1,605 research outputs found
Soft x-ray magnetic circular dichroism study on Gd-doped EuO thin films
We report on the growth and characterization of ferromagnetic Gd-doped EuO
thin films. We prepared samples with Gd concentrations up to 11% by means of
molecular beam epitaxy under distillation conditions, which allows a very
precise control of the doping concentration and oxygen stoichiometry. Using
soft x-ray magnetic circular dichroism at the Eu and Gd M4,5 edges, we found
that the Curie temperature ranged from 69 K for pure stoichiometric EuO to
about 170 K for the film with the optimal Gd doping of around 4%. We also show
that the Gd magnetic moment couples ferromagnetically to that of Eu.Comment: 4 pages, 4 figure
Discovery of a Second Millisecond Accreting Pulsar: XTE J1751-305
We report the discovery by the RXTE PCA of a second transient accreting
millisecond pulsar, XTE J1751-305, during regular monitoring observations of
the galactic bulge region. The pulsar has a spin frequency of 435 Hz, making it
one of the fastest pulsars. The pulsations contain the signature of orbital
Doppler modulation, which implies an orbital period of 42 minutes, the shortest
orbital period of any known radio or X-ray millisecond pulsar. The mass
function, f_x = (1.278 +/- 0.003) x 10^{-6} M_sun, yields a minimum mass for
the companion of between 0.013 and 0.017 M_sun, depending on the mass of the
neutron star. No eclipses were detected. A previous X-ray outburst in June,
1998, was discovered in archival All-Sky Monitor data. Assuming mass transfer
in this binary system is driven by gravitational radiation, we constrain the
orbital inclination to be in the range 30-85 deg, and the companion mass to be
0.013-0.035 M_sun. The companion is most likely a heated helium dwarf. We also
present results from the Chandra HRC-S observations which provide the best
known position of XTE J1751-305.Comment: Astrophysical Journal Letters, Accepted, (AASTeX
Can the UNAIDS modes of transmission model be improved? A comparison of the original and revised model projections using data from a setting in west Africa.
OBJECTIVE: The UNAIDS modes of transmission model (MoT) is a user-friendly model, developed to predict the distribution of new HIV infections among different subgroups. The model has been used in 29 countries to guide interventions. However, there is the risk that the simplifications inherent in the MoT produce misleading findings. Using input data from Nigeria, we compare projections from the MoT with those from a revised model that incorporates additional heterogeneity. METHODS: We revised the MoT to explicitly incorporate brothel and street-based sex-work, transactional sex, and HIV-discordant couples. Both models were parameterized using behavioural and epidemiological data from Cross River State, Nigeria. Model projections were compared, and the robustness of the revised model projections to different model assumptions, was investigated. RESULTS: The original MoT predicts 21% of new infections occur in most-at-risk-populations (MARPs), compared with 45% (40-75%, 95% Crl) once additional heterogeneity and updated parameterization is incorporated. Discordant couples, a subgroup previously not explicitly modelled, are predicted to contribute a third of new HIV infections. In addition, the new findings suggest that women engaging in transactional sex may be an important but previously less recognized risk group, with 16% of infections occurring in this subgroup. CONCLUSION: The MoT is an accessible model that can inform intervention priorities. However, the current model may be potentially misleading, with our comparisons in Nigeria suggesting that the model lacks resolution, making it challenging for the user to correctly interpret the nature of the epidemic. Our findings highlight the need for a formal review of the MoT
The correlations between the spin frequencies and kHz QPOs of Neutron Stars in LMXBs
We studied the correlations between spin frequencies and kilohertz
quasi-periodic oscillations (kHz QPOs) in neutron star low mass X-ray binaries.
The updated data of kHz QPOs and spin frequencies are statistically analyzed.
We found that when two simultaneous kHz QPOs are present in the power spectrum,
the minimum frequency of upper kHz QPO is at least 1.3 times larger than the
spin frequency, i.e. \nu_{s}<\nu_{2min}/1.3. We also found that the average kHz
QPO peak separation in 6 Atoll sources anti-correlates with the spin frequency
in the form \lan\dn\ran = -(0.19\pm0.05)\ns+(389.40\pm21.67)Hz. If we shifted
this correlation in the direction of the peak separation by a factor of 1.5,
this correlation matches the data points of the two accretion powered
millisecond X-ray pulsars, SAX J1808.4-3658 and XTE J1807-294.Comment: 4 pages, 2 figures, accepted for publication in A&
A note on fermions in holographic QCD
We study the fermionic sector of a probe D8-brane in the supergravity
background made of D4-branes compactified on a circle with supersymmetry broken
explicitly by the boundary conditions. At low energies the dual field theory is
effectively four-dimensional and has proved surprisingly successful in
recovering qualitative and quantitative properties of QCD. We investigate
fluctuations of the fermionic fields on the probe D8-brane and interpret these
as mesinos (fermionic superpartners of mesons). We demonstrate that the masses
of these modes are comparable to meson masses and show that their interactions
with ordinary mesons are not suppressed.Comment: 21+1 pp, 1 figure; v2: typos corrected, refs. adde
Observation of X-ray lines from a Gamma-Ray Burst (GRB991216): Evidence of Moving Ejecta from the Progenitor
We report on the discovery of two emission features observed in the X-ray
spectrum of the afterglow of the gamma-ray burst (GRB) of 16 Dec. 1999 by the
Chandra X-Ray Observatory. These features are identified with the Ly
line and the narrow recombination continuum by hydrogenic ions of iron at a
redshift , providing an unambiguous measurement of the distance
of a GRB. Line width and intensity imply that the progenitor of the GRB was a
massive star system that ejected, before the GRB event, \approx 0.01 \Ms of
iron at a velocity , probably by a supernova explosion.Comment: 11 pages,2 fig.s, link to the published paper in Science, 290, 955
(2000) through http://www.ias.rm.cnr.it/grb/gb991216.htm
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