184,181 research outputs found
The Angular Momenta of Neutron Stars and Black Holes as a Window on Supernovae
It is now clear that a subset of supernovae display evidence for jets and are
observed as gamma-ray bursts. The angular momentum distribution of massive
stellar endpoints provides a rare means of constraining the nature of the
central engine in core-collapse explosions. Unlike supermassive black holes,
the spin of stellar-mass black holes in X-ray binary systems is little affected
by accretion, and accurately reflects the spin set at birth. A modest number of
stellar-mass black hole angular momenta have now been measured using two
independent X-ray spectroscopic techniques. In contrast, rotation-powered
pulsars spin-down over time, via magnetic braking, but a modest number of natal
spin periods have now been estimated. For both canonical and extreme neutron
star parameters, statistical tests strongly suggest that the angular momentum
distributions of black holes and neutron stars are markedly different. Within
the context of prevalent models for core-collapse supernovae, the angular
momentum distributions are consistent with black holes typically being produced
in GRB-like supernovae with jets, and with neutron stars typically being
produced in supernovae with too little angular momentum to produce jets via
magnetohydrodynamic processes. It is possible that neutron stars are imbued
with high spin initially, and rapidly spun-down shortly after the supernova
event, but the available mechanisms may be inconsistent with some observed
pulsar properties.Comment: ApJ Letters, accepte
A Bundle Theory of Words
It has been a common assumption that words are substances that instantiate or have properties. In this paper, I question the assumption that our ontology of words requires posting substances by outlining a bundle theory of words, wherein words are bundles of various sorts of properties (such as semantic, phonetic, orthographic, and grammatical properties). I argue that this view can better account for certain phenomena than substance theories, is ontologically more parsimonious, and coheres with claims in linguistics
Estimation of deviation angle for axial-flow compressor blade sections using inviscid-flow solutions
Development of a method of estimating deviation angles by analytical procedures was begun. Solutions for inviscid, irrotational flow in the blade-to-blade plane were obtained with a finite-difference calculation method. Deviation angles for a plane cascade with a rounded trailing edge were estimated by using the inviscid-flow solutions and three trailing-edge hypotheses. The estimated deviation angles were compared with existing experimental data over a range of incidence angles at inlet flow angles of 30 deg and 60 deg. The results indicate that deviation angles can be estimated accurately (within 1 deg) by using one of the three trailing-edge hypotheses, but only when pressure losses are low. A new trailing-edge hypotheses is presented which is suitable (for the cascade considered) for both low- and high-loss operating points
Evidence for Antipodal Hot Spots During X-ray Bursts From 4U 1636-536
The discovery of high-frequency brightness oscillations in thermonuclear
X-ray bursts from several neutron-star low-mass X-ray binaries has important
implications for the beat frequency model of kilohertz quasi-periodic
brightness oscillations, the propagation of nuclear burning, the structure of
the subsurface magnetic fields in neutron stars, and the equation of state of
high-density matter. These implications depend crucially on whether the
observed frequency is the stellar spin frequency or its first overtone. Here we
report an analysis of five bursts from 4U 1636-536 which exhibit strong
oscillations at approximately 580 Hz. We show that combining the data from the
first 0.75 seconds of each of the five bursts yields a signal at 290 Hz that is
significant at the level when the number of trials is taken
into account. This strongly indicates that 290 Hz is the spin frequency of this
neutron star and that 580 Hz is its first overtone, in agreement with other
arguments about this source but in contrast to suggestions in the literature
that 580 Hz is the true spin frequency. The method used here, which is an
algorithm for combining time series data from the five bursts so that the
phases of the 580 Hz oscillations are aligned, may be used in any source to
search for weak oscillations that have frequencies related in a definite way to
the frequency of a strong oscillation.Comment: 9 pages including one figure, uses aaspp4.sty, submitted to The
Astrophysical Journal Letters on September 1
Measuring the Spin of GRS 1915+105 with Relativistic Disk Reflection
GRS 1915+105 harbors one of the most massive known stellar black holes in the
Galaxy. In May 2007, we observed GRS 1915+105 for 117 ksec in the low/hard
state using Suzaku. We collected and analyzed the data with the HXD/PIN and XIS
cameras spanning the energy range from 2.3-55 keV. Fits to the spectra with
simple models reveal strong disk reflection through an Fe K emission line and a
Compton back-scattering hump. We report constraints on the spin parameter of
the black hole in GRS 1915+105 using relativistic disk reflection models. The
model for the soft X-ray spectrum (i.e. < 10 keV) suggests a/M = 0.56(2) and
excludes zero spin at the 4 sigma level of confidence. The model for the full
broadband spectrum suggests that the spin may be higher, a/M = 0.98(1) (1 sigma
confidence), and again excludes zero spin at the 2 sigma level of confidence.
We discuss these results in the context of other spin constraints and inner
disk studies in GRS 1915+105.Comment: Accepted for publication in Ap
A systematic review and meta-analysis of Macroplastique for treating female stress urinary incontinence.
Introduction and hypothesisMacroplastique® (polydimethylsiloxane injection) is a minimally invasive urethral bulking agent with global clinical literature describing its use over 20 years. This study critically assessed the safety and effectiveness outcomes for adult women treated with Macroplastique for stress urinary incontinence (SUI) through a systematic review and meta-analysis.MethodsA systematic review of the scientific literature from 1990 to 2010 was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement to quantitatively summarize the safety and effectiveness of Macroplastique for female SUI. A total of 958 patients from 23 cohorts were eligible for inclusion and were analyzed. Random-effects models were used to estimate the improvement and cure rates following treatment at three time periods: short-term (<6 months), mid-term (6-18 months), and long-term (>18 months). Expanded models assessed the effect of reinjection rate on successful treatment outcomes. Adverse event rates were aggregated and reported.ResultsImprovement rates were 75 % [95 % confidence interval (CI), 69-81] in the short-term, 73 % (95 % CI, 62-83) in the mid-term, and 64 % (95 % CI, 57-71) long-term. Cure/dry rates were 43 % (95 % CI, 33-54), 37 % (95 % CI, 28-46), and 36 % (95 % CI, 27-46) over the same respective follow-up periods. Higher study reinjection rates were associated with improved long-term SUI outcomes. No serious adverse events were reported.ConclusionsThis quantitative review supports Macroplastique as an effective, durable, and safe treatment option for female SUI. Meta-analytic evidence suggests that long-term therapeutic benefit is frequently maintained, with some patients requiring reinjection
Band Symmetries and Singularities in Twisted Multilayer Graphene
The electronic spectra of rotationally faulted graphene bilayers are
calculated using a continuum formulation for small fault angles that identifies
two distinct electronic states of the coupled system. The low energy spectra of
one state features a Fermi velocity reduction which ultimately leads to
pairwise annihilation and regeneration of its low energy Dirac nodes. The
physics in the complementary state is controlled by pseudospin selection rules
that prevent a Fermi velocity renormalization and produce second generation
symmetry-protected Dirac singularities in the spectrum. These results are
compared with previous theoretical analyses and with experimental data.Comment: 5 pages, 3 figure
Second order parameter-uniform convergence for a finite difference method for a singularly perturbed linear reaction-diffusion system
A singularly perturbed linear system of second order ordinary differential
equations of reaction-diffusion type with given boundary conditions is
considered. The leading term of each equation is multiplied by a small positive
parameter. These singular perturbation parameters are assumed to be distinct.
The components of the solution exhibit overlapping layers. Shishkin
piecewise-uniform meshes are introduced, which are used in conjunction with a
classical finite difference discretisation, to construct a numerical method for
solving this problem. It is proved that the numerical approximations obtained
with this method is essentially second order convergent uniformly with respect
to all of the parameters
- …