571 research outputs found
Assessment of the risk of tuberculosis transmission for five housing prototypes designed for a Haitian community
The hospitalization of patients affected with tuberculosis (TB) can be particularly long and
burdensome, especially in poor countries where the disease remains a major issue and beds in
health care centres are a precious resource. Therefore a policy of decentralising TB treatment
from hospitals to residential environments is starting to be considered worldwide, and new
guidelines in support of such strategy are needed. This study illustrates a potential “risk
assessment model” for TB transmission in dwellings that might help analysing both existing
building stocks and new designs in order to apply the new policy, utilizing as a reference the
general frame of risk assessment for buildings developed by civil engineering; the model was
then tested on five housing prototypes proposed for the town of Saint Marc, Haiti, showing
that environmental features of a building such as ventilation, crowding, temperature and
relative humidity are among the most important parameters for the estimate of the risk. The
final outcome of the analysis, however, highlighted how he most influential factor on the risk
of spread of infectious diseases is the efficiency of the health care system operating in the
building urban context
Very hard states in neutron star low-mass X-ray binaries
We report on unusually very hard spectral states in three confirmed
neutron-star low-mass X-ray binaries (1RXS J180408.9-342058, EXO 1745-248, and
IGR J18245-2452) at a luminosity between ~ 10^{36-37} erg s^{-1}. When fitting
the Swift X-ray spectra (0.5 - 10 keV) in those states with an absorbed
power-law model, we found photon indices of \Gamma ~ 1, significantly lower
than the \Gamma = 1.5 - 2.0 typically seen when such systems are in their so
called hard state. For individual sources very hard spectra were already
previously identified but here we show for the first time that likely our
sources were in a distinct spectral state (i.e., different from the hard state)
when they exhibited such very hard spectra. It is unclear how such very hard
spectra can be formed; if the emission mechanism is similar to that operating
in their hard states (i.e., up-scattering of soft photons due to hot electrons)
then the electrons should have higher temperatures or a higher optical depth in
the very hard state compared to those observed in the hard state. By using our
obtained \Gamma as a tracer for the spectral evolution with luminosity, we have
compared our results with those obtained by Wijnands et al. (2015). We confirm
their general results in that also our sample of sources follow the same track
as the other neutron star systems, although we do not find that the accreting
millisecond pulsars are systematically harder than the non-pulsating systems.Comment: Accepted for publication in MNRA
The very faint X-ray binary IGR J17062-6143: a truncated disc, no pulsations, and a possible outflow
We present a comprehensive X-ray study of the neutron star low-mass X-ray binary IGR J17062-6143, which has been accreting at low luminosities since its discovery in 2006. Analysing NuSTAR, XMM–Newton, and Swift observations, we investigate the very faint nature of this source through three approaches: modelling the relativistic reflection spectrum to constrain the accretion geometry, performing high-resolution X-ray spectroscopy to search for an outflow, and searching for the recently reported millisecond X-ray pulsations. We find a strongly truncated accretion disc at
77+22−18
gravitational radii (∼164 km) assuming a high inclination, although a low inclination and a disc extending to the neutron star cannot be excluded. The high-resolution spectroscopy reveals evidence for oxygen-rich circumbinary material, possibly resulting from a blueshifted, collisionally ionized outflow. Finally, we do not detect any pulsations. We discuss these results in the broader context of possible explanations for the persistent faint nature of weakly accreting neutron stars. The results are consistent with both an ultra-compact binary orbit and a magnetically truncated accretion flow, although both cannot be unambiguously inferred. We also discuss the nature of the donor star and conclude that it is likely a CO or O–Ne–Mg white dwarf, consistent with recent multiwavelength modelling
High-resolution ellipsometric study of an n-alkane film, dotriacontane, adsorbed on a SiO2 surface
doi:10.1063/1.1429645Using high-resolution ellipsometry and stray light intensity measurements, we have investigated during successive heating-cooling cycles the optical thickness and surface roughness of thin dotriacontane (n-C32H66) films adsorbed from a heptane (n-C7H16) solution onto SiO2-coated Si(100) single-crystal substrates. Our results suggest a model of a solid dotriacontane film that has a phase closest to the SiO2 surface in which the long-axis of the molecules is oriented parallel to the interface. Above this "parallel film" phase, a solid monolayer adsorbs in which the molecules are oriented perpendicular to the interface. At still higher coverages and at temperatures below the bulk melting point at Tb = 341 K, solid bulk particles coexist on top of the "perpendicular film." For higher temperatures in the range TbTs, a uniformly thick fluid film wets to the parallel film phase. This structure of the alkane/SiO2 interfacial region differs qualitatively from that which occurs in the surface freezing effect at the bulk alkane fluid/vapor interface. In that case, there is again a perpendicular film phase adjacent to the air interface but no parallel film phase intervenes between it and the bulk alkane fluid. Similarities and differences between our model of the alkane/SiO2 interface and one proposed recently will be discussed. Our ellipsometric measurements also show evidence of a crystalline-to-plastic transition in the perpendicular film phase similar to that occurring in the solid bulk particles present at higher coverages. In addition, we have performed high-resolution ellipsometry and stray-light measurements on dotriacontane films deposited from solution onto highly oriented pyrolytic graphite substrates. After film deposition, these substrates proved to be less stable in air than SiO2.This work was supported by the Chilean government under CONICYT Grant No. 018/AT/005NSF and FONDECYT Grant No. 1980586 and by the U.S. National Science Foundation under Grant Nos. INT-9605227, DMR-9802476, and DMR-0109057
The LOFT perspective on neutron star thermonuclear bursts
This is a White Paper in support of the mission concept of the Large
Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We
discuss the potential of LOFT for the study of thermonuclear X-ray bursts on
accreting neutron stars. For a summary, we refer to the paper.Comment: White Paper in Support of the Mission Concept of the Large
Observatory for X-ray Timin
Modeling of Disk-Star Interaction: Different Regimes of Accretion and Variability
The appearance and time variability of accreting millisecond X-ray pulsars
(hereafter AMXPs, e.g. Wijnands & van der Klis 1998) depends strongly on the
accretion rate, the effective viscosity and the effective magnetic diffusivity
of the disk-magnetosphere boundary. The accretion rate is the main parameter
which determines the location of the magnetospheric radius of the star for a
given stellar magnetic field. We introduce a classification of accreting
neutron stars as a function of the accretion rate and show the corresponding
stages obtained from our global 3D magnetohydrodynamic (MHD) simulations and
from our axisymmetric MHD simulations. We discuss the expected variability
features in each stage of accretion, both periodic and quasi-periodic (QPOs).
We conclude that the periodicity may be suppressed at both very high and very
low accretion rates. In addition the periodicity may disappear when ordered
funnel flow accretion is replaced by disordered accretion through the
interchange instability.Comment: 10 pages, 6 figures, Invited review, to appear in the proceedings of
the workshop 'A Decade of Accreting Millisecond X-ray Pulsars' (Amsterdam,
April 2008); see animations at
http://www.astro.cornell.edu/~romanova/projects.ht
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