1,565 research outputs found
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
On the Nature of the Bright Short-Period X-ray Source in the Circinus Galaxy Field
The spectrum and light curve of the bright X-ray source CG X-1 in the field
of the Circinus galaxy are re-examined. Previous analyses have concluded that
the source is an accreting black hole of about 50 solar masses although it was
noted that the light curve resembles that of an AM Her system. Here we show
that the short period and an assumed main sequence companion constrain the mass
of the companion to less than one solar mass. Further a possible eclipse seen
during one of the Chandra observations and a subsequent XMM-Newton observation
constrains the mass of the compact object to less than about 60 solar masses.
If such a system lies in the Circinus galaxy, then the accreting object must
either radiate anisotropically or strongly violate the Eddington limit. Even if
the emission is beamed, then the companion star which intercepts this flux
during eclipse will be driven out of thermal equilibrium and evaporate within
about 1000 years. We find that the observations cannot rule out an AM Her
system in the Milky Way and that such a system can account for the variations
seen in the light curve.Comment: 25 pages, 8 figures, accepted for publication in the Astrophysical
Journa
XTE J1739-302: An Unusual New X-ray Transient
A new x-ray transient, designated XTE J1739-302, was discovered with the
Proportional Counter Array (PCA) on the Rossi X-ray Timing Explorer (RXTE) in
data from 12 August 1997. Although it was the brightest source in the Galactic
Center region while active (about 3.0 x 10^-9 ergs/cm2/s from 2 to 25 keV), it
was only observed on that one day; it was not detectable nine days earlier or
two days later. There is no known counterpart at other wavelengths, and its
proximity to the Galactic Center will make such an identification difficult due
to source confusion and extinction. The x-ray spectrum and intensity suggest a
giant outburst of a Be/neutron star binary, although no pulsations were
observed and the outburst was shorter than is usual from these systems.Comment: 11 pages incorporating 6 figures, AAStex; accepted for The
Astrophysical Journal, Part 2 (Letters
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
RXTE Observations of 1A 1744-361: Correlated Spectral and Timing Behavior
We analyze Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array
(PCA) data of the transient low mass X-ray binary (LMXB) system 1A 1744-361. We
explore the X-ray intensity and spectral evolution of the source, perform
timing analysis, and find that 1A 1744-361 shows `atoll' behavior during the
outbursts. The color-color diagram indicates that this LMXB was observed in a
low intensity spectrally hard (low-hard) state and in a high intensity `banana'
state. The low-hard state shows a horizontal pattern in the color-color
diagram, and the previously reported `dipper QPO' appears only during this
state. We also perform energy spectral analyses, and report the first detection
of broad iron emission line and iron absorption edge from 1A 1744-361.Comment: 20 pages, 4 tables, 4 figures, accepted for publication in Ap
BeppoSAX measurements of the bright gamma-ray burst 010222
We analyze the BeppoSAX measurements of the prompt and afterglow emission of
the gamma-ray burst GRB010222. Among 45 GRBs detected with the Wide Field
Cameras on BeppoSAX, the 40-700 keV fluence of (9.3+/-0.3)E-5 erg cm-2 is only
surpassed by GRB990123. In terms of the isotropic 20-2000 keV energy output of
7.8E53 erg, it ranks third of all GRBs with measured distances. Since this
burst is so bright, the data provide complete and valuable coverage up to 65 hr
after the event, except for a gap between 3.5 and 8.0 hr. The 2-10 keV flux
history shows clear signs of a break which is consistent with a break seen in
the optical, and provides supporting evidence for the achromatic nature of the
break. An explanation for the break in the context of a collimated expansion is
not straightforward. Rather, a model is favored whereby the fireball is braked
to the non-relativistic regime quickly (within a fraction of day) by a dense
1E6 cm-3 circumburst medium. This implies that, after a mild beaming
correction, GRB010222 may be the most energetic burst observed thus far. The
X-ray decay index after the break is 1.33+/-0.04, the spectral index
0.97+/-0.05. The decay is, with unprecedented accuracy, identical to that
observed in the optical.Comment: Accepted on June 6 for publication in ApJ part I. Publication due in
October 2001. Accepted version has only minor modification
X-ray afterglow detection of the short gamma-ray burst 991014
GRB 991014 is one of the shortest gamma-ray bursts detected so far with the
Wide Field Cameras aboard BeppoSAX, both in gamma-rays and X-rays. The duration
is 9.6 sec in 2-28 keV and 3.2 sec in 40 to 700 keV (as measured between the
times when 5 and 95% of the burst photons have been accumulated). We refine the
InterPlanetary Network annulus of the burst, present the detection of the X-ray
afterglow of GRB 991014 within this refined annulus, and discuss X-ray and
gamma-ray observations of the prompt and afterglow emission. Except for the
briefness of the prompt event, no other unusual aspects were found in the
prompt and afterglow observations as compared to such measurements in previous
gamma-ray bursts.Comment: accepted for publication in Astrophysical Journa
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