1,056 research outputs found
Core-Level Spectroscopy of the Ni/W(110) Interface: Correlation of W Interfacial Core LevelShifts with First-Layer Ni Phases
We have measured W 4f7/2 core-level photoemission spectra from W(1 1 0) in the presence of Ni overlayers, from ∼0.2 to ∼3 monolayers. Interfacial core-level shifts associated with first-layer Ni phases have been identified: −230 ± 15 meV for the 1 × 1 pseudomorphic phase and −70 ± 7 meV for the 7 × 1 close-packed commensurate phase. At higher Ni coverages the interfacial core-level shift is −100 ± 10 meV. These shifts are analyzed using the partial-shift model of Nilsson et al. [Phys. Rev. B 38 (1988) 10357]; the analysis indicates that the difference in binding energies between the 1 × 1 and 7 × 1 phases has a large contribution from structural differences between the two phases
Fermi-LAT Observations of High- and Intermediate-Velocity Clouds: Tracing Cosmic Rays in the Halo of the Milky Way
It is widely accepted that cosmic rays (CRs) up to at least PeV energies are
Galactic in origin. Accelerated particles are injected into the interstellar
medium where they propagate to the farthest reaches of the Milky Way, including
a surrounding halo. The composition of CRs coming to the solar system can be
measured directly and has been used to infer the details of CR propagation that
are extrapolated to the whole Galaxy. In contrast, indirect methods, such as
observations of gamma-ray emission from CR interactions with interstellar gas,
have been employed to directly probe the CR densities in distant locations
throughout the Galactic plane. In this article we use 73 months of data from
the Fermi Large Area Telescope in the energy range between 300 MeV and 10 GeV
to search for gamma-ray emission produced by CR interactions in several high-
and intermediate-velocity clouds located at up to ~ 7 kpc above the Galactic
plane. We achieve the first detection of intermediate-velocity clouds in gamma
rays and set upper limits on the emission from the remaining targets, thereby
tracing the distribution of CR nuclei in the halo for the first time. We find
that the gamma-ray emissivity per H atom decreases with increasing distance
from the plane at 97.5% confidence level. This corroborates the notion that CRs
at the relevant energies originate in the Galactic disk. The emissivity of the
upper intermediate-velocity Arch hints at a 50% decline of CR densities within
2 kpc from the plane. We compare our results to predictions of CR propagation
models.Comment: Accepted for publication in the Astrophysical Journa
The Origin of the Extragalactic Gamma-Ray Background and Implications for Dark-Matter Annihilation
The origin of the extragalactic -ray background (EGB) has been
debated for some time. { The EGB comprises the -ray emission from
resolved and unresolved extragalactic sources, such as blazars, star-forming
galaxies and radio galaxies, as well as radiation from truly diffuse
processes.} This letter focuses on the blazar source class, the most numerous
detected population, and presents an updated luminosity function and spectral
energy distribution model consistent with the blazar observations performed by
the {\it Fermi} Large Area Telescope (LAT). We show that blazars account for
50\,\% of the EGB photons (0.1\,GeV), and that {\it Fermi}-LAT
has already resolved 70\,\% of this contribution. Blazars, and in
particular low-luminosity hard-spectrum nearby sources like BL Lacs, are
responsible for most of the EGB emission above 100\,GeV. We find that the
extragalactic background light, which attenuates blazars' high-energy emission,
is responsible for the high-energy cut-off observed in the EGB spectrum.
Finally, we show that blazars, star-forming galaxies and radio galaxies can
naturally account for the amplitude and spectral shape of the background in the
0.1--820\,GeV range, leaving only modest room for other contributions. This
allows us to set competitive constraints on the dark-matter annihilation cross
section.Comment: On behalf of the Fermi-LAT collaboration. Contact authors: M. Ajello,
D. Gasparrini, M. Sanchez-Conde, G. Zaharijas, M. Gustafsson. Accepted for
publication on ApJ
The Antares Neutrino Telescope and Multi-Messenger Astronomy
Antares is currently the largest neutrino telescope operating in the Northern
Hemisphere, aiming at the detection of high-energy neutrinos from astrophysical
sources. Such observations would provide important clues about the processes at
work in those sources, and possibly help solve the puzzle of ultra-high energy
cosmic rays. In this context, Antares is developing several programs to improve
its capabilities of revealing possible spatial and/or temporal correlations of
neutrinos with other cosmic messengers: photons, cosmic rays and gravitational
waves. The neutrino telescope and its most recent results are presented,
together with these multi-messenger programs.Comment: 10 pages, 7 figures. Proceedings of the 14th Gravitational Wave Data
Analysis Workshop (GWDAW-14) in Roma - January 26th-29th, 201
Identification of QTLs Associated with Fusarium Head Blight Resistance in Barley Accession CIho 4196
Fusarium head blight (FHB), incited by Fusarium graminearum Schwabe [teleomorph Gibberella zea (Schwein)], reduces quality of harvested barley (Hordeum vulgare L.) because of blighted kernels and the presence of deoxynivalenol (DON), a mycotoxin produced by the pathogen. CIho 4196, a two-rowed type, is one of the most resistant accessions known in barley; however, it possesses many undesirable agronomic traits. To better understand the genetics of reduced FHB severity and DON accumulation conferred by CIho 4196, a genetic map was generated using a population of recombinant inbred lines derived from a cross between Foster (a six-rowed malting cultivar) and CIho 4196. Quantitative trait loci (QTL) analyses were performed using data obtained from 10 field environments. The possible associations of resistance QTLs and various agronomic and morphological traits in barley also were investigated. The centromeric region of chromosome 2H flanked by the markers ABG461C and MWG882A (bins 6–10) likely (P\u3c0.001) contains two QTLs contributing to lower FHB severity and plant height, and one QTL each for DON accumulation, days to heading, and rachis node number. The QTL for low FHB severity in the bin 8 region explained from 3 to 9% of the variation, while the QTL in the bin 10 region explained from 17 to 60% of the variation. A QTL for DON accumulation that explained 9 to 14% of the variation was found in the bin 2 region of chromosome 4H. This may represent a new QTL not present in other FHB resistant sources. Resistance QTLs in the bin 8 region and bin 10 region of chromosome 2HL were provisionally designated Qrgz-2H-8 and Qrgz-2H-10, respectively. The QTL for DON accumulation in chromosome 4H was provisionally named QDON-4H-2
Spot form of net blotch resistance in barley is under complex genetic control
Key message: Evaluation of resistance toPyrenophora teresf.maculatain barley breeding populations via association mapping revealed a complex genetic architecture comprising a mixture of major and minor effect genes. Abstract: In the search for stable resistance to spot form of net blotch (Pyrenophora teres f. maculata, SFNB), association mapping was conducted on four independent barley (Hordeum vulgare L.) breeding populations comprising a total of 898 unique elite breeding lines from the Northern Region Barley Breeding Program in Australia for discovery of quantitative trait loci (QTL) influencing resistance at seedling and adult plant growth stages. A total of 29 significant QTL were validated across multiple breeding populations, with 22 conferring resistance at both seedling and adult plant growth stages. The remaining 7 QTL conferred resistance at either seedling (2 QTL) or adult plant (5 QTL) growth stages only. These 29 QTL represented 24 unique genomic regions, of which five were found to co-locate with previously identified QTL for SFNB. The results indicated that SFNB resistance is controlled by a large number of QTL varying in effect size with large effects QTL on chromosome 7H. A large proportion of the QTL acted in the same direction for both seedling and adult responses, suggesting that phenotypic selection for SFNB resistance performed at either growth stage could achieve adequate levels of resistance. However, the accumulation of specific resistance alleles on several chromosomes must be considered in molecular breeding selection strategies
Searching for prompt signatures of nearby core-collapse supernovae by a joint analysis of neutrino and gravitational-wave data
We discuss the science motivations and prospects for a joint analysis of
gravitational-wave (GW) and low-energy neutrino data to search for prompt
signals from nearby supernovae (SNe). Both gravitational-wave and low-energy
neutrinos are expected to be produced in the innermost region of a
core-collapse supernova, and a search for coincident signals would probe the
processes which power a supernova explosion. It is estimated that the current
generation of neutrino and gravitational-wave detectors would be sensitive to
Galactic core-collapse supernovae, and would also be able to detect
electromagnetically dark SNe. A joint GW-neutrino search would enable
improvements to searches by way of lower detection thresholds, larger distance
range, better live-time coverage by a network of GW and neutrino detectors, and
increased significance of candidate detections. A close collaboration between
the GW and neutrino communities for such a search will thus go far toward
realizing a much sought-after astrophysics goal of detecting the next nearby
supernova.Comment: 10 pages, 3 figures. To appear in Class. Quantum Gra
Detecting a stochastic gravitational wave background with the Laser Interferometer Space Antenna
The random superposition of many weak sources will produce a stochastic
background of gravitational waves that may dominate the response of the LISA
(Laser Interferometer Space Antenna) gravitational wave observatory. Unless
something can be done to distinguish between a stochastic background and
detector noise, the two will combine to form an effective noise floor for the
detector. Two methods have been proposed to solve this problem. The first is to
cross-correlate the output of two independent interferometers. The second is an
ingenious scheme for monitoring the instrument noise by operating LISA as a
Sagnac interferometer. Here we derive the optimal orbital alignment for
cross-correlating a pair of LISA detectors, and provide the first analytic
derivation of the Sagnac sensitivity curve.Comment: 9 pages, 11 figures. Significant changes to the noise estimate
Search for Early Gamma-ray Production in Supernovae Located in a Dense Circumstellar Medium with the Fermi LAT
Supernovae (SNe) exploding in a dense circumstellar medium (CSM) are
hypothesized to accelerate cosmic rays in collisionless shocks and emit GeV
gamma rays and TeV neutrinos on a time scale of several months. We perform the
first systematic search for gamma-ray emission in Fermi LAT data in the energy
range from 100 MeV to 300 GeV from the ensemble of 147 SNe Type IIn exploding
in dense CSM. We search for a gamma-ray excess at each SNe location in a one
year time window. In order to enhance a possible weak signal, we simultaneously
study the closest and optically brightest sources of our sample in a
joint-likelihood analysis in three different time windows (1 year, 6 months and
3 months). For the most promising source of the sample, SN 2010jl (PTF10aaxf),
we repeat the analysis with an extended time window lasting 4.5 years. We do
not find a significant excess in gamma rays for any individual source nor for
the combined sources and provide model-independent flux upper limits for both
cases. In addition, we derive limits on the gamma-ray luminosity and the ratio
of gamma-ray-to-optical luminosity ratio as a function of the index of the
proton injection spectrum assuming a generic gamma-ray production model.
Furthermore, we present detailed flux predictions based on multi-wavelength
observations and the corresponding flux upper limit at 95% confidence level
(CL) for the source SN 2010jl (PTF10aaxf).Comment: Accepted for publication in ApJ. Corresponding author: A. Franckowiak
([email protected]), updated author list and acknowledgement
Determining neutrino oscillation parameters from atmospheric muon neutrino disappearance with three years of IceCube DeepCore data
We present a measurement of neutrino oscillations via atmospheric muon
neutrino disappearance with three years of data of the completed IceCube
neutrino detector. DeepCore, a region of denser instrumentation, enables the
detection and reconstruction of atmospheric muon neutrinos between 10 GeV and
100 GeV, where a strong disappearance signal is expected. The detector volume
surrounding DeepCore is used as a veto region to suppress the atmospheric muon
background. Neutrino events are selected where the detected Cherenkov photons
of the secondary particles minimally scatter, and the neutrino energy and
arrival direction are reconstructed. Both variables are used to obtain the
neutrino oscillation parameters from the data, with the best fit given by
and
(normal mass hierarchy assumed). The
results are compatible and comparable in precision to those of dedicated
oscillation experiments.Comment: 10 pages, 7 figure
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