2,417 research outputs found
Emergence of ferromagnetism and Jahn-Teller distortion in low Cr-substituted LaMnO3
The emergence of a ferromagnetic component in with low Cr-for-Mn
substitution has been studied by x-ray absorption spectroscopy and x-ray
magnetic circular dichroism at the Mn and Cr K edges. The local magnetic moment
strength for the Mn and Cr are proportional to each other and follows the
macroscopic magnetization. The net ferromagnetic components of and
are found antiferromagnetically coupled. Unlike hole doping by La
site substitution, the inclusion of ions up to x = 0.15 does not
decrease the Jahn-Teller (JT) distortion and consequently does not
significantly affect the orbital ordering. This demonstrates that the emergence
of the ferromagnetism is not related to JT weakening and likely arises from a
complex orbital mixing.Comment: 5 figure
Prospects for Pulsar Studies at MeV Energies
Enabled by the Fermi Large Area Telescope, we now know young and recycled
pulsars fill the gamma-ray sky, and we are beginning to understand their
emission mechanism and their distribution throughout the Galaxy. However, key
questions remain: Is there a large population of pulsars near the Galactic
center? Why do the most energetic pulsars shine so brightly in MeV gamma rays
but not always at GeV energies? What is the source and nature of the pair
plasma in pulsar magnetospheres, and what role does the polar cap accelerator
play? Addressing these questions calls for a sensitive, wide-field MeV
telescope, which can detect the population of MeV-peaked pulsars hinted at by
Fermi and hard X-ray telescopes and characterize their spectral shape and
polarization.Comment: 8 pages, 4 figures, Astro2020 Science White Paper submitted to the
National Academies of Science
Réalisation d'un système d'émission-réception 4 canaux dédié au cerveau de rat pour un système RM à 7T
International audienceUn système ainsi qu'une bobine d'émission-réception ont été réalisés pour un système RM à 7 T. Ce système d'émission-réception 4 canaux permet de créer un champ magnétique RF B1+ polarisé circulairement. La combinaison constructive des phases des 4 canaux a été démontrée par des images RM
A population of gamma-ray emitting globular clusters seen with the Fermi Large Area Telescope
Globular clusters with their large populations of millisecond pulsars (MSPs)
are believed to be potential emitters of high-energy gamma-ray emission. Our
goal is to constrain the millisecond pulsar populations in globular clusters
from analysis of gamma-ray observations. We use 546 days of continuous
sky-survey observations obtained with the Large Area Telescope aboard the Fermi
Gamma-ray Space Telescope to study the gamma-ray emission towards 13 globular
clusters. Steady point-like high-energy gamma-ray emission has been
significantly detected towards 8 globular clusters. Five of them (47 Tucanae,
Omega Cen, NGC 6388, Terzan 5, and M 28) show hard spectral power indices and clear evidence for an exponential cut-off in the range
1.0-2.6 GeV, which is the characteristic signature of magnetospheric emission
from MSPs. Three of them (M 62, NGC 6440 and NGC 6652) also show hard spectral
indices , however the presence of an exponential cut-off
can not be unambiguously established. Three of them (Omega Cen, NGC 6388, NGC
6652) have no known radio or X-ray MSPs yet still exhibit MSP spectral
properties. From the observed gamma-ray luminosities, we estimate the total
number of MSPs that is expected to be present in these globular clusters. We
show that our estimates of the MSP population correlate with the stellar
encounter rate and we estimate 2600-4700 MSPs in Galactic globular clusters,
commensurate with previous estimates. The observation of high-energy gamma-ray
emission from a globular cluster thus provides a reliable independent method to
assess their millisecond pulsar populations that can be used to make
constraints on the original neutron star X-ray binary population, essential for
understanding the importance of binary systems in slowing the inevitable core
collapse of globular clusters.Comment: Accepted for publication in A&A. Corresponding authors: J.
Kn\"odlseder, N. Webb, B. Pancraz
Fermi Large Area Telescope Constraints on the Gamma-ray Opacity of the Universe
The Extragalactic Background Light (EBL) includes photons with wavelengths
from ultraviolet to infrared, which are effective at attenuating gamma rays
with energy above ~10 GeV during propagation from sources at cosmological
distances. This results in a redshift- and energy-dependent attenuation of the
gamma-ray flux of extragalactic sources such as blazars and Gamma-Ray Bursts
(GRBs). The Large Area Telescope onboard Fermi detects a sample of gamma-ray
blazars with redshift up to z~3, and GRBs with redshift up to z~4.3. Using
photons above 10 GeV collected by Fermi over more than one year of observations
for these sources, we investigate the effect of gamma-ray flux attenuation by
the EBL. We place upper limits on the gamma-ray opacity of the Universe at
various energies and redshifts, and compare this with predictions from
well-known EBL models. We find that an EBL intensity in the optical-ultraviolet
wavelengths as great as predicted by the "baseline" model of Stecker et al.
(2006) can be ruled out with high confidence.Comment: 42 pages, 12 figures, accepted version (24 Aug.2010) for publication
in ApJ; Contact authors: A. Bouvier, A. Chen, S. Raino, S. Razzaque, A.
Reimer, L.C. Reye
Search for Neutral Higgs Bosons in Events with Multiple Bottom Quarks at the Tevatron
The combination of searches performed by the CDF and D0 collaborations at the
Fermilab Tevatron Collider for neutral Higgs bosons produced in association
with b quarks is reported. The data, corresponding to 2.6 fb-1 of integrated
luminosity at CDF and 5.2 fb-1 at D0, have been collected in final states
containing three or more b jets. Upper limits are set on the cross section
multiplied by the branching ratio varying between 44 pb and 0.7 pb in the Higgs
boson mass range 90 to 300 GeV, assuming production of a narrow scalar boson.
Significant enhancements to the production of Higgs bosons can be found in
theories beyond the standard model, for example in supersymmetry. The results
are interpreted as upper limits in the parameter space of the minimal
supersymmetric standard model in a benchmark scenario favoring this decay mode.Comment: 10 pages, 2 figure
Search for the standard model Higgs boson decaying to a pair in events with no charged leptons and large missing transverse energy using the full CDF data set
We report on a search for the standard model Higgs boson produced in
association with a vector boson in the full data set of proton-antiproton
collisions at TeV recorded by the CDF II detector at the
Tevatron, corresponding to an integrated luminosity of 9.45 fb. We
consider events having no identified charged lepton, a transverse energy
imbalance, and two or three jets, of which at least one is consistent with
originating from the decay of a quark. We place 95% credibility level upper
limits on the production cross section times standard model branching fraction
for several mass hypotheses between 90 and . For a Higgs
boson mass of , the observed (expected) limit is 6.7
(3.6) times the standard model prediction.Comment: Accepted by Phys. Rev. Let
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