134 research outputs found
Correlating atom probe tomography with x-ray and electron spectroscopies to understand microstructure-activity relationships in electrocatalysts
The search for a new energy paradigm with net-zero carbon emissions requires
new technologies for energy generation and storage that are at the crossroad
between engineering, chemistry, physics, surface and materials sciences. To
keep pushing the inherent boundaries of device performance and lifetime, we
need to step away from a cook-and-look approach and aim to establish the
scientific ground to guide the design of new materials. This requires strong
efforts in establishing bridges between microscopy and spectroscopy techniques,
across multiple scales. Here, we discuss how the complementarities of X-ray-
and electron-based spectroscopies and atom probe tomography can be exploited in
the study of surfaces and sub-surfaces to understand structure-property
relationships in electrocatalysts
Radio emission models of Colliding-Wind Binary Systems
We present calculations of the spatial and spectral distribution of the radio
emission from a wide WR+OB colliding-wind binary system based on
high-resolution hydrodynamical simulations and solutions to the radiative
transfer equation. We account for both thermal and synchrotron radio emission,
free-free absorption in both the unshocked stellar wind envelopes and the
shocked gas, synchrotron self-absorption, and the Razin effect. The
applicability of these calculations to modelling radio images and spectra of
colliding-wind systems is demonstrated with models of the radio emission from
the wide WR+OB binary WR147. Its synchrotron spectrum follows a power-law
between 5 and 15 GHz but turns down to below this at lower and higher
frequencies. We find that while free-free opacity from the circum-binary
stellar winds can potentially account for the low-frequency turnover, models
that also include a combination of synchrotron self-absorption and Razin effect
are favoured. We argue that the high-frequency turn down is a consequence of
inverse-Compton cooling. We present our resulting spectra and intensity
distributions, along with simulated MERLIN observations of these intensity
distributions. From these we argue that the inclination of the WR147 system to
the plane of the sky is low. We summarise by considering extensions of the
current model that are important for models of the emission from closer
colliding wind binaries, in particular the dramatically varying radio emission
of WR140.Comment: 18 pages, 18 figures; Accepted by Astronomy and Astrophysics, July 8,
200
Arecibo Pulsar Survey Using ALFA. I. Survey Strategy and First Discoveries
We report results from the initial stage of a long-term pulsar survey of the
Galactic plane using the Arecibo L-band Feed Array (ALFA), a seven-beam
receiver operating at 1.4 GHz with 0.3 GHz bandwidth. The search targets
Galactic latitudes |b| < 5 deg in the longitude ranges 32 deg < l < 77 deg and
168 deg < l < 77 deg. Data discussed here were collected over a 100 MHz
passband centered on 1.42 GHz using a spectrometer that recorded 256 channels
every 64 microsec. In a preliminary, standard period-DM analysis, we have
detected 29 previously known pulsars and discovered 11 new ones. One of these,
with a period of 69 ms and a low characteristic age of 82 kyr, is a plausible
candidate for association with the unidentified EGRET source 3EG J1928+1733.
Another is a non-recycled pulsar in a relativistic binary with orbital period
of 3.98 hr. We also search the data for isolated dispersed pulses, a technique
that yielded discovery of an extremely sporadic radio emitter with a spin
period of 1.2 s. Simulations we have carried out indicate that about 1000 new
pulsars will be found in the ALFA survey. In addition to providing a large
sample for use in population analyses and for probing the magnetoionic
interstellar medium, the survey maximizes the chances of finding rapidly
spinning millisecond pulsars and pulsars in compact binary systems. Our search
algorithms will exploit the multiple data streams from ALFA to discriminate
between radio frequency interference and celestial signals, including pulsars
and possibly new classes of transient radio sources.Comment: 10 pp, 9 figures, accepted by the Astrophysical Journa
An Eccentric Binary Millisecond Pulsar in the Galactic Plane
Binary pulsar systems are superb probes of stellar and binary evolution and
the physics of extreme environments. In a survey with the Arecibo telescope, we
have found PSR J1903+0327, a radio pulsar with a rotational period of 2.15 ms
in a highly eccentric (e = 0.44) 95-day orbit around a solar mass companion.
Infrared observations identify a possible main-sequence companion star.
Conventional binary stellar evolution models predict neither large orbital
eccentricities nor main-sequence companions around millisecond pulsars.
Alternative formation scenarios involve recycling a neutron star in a globular
cluster then ejecting it into the Galactic disk or membership in a hierarchical
triple system. A relativistic analysis of timing observations of the pulsar
finds its mass to be 1.74+/-0.04 Msun, an unusually high value.Comment: 28 pages, 4 figures inc Supplementary On-Line Material. Accepted for
publication in Science, published on Science Express: 10.1126/science.115758
The epidemiology of Plasmodium falciparum and Plasmodium vivax in East Sepik Province, Papua New Guinea, pre- and post-implementation of national malaria control efforts
Background
In the past decade, national malaria control efforts in Papua New Guinea (PNG) have received renewed support, facilitating nationwide distribution of free long-lasting insecticidal nets (LLINs), as well as improvements in access to parasite-confirmed diagnosis and effective artemisinin-combination therapy in 2011â2012.
Methods
To study the effects of these intensified control efforts on the epidemiology and transmission of Plasmodium falciparum and Plasmodium vivax infections and investigate risk factors at the individual and household level, two cross-sectional surveys were conducted in the East Sepik Province of PNG; one in 2005, before the scale-up of national campaigns and one in late 2012-early 2013, after 2 rounds of LLIN distribution (2008 and 2011â2012). Differences between studies were investigated using Chi square (Ï2), Fischerâs exact tests and Studentâs t-test. Multivariable logistic regression models were built to investigate factors associated with infection at the individual and household level.
Results
The prevalence of P. falciparum and P. vivax in surveyed communities decreased from 55% (2005) to 9% (2013) and 36% to 6%, respectively. The mean multiplicity of infection (MOI) decreased from 1.8 to 1.6 for P. falciparum (pâ=â0.08) and from 2.2 to 1.4 for P. vivax (pââ50% of household members with Plasmodium infection).
Conclusion
After the scale-up of malaria control interventions in PNG between 2008 and 2012, there was a substantial reduction in P. falciparum and P. vivax infection rates in the studies villages in East Sepik Province. Understanding the extent of local heterogeneity in malaria transmission and the driving factors is critical to identify and implement targeted control strategies to ensure the ongoing success of malaria control in PNG and inform the development of tools required to achieve elimination. In household-based interventions, diagnostics with a sensitivity similar to (expert) microscopy could be used to identify and target high rate households
PSR J1856+0245: Arecibo Discovery of a Young, Energetic Pulsar Coincident with the TeV Gamma-ray Source HESS J1857+026
We present the discovery of the Vela-like radio pulsar J1856+0245 in the
Arecibo PALFA survey. PSR J1856+0245 has a spin period of 81ms, a
characteristic age of 21kyr, and a spin-down luminosity Edot = 4.6 x 10^36
ergs/s. It is positionally coincident with the TeV gamma-ray source HESS
J1857+026, which has no other known counterparts. Young, energetic pulsars
create wind nebulae, and more than a dozen pulsar wind nebulae have been
associated with very-high-energy (100GeV-100TeV) gamma-ray sources discovered
with the HESS telescope. The gamma-ray emission seen from HESS J1857+026 is
potentially produced by a pulsar wind nebula powered by PSR J1856+0245; faint
X-ray emission detected by ASCA at the pulsar's position supports this
hypothesis. The inferred gamma-ray efficiency is epsilon_gamma = L_gamma/Edot =
3.1% (1-10TeV, for a distance of 9kpc), comparable to that observed in similar
associations.Comment: 13 pages, 1 figure, accepted for publication in The Astrophysical
Journal Letter
Probing catalytic surfaces by correlative scanning photoemission electron microscopy and atom probe tomography
The chemical composition and the electronic state of the surface of alloys or mixed oxides with enhanced electrocatalytic properties are usually heterogeneous at the nanoscale. The non-uniform distribution of the potential across their surface affects both activity and stability. Studying such heterogeneities at the relevant length scale is crucial for understanding the relationships between structure and catalytic behaviour. Here, we demonstrate an experimental approach combining scanning photoemission electron microscopy and atom probe tomography performed at identical locations to characterise the surface's structure and oxidation states, and the chemical composition of the surface and sub-surface regions. Showcased on an Ir-Ru thermally grown oxide, an efficient catalyst for the anodic oxygen evolution reaction, the complementary techniques yield consistent results in terms of the determined surface oxidation states and local oxide stoichiometry. Significant chemical heterogeneities in the sputter-deposited Ir-Ru alloy thin films govern the oxide's chemistry, observed after thermal oxidation both laterally and vertically. While the oxide grains have a composition of Ir0.94Ru0.06O2, the composition in the grain boundary region varies from Ir0.70Ru0.30O2 to Ir0.40Ru0.60O2 and eventually to Ir0.75Ru0.25O2 from the top surface into the depth. The influence of such compositional non-uniformities on the catalytic performance of the material is discussed, along with possible engineering levers for the synthesis of more stable and reactive mixed oxides. The proposed method provides a framework for investigating materials of interest in the field of electrocatalysis and beyond
The Binary Companion of Young, Relativistic Pulsar J1906+0746
PSR J1906+0746 is a young pulsar in the relativistic binary with the
second-shortest known orbital period, of 3.98 hours. We here present a timing
study based on five years of observations, conducted with the 5 largest radio
telescopes in the world, aimed at determining the companion nature. Through the
measurement of three post-Keplerian orbital parameters we find the pulsar mass
to be 1.291(11) M_sol, and the companion mass 1.322(11) M_sol respectively.
These masses fit well in the observed collection of double neutron stars, but
are also compatible with other white dwarfs around young pulsars such as
J1906+0746. Neither radio pulsations nor dispersion-inducing outflows that
could have further established the companion nature were detected. We derive an
HI-absorption distance, which indicates that an optical confirmation of a white
dwarf companion is very challenging. The pulsar is fading fast due to geodetic
precession, limiting future timing improvements. We conclude that young pulsar
J1906+0746 is likely part of a double neutron star, or is otherwise orbited by
an older white dwarf, in an exotic system formed through two stages of mass
transfer.Comment: 13 pages, 10 figures. Accepted for Ap
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