9,741 research outputs found
Chandra Observations of X-ray Weak Narrow-Line Seyfert 1 Galaxies
We present Chandra observations of 17 optically-selected, X-ray weak
narrow-line Seyfert 1 (NLS1) galaxies. These objects were optically identified
by Williams et al. (2002) in the Sloan Digital Sky Survey Early Data Release,
but were not found in the ROSAT All-Sky Survey (RASS) despite having optical
properties similar to RASS-detected NLS1s. All objects in this sample were
detected by Chandra and exhibit a range of 0.5-2 keV photon indices
Gamma=1.1-3.4. One object was not detected in the soft band, but has a best-fit
Gamma=0.25 over the full 0.5-8 keV range. These photon indices extend to values
far below what are normally observed in NLS1s. A composite X-ray spectrum of
the hardest objects in this sample does not show any signs of absorption,
although the data do not prohibit one or two of the objects from being highly
absorbed. We also find a strong correlation between Gamma and L_1keV; this may
be due to differences in L_bol/L_Edd among the NLS1s in this sample. Such
variations are seemingly in conflict with the current paradigm that NLS1s
accrete near the Eddington limit. Most importantly, this sample shows that
strong, ultrasoft X-ray emission is not a universal characteristic of NLS1s; in
fact, a substantial number may exhibit weak and/or low-Gamma X-ray emission.Comment: Minor changes, added section on X-ray weakness. 25 pages incl. 6
figures, 3 tables, accepted to Ap
The decay pi0 to gamma gamma to next to leading order in Chiral Perturbation Theory
The two photon decay width of the neutral pion is analyzed within the
combined framework of Chiral Perturbation Theory and the 1/Nc expansion up to
order p^6 and p^4 times 1/Nc in the decay amplitude. The eta' is explicitly
included in the analysis. It is found that the decay width is enhanced by about
4.5% due to the isospin-breaking induced mixing of the pure U(3) states. This
effect, which is of leading order in the low energy expansion, is shown to
persist nearly unchanged at next to leading order. The chief prediction for the
width with its estimated uncertainty is 8.10+-0.08 eV. This prediction at the
1% level makes the upcomming precision measurement of the decay width even more
urgent.
Observations on the eta and eta' can also be made, especially about their
mixing, which is shown to be significantly affected by next to leading order
corrections.Comment: 21 pages, two figure
Advances in reproductive biology and seed production systems of Eucalyptus: The case of Eucalyptus globulus
Eucalyptus globulus is the main eucalypt species grown in Australian plantations. The focus
on seedling deployment systems, coupled with exploitation of large, open-pollinated base
populations for breeding purposes over the last two decades, has required a detailed
understanding of the reproductive biology of this species. We review our research on the
reproductive biology of E. globulus, with a focus on it’s breeding system and advances made
in seed production systems. While most improved seed is still obtained from open-pollinated
seedling or grafted seed orchards, the development of the one-stop/single-visit pollination
procedure has revolutionised the breeding and deployment of this species. The reduced
costs of controlled pollination has meant full pedigree-control can now be maintained in large
advanced generation breeding populations and E. globulus is one of the few eucalypt
species where large-scale production of manually pollinated seed for family forestry is being
undertaken
Schematic Models for Active Nonlinear Microrheology
We analyze the nonlinear active microrheology of dense colloidal suspensions
using a schematic model of mode-coupling theory. The model describes the
strongly nonlinear behavior of the microscopic friction coefficient as a
function of applied external force in terms of a delocalization transition. To
probe this regime, we have performed Brownian dynamics simulations of a system
of quasi-hard spheres. We also analyze experimental data on hard-sphere-like
colloidal suspensions [Habdas et al., Europhys. Lett., 2004, 67, 477]. The
behavior at very large forces is addressed specifically
Quantum-mechanical calculation of Stark widths of Ne VII n=3, transitions
The Stark widths of the Ne VII 2s3s-2s3p singlet and triplet lines are
calculated in the impact approximation using quantum-mechanical Convergent
Close-Coupling and Coulomb-Born-Exchange approximations. It is shown that the
contribution from inelastic collisions to the line widths exceeds the elastic
width contribution by about an order of magnitude. Comparison with the line
widths measured in a hot dense plasma of a gas-liner pinch indicates a
significant difference which may be naturally explained by non-thermal Doppler
effects from persistent implosion velocities or turbulence developed during the
pinch implosion. Contributions to the line width from different partial waves
and types of interactions are discussed as well.Comment: 8 pages, 3 figures; accepted by Phys. Rev.
Weekend admission and mortality for gastrointestinal disorders across England and Wales
BACKGROUND: Little has been reported on mortality following admissions at weekends for many gastrointestinal (GI) disorders. The aim was to establish whether GI disorders are susceptible to increased mortality following unscheduled admission on weekends compared with weekdays. METHODS: Record linkage was undertaken of national administrative inpatient and mortality data for people in England and Wales who were hospitalized as an emergency for one of 19 major GI disorders. RESULTS: The study included 2 254 701 people in England and 155 464 in Wales. For 11 general surgical and medical GI disorders there were little, or no, significant weekend effects on mortality at 30 days in either country. There were large consistent weekend effects in both countries for severe liver disease (England: 26·2 (95 per cent c.i. 21·1 to 31·6) per cent; Wales: 32·0 (12·4 to 55·1 per cent) and GI cancer (England: 21·8 (19·1 to 24·5) per cent; Wales: 25·0 (15·0 to 35·9) per cent), which were lower in patients managed by surgeons. Admission rates were lower at weekends than on weekdays, most strongly for severe liver disease (by 43·3 per cent in England and 51·4 per cent in Wales) and GI cancer (by 44·6 and 52·8 per cent respectively). Both mortality and the weekend mortality effect for GI cancer were lower for patients managed by surgeons. DISCUSSION: There is little, or no, evidence of a weekend mortality effect for most major general surgical or medical GI disorders, but large weekend effects for GI cancer and severe liver disease. Lower admission rates at weekends indicate more severe cases. The findings for severe liver disease may suggest a lack of specialist hepatological resources. For cancers, reduced availability of end-of-life care in the community at weekends may be the cause
Dust in the bright supernova remnant N49 in the LMC
We investigate the dust associated with the supernova remnant (SNR) N49 in
the Large Magellanic Cloud (LMC) as observed with the Herschel Space
Observatory. N49 is unusually bright because of an interaction with a molecular
cloud along its eastern edge. We have used PACS and SPIRE to measure the far IR
flux densities of the entire SNR and of a bright region on the eastern edge of
the SNR where the SNR shock is encountering the molecular cloud. Using these
fluxes supplemented with archival data at shorter wavelengths, we estimate the
dust mass associated with N49 to be about 10 Msun. The bulk of the dust in our
simple two-component model has a temperature of 20-30 K, similar to that of
nearby molecular clouds. Unfortunately, as a result of the limited angular
resolution of Herschel at the wavelengths sampled with SPIRE, the uncertainties
are fairly large. Assuming this estimate of the dust mass associated with the
SNR is approximately correct, it is probable that most of the dust in the SNR
arises from regions where the shock speed is too low to produce significant
X-ray emission. The total amount of warm 50-60 K dust is ~0.1 or 0.4 Msun,
depending on whether the dust is modeled in terms of carbonaceous or silicate
grains. This provides a firm lower limit to the amount of shock heated dust in
N49.Comment: accepted by the Astronomy & Astrophysics Lette
Alpha-Relaxation Processes in Binary Hard-Sphere Mixtures
Molecular-dynamics simulations are presented for two correlation functions
formed with the partial density fluctuations of binary hard-sphere mixtures in
order to explore the effects of mixing on the evolution of glassy dynamics upon
compressing the liquid into high-density states. Partial-density-fluctuation
correlation functions for the two species are reported. Results for the
alpha-relaxation process are quantified by parameters for the strength, the
stretching, and the time scale, where the latter varies over almost four orders
of magnitude upon compression. The parameters exhibit an appreciable dependence
on the wave vector; and this dependence is different for the correlation
function referring to the smaller and that for the larger species. These
features are shown to be in semi-quantitative agreement with those calculated
within the mode-coupling theory for ideal liquid-glass transitions.Comment: 14 pages, 20 figures, RevTe
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