112 research outputs found
The nature of the intranight variability of radio-quiet quasars
We select a sample of 10 radio-quiet quasars with confirmed intranight
optical variability and with available X-ray data. We compare the variability
properties and the broad band spectral constraints to the predictions of
intranight variability by three models: (i) irradiation of an accretion disk by
a variable X-ray flux (ii) an accretion disk instability (iii) the presence of
a weak blazar component. We concluded that the third model, e.g. the blazar
component model, is the most promising if we adopt a cannonball model for the
jet variable emission. In this case, the probability of detecting the
intranight variability is within 20-80%, depending on the ratio of the disk to
the jet optical luminosity. Variable X-ray irradiation mechanism is also
possible but only under additional requirement: either the source should have a
very narrow Hbeta line or occasional extremely strong flares should appear at
very large disk radii.Comment: MNRAS (in press
A multiple-beam CLEAN for imaging intra-day variable radio sources
The CLEAN algorithm, widely used in radio interferometry for the
deconvolution of radio images, performs well only if the raw radio image (dirty
image) is, to good approximation, a simple convolution between the instrumental
point-spread function (dirty beam) and the true distribution of emission across
the sky. An important case in which this approximation breaks down is during
frequency synthesis if the observing bandwidth is wide enough for variations in
the spectrum of the sky to become significant. The convolution assumption also
breaks down, in any situation but snapshot observations, if sources in the
field vary significantly in flux density over the duration of the observation.
Such time-variation can even be instrumental in nature, for example due to
jitter or rotation of the primary beam pattern on the sky during an
observation. An algorithm already exists for dealing with the spectral
variation encountered in wide-band frequency synthesis interferometry. This
algorithm is an extension of CLEAN in which, at each iteration, a set of N
`dirty beams' are fitted and subtracted in parallel, instead of just a single
dirty beam as in standard CLEAN. In the wide-band algorithm the beams are
obtained by expanding a nominal source spectrum in a Taylor series, each term
of the series generating one of the beams. In the present paper this algorithm
is extended to images which contain sources which vary over both frequency and
time. Different expansion schemes (or bases) on the time and frequency axes are
compared, and issues such as Gibbs ringing and non-orthogonality are discussed.
It is shown that practical considerations make it often desirable to
orthogonalize the set of beams before commencing the cleaning. This is easily
accomplished via a Gram-Schmidt technique.Comment: 9 pages, 7 figures. Accepted for publication in A&
A Search for Propylene Oxide and Glycine in Sagittarius B2 (LMH) and Orion
We have used the Mopra Telescope to search for glycine and the simple chiral
molecule propylene oxide in the Sgr B2 (LMH) and Orion KL, in the 3-mm band. We
have not detected either species, but have been able to put sensitive upper
limits on the abundances of both molecules. The 3-sigma upper limits derived
for glycine conformer I are 3.7 x 10^{14} cm^{-2} in both Orion-KL and Sgr B2
(LMH), comparable to the reported detections of conformer I by Kuan et al.
However, as our values are 3-sigma upper limits rather than detections we
conclude that this weighs against confirming the detection of Kuan et al. We
find upper limits for the glycine II column density of 7.7 x 10^{12} cm^{-2} in
both Orion-KL and Sgr B2 (LMH), in agreement with the results of Combes et al.
The results presented here show that glycine conformer II is not present in the
extended gas at the levels detected by Kuan et al. for conformer I. Our ATCA
results (Jones et al.) have ruled out the detection of glycine (both conformers
I and II) in the compact hot core of the LMH at the levels reported, so we
conclude that it is unlikely that Kuan et al. have detected glycine in either
Sgr B2 or Orion-KL. We find upper limits for propylene oxide abundance of 3.0 x
10^{14} cm^{-2} in Orion-KL and 6.7 x 10^{14} cm^{-2} in Sgr B2 (LMH). We have
detected fourteen features in Sgr B2 and four features in Orion-KL which have
not previously been reported in the ISM, but have not be able to plausibly
assign these transitions to any carrier.Comment: 12 pages, 3 figures. Accepted by MNRAS 12th January 200
A panchromatic view of PKS 0558-504: an ideal laboratory to study the disk-jet link
PKS 0558-504 is the brightest radio-loud Narrow-Line Seyfert 1 galaxy at
X-ray energies. Here we present results from the radio, optical, UV, and X-ray
bands obtained with Swift, XMM, and ATCA during a 10-day monitoring campaign in
September 2008. The simultaneous coverage at several wavelengths makes it
possible to investigate in detail the broadband spectral energy distribution
(SED) and the energetic of this source. The main results can be summarized as
follows. The ATCA reveals the presence of an extended radio emission in PKS
0558-504 with two lobe-like structures at ~7" from the bright central source.
The extended radio structure and the low value of the radio-loudness similar to
radio-quiet Seyfert galaxies coupled with constraints from higher energy bands
argue against a jet-dominated emission. The study of the SED, which is
dominated by a nearly constant optical-UV emission, supports the conclusion
that PKS 0558-504 is accreting at super-Eddington rate. This conclusion was
reached assuming M_BH=2.5e8 M_sun, which was obtained with a new scaling method
based on X-ray spectral variability results. A comparison between the accretion
luminosity and the kinetic power associated with the jet suggests that in this
source the accretion power dominates in agreement with the results obtained
from Radiation-MHD simulations of Galactic black holes (GBHs) accreting at the
Eddington rate. The combined findings from this panchromatic investigation
strongly suggest that PKS 0558-504 is a large-scale analog of GBHs in their
highly accreting intermediate state. Importantly, PKS 0558-504 may also be the
prototype of the parent population of the very radio-loud NLS1s recently
detected at gamma-ray energies.Comment: 11 pages, 5 figures, 3 tables, accepted for publication in Ap
Sub-Milliarcsecond Imaging of Quasars and Active Galactic Nuclei. II. Additional Sources
We report further results from our imaging survey at 15GHz (lambda=2 cm) with
the Very Long Baseline Array. This paper presents single epoch images for 39
sources, bringing the total number of objects in the sample to 171. Our sample
is representative of a complete unbiased sample and it will be used for
statistical analysis of source properties. We compare the observed brightness
temperatures derived from our VLBA observations to those derived from total
intensity variations at 22 and 37 GHz. These are consistent with intrinsic
brightness temperatures in the range 10^10 to 10^12 K. We also present three
new spectroscopic redshift values: z=0.517+/-0.001 for 0026+346,
z=1.591+/-0.003 for 0727-115, and z=0.2016+/-0.0004 for 1155+251.
Images from this VLBA 2 cm survey are available on the Internet under
http://www.cv.nrao.edu/2cmsurvey.Comment: 21 pages, 6 figures, 4 tables, 1 appendix. Accepted for publication
(August 2002 issue) at The Astronomical Journal. Needs aastex.cls,
amsfonts.sty, amssymb.sty, apj.bst, natbib.st
Getting it right when budgets are tight: Using optimal expansion pathways to prioritize responses to concentrated and mixed HIV epidemics.
BACKGROUND: Prioritizing investments across health interventions is complicated by the nonlinear relationship between intervention coverage and epidemiological outcomes. It can be difficult for countries to know which interventions to prioritize for greatest epidemiological impact, particularly when budgets are uncertain. METHODS: We examined four case studies of HIV epidemics in diverse settings, each with different characteristics. These case studies were based on public data available for Belarus, Peru, Togo, and Myanmar. The Optima HIV model and software package was used to estimate the optimal distribution of resources across interventions associated with a range of budget envelopes. We constructed "investment staircases", a useful tool for understanding investment priorities. These were used to estimate the best attainable cost-effectiveness of the response at each investment level. FINDINGS: We find that when budgets are very limited, the optimal HIV response consists of a smaller number of 'core' interventions. As budgets increase, those core interventions should first be scaled up, and then new interventions introduced. We estimate that the cost-effectiveness of HIV programming decreases as investment levels increase, but that the overall cost-effectiveness remains below GDP per capita. SIGNIFICANCE: It is important for HIV programming to respond effectively to the overall level of funding availability. The analytic tools presented here can help to guide program planners understand the most cost-effective HIV responses and plan for an uncertain future
Optima TB: A tool to help optimally allocate tuberculosis spending.
Approximately 85% of tuberculosis (TB) related deaths occur in low- and middle-income countries where health resources are scarce. Effective priority setting is required to maximise the impact of limited budgets. The Optima TB tool has been developed to support analytical capacity and inform evidence-based priority setting processes for TB health benefits package design. This paper outlines the Optima TB framework and how it was applied in Belarus, an upper-middle income country in Eastern Europe with a relatively high burden of TB. Optima TB is a population-based disease transmission model, with programmatic cost functions and an optimisation algorithm. Modelled populations include age-differentiated general populations and higher-risk populations such as people living with HIV. Populations and prospective interventions are defined in consultation with local stakeholders. In partnership with the latter, demographic, epidemiological, programmatic, as well as cost and spending data for these populations and interventions are then collated. An optimisation analysis of TB spending was conducted in Belarus, using program objectives and constraints defined in collaboration with local stakeholders, which included experts, decision makers, funders and organisations involved in service delivery, support and technical assistance. These analyses show that it is possible to improve health impact by redistributing current TB spending in Belarus. Specifically, shifting funding from inpatient- to outpatient-focused care models, and from mass screening to active case finding strategies, could reduce TB prevalence and mortality by up to 45% and 50%, respectively, by 2035. In addition, an optimised allocation of TB spending could lead to a reduction in drug-resistant TB infections by 40% over this period. This would support progress towards national TB targets without additional financial resources. The case study in Belarus demonstrates how reallocations of spending across existing and new interventions could have a substantial impact on TB outcomes. This highlights the potential for Optima TB and similar modelling tools to support evidence-based priority setting
Optimizing Investments for a Sustainable and Efficient HIV Response in Togo: Findings From an HIV Allocative Efficiency Study
This report summarizes the findings of an allocative efficiency analysis of Togo’s HIV response. The Government of Togo indicated a desire to mobilize additional resources, including domestic and private resources, for comprehensive HIV services to respond to the goals of the national HIV Strategic Plan. To ensure that the resources that have been, or will be, mobilized are used in the most efficient way, and to determine the allocation of resources that brings the greatest health benefit, the Government of Togo asked the World Bank to conduct an allocative efficiency analysis using the Optima HIV mathematical model. The findings highlighted a significant treatment gap, and argue strongly for additional funding to scale up ART and increase coverage, in particular for key populations. In order to reduce incidence and deaths by 50 percent, resources should be shifted from prevention programs targeting the general low risk population to ART, PMTCT, and non-ART prevention programs targeting key populations
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