4,465 research outputs found
Radio Quiet Pulsars with Ultra-Strong Magnetic Fields
The notable absence of radio pulsars having measured magnetic dipole surface
field strengths above Gauss naturally raises the
question of whether this forms an upper limit to pulsar magnetization. Recently
there has been increasing evidence that neutron stars possessing higher dipole
spin-down fields do in fact exist, including a growing list of anomalous X-ray
pulsars (AXPs) with long periods and spinning down with high period
derivatives, implying surface fields of -- Gauss.
Furthermore, the recently reported X-ray period and period derivative for the
Soft Gamma-ray Repeater (SGR) source SGR1806-20 suggest a surface field around
Gauss. None of these high-field pulsars have yet been detected as
radio pulsars. We propose that high-field pulsars should be radio-quiet because
electron-positron pair production in their magnetospheres, thought to be
essential for radio emission, is efficiently suppressed in ultra-strong fields
( Gauss) by the action of photon splitting, a
quantum electrodynamical process in which a photon splits into two. Our
computed radio quiescence boundary in the radio pulsar diagram,
where photon splitting overtakes pair creation, is located just above the
boundary of the known radio pulsar population, neatly dividing them from the
AXPs. We thus identify a physical mechanism that defines a new class of
high-field radio-quiet neutron stars that should be detectable by their pulsed
emission at X-ray and perhaps -ray energies.Comment: 4 pages, including one figure and one table, in AASTeX emulatapj
format, Astrophysical Journal Letters, in pres
Online Pattern Recognition for the ALICE High Level Trigger
The ALICE High Level Trigger has to process data online, in order to select
interesting (sub)events, or to compress data efficiently by modeling
techniques.Focusing on the main data source, the Time Projection Chamber (TPC),
we present two pattern recognition methods under investigation: a sequential
approach "cluster finder" and "track follower") and an iterative approach
("track candidate finder" and "cluster deconvoluter"). We show, that the former
is suited for pp and low multiplicity PbPb collisions, whereas the latter might
be applicable for high multiplicity PbPb collisions, if it turns out, that more
than 8000 charged particles would have to be reconstructed inside the TPC.
Based on the developed tracking schemes we show, that using modeling techniques
a compression factor of around 10 might be achievableComment: Realtime Conference 2003, Montreal, Canada to be published in IEEE
Transactions on Nuclear Science (TNS), 6 pages, 8 figure
Si and Mn Abundances in Damped Lya Systems with Low Dust Content
We have measured the abundances of Zn, Si, Mn, Cr, Fe, and Ni in three damped
Lyman alpha systems at redshifts z < 1 from high resolution echelle spectra of
QSOs recorded with the Keck I telescope. In all three cases the abundances of
Cr, Fe, and Ni relative to Zn indicate low levels of dust depletions. We
propose that when the proportion of refractory elements locked up in dust
grains is less than about 50 percent, it is plausible to assume an
approximately uniform level of depletion for all grain constituents and, by
applying a small dust correction, recover the intrisic abundances of Si and Mn.
We use this approach on a small sample of damped systems for which it is
appropriate, with the aim of comparing the metallicity dependence of the ratios
[Si/Fe] and [Mn/Fe] with analogous measurements in Milky Way stars. The main
conclusion is that the relative abundances of both elements in distant galaxies
are broadly in line with expectations based on Galactic data. Si displays a
mild enhancement at low metallicities, as expected for an alpha-capture
element, but there are also examples of near-solar [Si/Fe] at [Fe/H] < -1. The
underabundance of Mn at low metallicities is possibly even more pronounced than
that in metal-poor stars, and no absorption system has yet been found where
[Mn/Fe] is solar. The heterogeneous chemical properties of damped Lyman alpha
systems, evident even from this limited set of measurements, provide further
support for the conclusion from imaging studies that a varied population of
galaxies gives rise to this class of QSO absorbers.Comment: 29 pages, LaTex, 7 Postscript Figures. Accepted for Publication in
the Astrophysical Journa
May car washing represent a risk for Legionella infection?
Background. Legionella is a ubiquitous Gram-negative bacterium naturally found in aquatic environments. It can pose a health problem when it grows and spreads in man-made water systems. Legionella pneumophila is the most common cause of Legionnaires\u2019 disease nowadays, a community-acquired pneumonia with pulmonary symptoms and chest radiography no different from any other form of infectious pneumonia. Legionella monitoring is important for public health reasons, including the identification of unusual environmental sources of Legionella.
Methods. We report two cases of Legionnaires\u2019 disease associated with two different car wash installations in the province of Vicenza, in the Veneto region, northeastern Italy. Patients were not employees of the car wash installations, but users of the service. In both cases, Legionella antigen was detected in urine using the Alere BinaxNOW\uae Legionella Urinary Antigen, and Legionella antibodies were detected in serum using SERION ELISA classic Legionella pneumophila 1-7 IgG and IgM. Water samples were also analyzed as part of the surveillance program for Legionella prevention and control in compliance with the Italian guidelines.
Results. Both patients had clinical symptoms and chest radiography compatible with pneumonia, and only one of them had diabetes as a risk factor. Legionella urinary antigen and serological test on serum samples were positive for Legionella in both patients, even if much slighter in the case A due to the retrospective serological investigation performed a year later the episode and after the second clinical case occurred in the same district. The environmental investigations highlighted two different car wash plants as potential source of infection. A certified company using shock hyperchlorination was asked to disinfect the two plants and, subsequently, control samples resulted negative for Legionella pneumophila.
Conclusions. Any water source producing aerosols should be considered at risk for the transmission of Legionella bacteria, including car wash installations frequently used by a large number of customers and where poor maintenance probably creates favorable conditions for Legionella overgrowth and spreading. Additional research is needed to ascertain optimal strategies for Legionella monitoring and control, but environmental surveillance, paying careful attention to possible unconventional sources, should remain an important component of any Legionnaires\u2019 disease prevention program. Additionally, all available diagnostic methods would be recommended for the confirmation of all cases even in the event of non-serogroup 1 Legionella pneumophila infection, probably underestimated at this time
Full polar cap cascade scenario: -ray and X-ray luminosities from spin-powered pulsars
We modify polar cap cascade picture to include the ICS of the higher
generation pairs. In such a ``full-cascade'' scenario, not only the
perpendicular portion of the energy of the pairs goes to high energy radiation
via SR, but the parallel portion of the energy of the pairs can also contribute
to high energy emission via ICS with the soft thermal photons from either the
full neutron star surface or the hot polar cap. An important output of such a
scenario is that the soft tail of the ICS spectrum can naturally result in a
non-thermal X-ray component which can contribute to the luminosities observed
by ROSAT and ASCA. Here we present an analytic description of such a full polar
cap cascade scenario within the framework of Harding & Muslimov acceleration
model. We present the theoretical predictions of the -ray luminosities,
the thermal and non-thermal X-ray luminosities for the known spin-powered X-ray
pulsars. Our results show that the observed different dependences of the high
energy luminosities on the pulsar spin-down luminosities, i.e., and , are well
reproduced. Our model predicts that the {\em pulsed} soft X-rays in the ROSAT
band from most of the millisecond pulsars might be of thermal origin if there
is no strong multipole field components near the surfaces of these pulsars.Comment: 23 pages, emulateapj style, final version to appear in the
Astrophysical Journa
An Economic Analysis of a Simple Structural Method to Reduce Road-Kills of Royal Terns at Bridges
Royal terns (Sterna maxima) in Florida are listed as a âspecies of special concernâ by the Florida Committee on Rare and Endangered Plants and Animals (Egensteiner et al., 1996). These birds live along the Atlantic and Gulf coasts of Florida throughout the year (Egensteiner et al., 1996); their winter range along the Atlantic Coast of the United States is from North Carolina south through Florida (Clapp et al., 1983), with the Florida population augmented by terns migrating from breeding colonies farther north. Collisions with vehicles cause many marine bird road-kills at some coastal roads and bridges in Florida (Skoog, 1982; Smith et al., 1994; Bard et al., 2002b). General methods to reduce wildlife resource road kills have included reduced speed limits, physical barriers, and public education (Bertwistle, 1999; Brown et al., 1999; Evink, 1999). We examine herein the benefit-costs from a multi-year trial of a simple hazard reduction method applied to a bridge in east-central Florida
Ionic conductivity in multiply substituted ceria-based electrolytes
The authors thank the University of St Andrews and the UK Engineering and Physical Sciences Research Council for the PhD studentship for AVC-A (grant code: EP/M506631/1).Cerias, appropriately doped with trivalent rare earth ions, have high oxide ion conductivity and are attractive SOFC (solid oxide fuel cell) electrolytes. Here, seven compositions of Ce0.8SmxGdyNdzO1.9 (where x, y and z = 0.2, 0.1, 0.0667 or 0 and x + y + z = 0.2) are synthesised using a low temperature method in order to determine the effect of multiple doping on microstructure and conductivity. Analysis using scanning and transmission electron microscopy, inductively coupled plasma mass spectrometry, X-ray diffraction and impedance spectroscopy is carried out. Crystallite sizes are determined in the powders and relative densities and grain size distributions were obtained in sintered pellets. Total, bulk and grain boundary conductivities are obtained using impedance spectroscopy and corresponding activation energies and enthalpies of ion migration and defect association are calculated. The highest total conductivity observed at 600 °C is 1.80 Smâ1 for Ce0.8Sm0.1Gd0.1O1.9 and an enhancement effect on conductivity for this combination of co-dopants between 300 °C and 700 °C relative to the singly doped compounds - Ce0.8Sm0.2O1.9 and Ce0.8Gd0.2O1.9 - is seen. This has interesting implications for their application as SOFC electrolytes, especially at intermediate temperatures.PostprintPeer reviewe
Galactic populations of radio and gamma-ray pulsars in the polar cap model
We simulate the characteristics of the Galactic population of radio and
-ray pulsars using Monte Carlo techniques. At birth, neutron stars are
spatially distributed in the Galactic disk, with supernova-kick velocities, and
randomly dispersed in age back to years. They are evolved in the
Galactic gravitational potential to the present time. From a radio luminosity
model, the radio flux is filtered through a selected set of radio-survey
parameters. -ray luminosities are assigned using the features of recent
polar cap acceleration models invoking space-charge-limited flow, and a pulsar
death valley further attenuates the population of radio-loud pulsars. Assuming
a simple emission geometry with aligned radio and -ray beams of 1
steradian solid angle, our model predicts that EGRET should have seen 7
radio-loud and 1 radio-quiet, -ray pulsars. With much improved
sensitivity, GLAST, on the other hand, is expected to observe 76 radio-loud and
74 radio-quiet, -ray pulsars of which 7 would be identified as pulsed
sources. We also explore the effect of magnetic field decay on the
characteristics of the radio and -ray pulsar populations. Including
magnetic field decay on a timescale of 5 Myr improves agreement with the radio
pulsar population and increases the predicted number of GLAST detected pulsars
to 90 radio-loud and 101 radio-quiet (9 pulsed) -ray pulsars. The lower
flux threshold allows GLAST to detect -ray pulsars at larger distances
than those observed by the radio surveys used in this study.Comment: 38 pages, 11 figures, accepted for publication v565 n1 Ap
Comparison of bitumenârubber use in extreme conditions in Russia and South Africa
The development of the use of crumb rubber from used tyres in asphalt started in
the USSR during the 1960s independently and in parallel with the research in the USA and
Western World during the 1970s. The result is that different processes were developed for
making bitumen-rubber asphalt. The objective of the paper is to present and compare the
bitumen-rubber specifications and asphalt experience in Russia and in South Africa (which
was derived from the USA approach). The focus is on the wet process even though more
recently the dry method has been used. Firstly, the properties of the constituent
components are presented and discussed, where after the properties and manufacture of
bitumen-rubber are compared. Finally, the properties of bitumen-rubber asphalt are
compared and the experience and performance is presented. It was found that although the
initial approach from the two schools differed, there are currently major similarities, even
though the temperature ranges of use are vastly different. Bitumen-rubber has been shown
to cater effectively for a wide range of environmental conditions and is superior to
unmodified bitumen.http://www.tandfonline.com/loi/trmp202018-01-31hb2016Civil Engineerin
Physics at the LHC: a short overview
The CERN Large Hadron Collider (LHC) started operation a few months ago. The
machine will deliver proton-proton and nucleus-nucleus collisions at energies
as high as sqrt(s)=14 TeV and luminosities up to L~10^{34} cm^{-2}s^{-1}, never
reached before. The main open scientific questions that the seven LHC
experiments -- ATLAS, CMS, ALICE, LHCb, TOTEM, LHCf and MOEDAL -- aim to solve
in the coming years are succinctly reviewed.Comment: 9 pages, 16 plots. Invited review talk Hot-Quarks 2010, La
Londe-Les-Maures, July 2010. J. Phys. Conf. Ser. 270, 012001 (2011). Minor
typos correcte
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