
profiles) to a granular flow with an exponential profile, and evaluate its
ability to describe the observed segregation dynamics. Our experiment is
conducted in an annular Couette cell with a moving lower boundary. The granular
material is initially prepared in an unstable configuration with a layer of
small particles above a layer of large particles. Under shear, the sample mixes
and then re-segregates so that the large particles are located in the top half
of the system in the final state. During this segregation process, we measure
the velocity profile and use the resulting exponential fit as input parameters
to the model. To make a direct comparison between the continuum model and the
observed segregation dynamics, we locally map the measured height of the
experimental sample (which indicates the degree of segregation) to the local
packing density. We observe that the model successfully captures the presence
of a fast mixing process and relatively slower re-segregation process, but the
model predicts a finite re-segregation time, while in the experiment
re-segregation occurs only exponentially in time
Segregation by thermal diffusion in granular shear flows
Segregation by thermal diffusion of an intruder immersed in a sheared
granular gas is analyzed from the (inelastic) Boltzmann equation. Segregation
is induced by the presence of a temperature gradient orthogonal to the shear
flow plane and parallel to gravity. We show that, like in analogous systems
without shear, the segregation criterion yields a transition between upwards
segregation and downwards segregation. The form of the phase diagrams is
illustrated in detail showing that they depend sensitively on the value of
gravity relative to the thermal gradient. Two specific situations are
considered: i) absence of gravity, and ii) homogeneous temperature. We find
that both mechanisms (upwards and downwards segregation) are stronger and more
clearly separated when compared with segregation criteria in systems without
shear.Comment: 8 figures. To appear in J. Stat. Mec
Observations of three young gamma-ray pulsars with the Gran Telescopio Canarias
We report the analysis of the first deep optical observations of three
isolated -ray pulsars detected by the {\em Fermi Gamma-ray Space
Telescope}: the radio-loud PSR\, J0248+6021 and PSR\, J0631+1036, and the
radio-quiet PSR\, J0633+0632. The latter has also been detected in the X rays.
The pulsars are very similar in their spin-down age (40--60 kyrs),
spin-down energy ( erg s), and dipolar surface
magnetic field (-- G). These pulsars are promising
targets for multi-wavelength observations, since they have been already
detected in rays and in radio or X-rays. None of them has been
detected yet in the optical band. We observed the three pulsar fields in 2014
with the Spanish 10.4m Gran Telescopio Canarias (GTC). We could not find any
candidate optical counterpart to the three pulsars close to their most recent
radio or {\em Chandra} positions down to limits of ,
, for PSR\, J0248+6021, J0631+1036, and J0633+0632,
respectively. From the inferred optical upper limits and estimated distance and
interstellar extinction, we derived limits on the pulsar optical luminosity. We
also searched for the X-ray counterpart to PSR\, J0248+6021 with \chan\ but we
did not detect the pulsar down to a 3 flux limit of
erg cm s (0.3--10 keV). For all these pulsars, we compared the
optical flux upper limits with the extrapolations in the optical domain of the
-ray spectra and compared their multi-wavelength properties with those
of other -ray pulsars of comparable age.Comment: 12 pages, 5 figures, accepted for publication in MNRA
Reducing the carbon footprint of lightweight aggregate concrete
Lightweight aggregate concrete (LWAC) is a special concrete type with density of no more than 2200 kg/m³. Lower densities than normal weight concrete (2400-2500 kg/m³) are achieved using lightweight aggregates, which may originate from by-products of industrial manufacture such as fly ash, for example Lytag. Currently there is an increasing demand for LWAC for the construction of lightweight composite flooring systems, particularly in commercial buildings. Despite the well-recognized issues and challenges associated with the carbon dioxide (CO²) emissions from cement production, LWAC still contains high quantities of Portland cement (Type I or CEM I) as well as high quantities of total cementitious materials content. This has been primarily utilized to attain a certain workability and pumpability, as well as to not compromise the strength development. As such, the carbon footprint of LWAC is generally higher than that of normal weight concrete, owing also to the carbon intensive lightweight aggregates. In this work, several alternative lightweight aggregate mixes were optimized to maximize Portland cement replacement and reduce the total cementitious materials content without compromising the strength, workability and pumpability of a standard, to Eurocode 2, LC 30/33. The developed mixes contained up to 60% of ground granulated blast-furnace slag, as well as limestone powder, which resulted in a reduced carbon footprint compared to the conventional LWAC mixes. It was possible to reduce the Portland cement content by approximately 40%, the total cementitious materials content by 22% and embodied carbon (life cycle stages A1-3) by 12% compared to the initial, conventional LWAC mixes
Beyond deficiency:Potential benefits of increased intakesof vitamin K for bone and vascular health
Vitamin K is wellknown for its role in the synthesisof a number of blood coagulationfactors.During recent years vitaminK-dependent proteins werediscovered to be of vital importancefor bone and vascular health.Recommendations for dietary vitaminK intake have been made onthe basis of the hepatic requirementsfor the synthesis of bloodcoagulation factors.Accumulatingevidence suggests that the requirementsfor other functions thanblood coagulation may be higher.This paper is the result of a closedworkshop (Paris,November 2002)in which a number of Europeanvitamin K experts reviewed theavailable data and formulated theirstandpoint with respect to recommendeddietary vitamin K intakeand the use of vitamin K-containingsupplements
Production of a high-density state-selected metastable neon beam
We have developed a high-density source of metastable neon and have selectively quenched both metastable species using a standing-wave dye laser. The source is compact, stable, and produces an average intensity of 3.6 x 1014 sr -1 s -1 and a density on target of 7.7 x 106 cm-3
Brief of Tennessee Valley Authority in Support of Motion to Dismiss and in Opposition to Motion for Injunction, \u3cem\u3eTVA v. Hill et al\u3c/em\u3e, Civil Action No. 3-71-48
Brief for the defendants in support of a motion to dismiss and in opposition to a motion for an injunction in the case of TVA v. Hill et al in the United States District Court for the Eastern District of Tennessee, Northern Division
Production of a high-density state-selected metastable neon beam
We have developed a high-density source of metastable neon and have selectively quenched both metastable species using a standing-wave dye laser. The source is compact, stable, and produces an average intensity of 3.6 x 1014 sr -1 s -1 and a density on target of 7.7 x 106 cm-3
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