2,829 research outputs found
The Brera Multi-scale Wavelet (BMW) ROSAT HRI source catalog. I: the algorithm
We present a new detection algorithm based on the wavelet transform for the
analysis of high energy astronomical images. The wavelet transform, due to its
multi-scale structure, is suited for the optimal detection of point-like as
well as extended sources, regardless of any loss of resolution with the
off-axis angle. Sources are detected as significant enhancements in the wavelet
space, after the subtraction of the non-flat components of the background.
Detection thresholds are computed through Monte Carlo simulations in order to
establish the expected number of spurious sources per field. The source
characterization is performed through a multi-source fitting in the wavelet
space. The procedure is designed to correctly deal with very crowded fields,
allowing for the simultaneous characterization of nearby sources. To obtain a
fast and reliable estimate of the source parameters and related errors, we
apply a novel decimation technique which, taking into account the correlation
properties of the wavelet transform, extracts a subset of almost independent
coefficients. We test the performance of this algorithm on synthetic fields,
analyzing with particular care the characterization of sources in poor
background situations, where the assumption of Gaussian statistics does not
hold. For these cases, where standard wavelet algorithms generally provide
underestimated errors, we infer errors through a procedure which relies on
robust basic statistics. Our algorithm is well suited for the analysis of
images taken with the new generation of X-ray instruments equipped with CCD
technology which will produce images with very low background and/or high
source density.Comment: 8 pages, 6 figures, ApJ in pres
A spherical model with directional interactions: I. Static properties
We introduce a simple spherical model whose structural properties are similar
to the ones generated by models with directional interactions, by employing a
binary mixture of large and small hard spheres, with a square-well attraction
acting only between particles of different size. The small particles provide
the bonds between the large ones. With a proper choice of the interaction
parameters, as well as of the relative concentration of the two species, it is
possible to control the effective valence. Here we focus on a specific choice
of the parameters which favors tetrahedral ordering and study the equilibrium
static properties of the system in a large window of densities and
temperatures. Upon lowering the temperature we observe a progressive increase
in local order, accompanied by the formation of a four-coordinated network of
bonds. Three different density regions are observed: at low density the system
phase separates into a gas and a liquid phase; at intermediate densities a
network of fully bonded particles develops; at high densities -- due to the
competition between excluded volume and attractive interactions -- the system
forms a defective network. The very same behavior has been previously observed
in numerical studies of non-spherical models for molecular liquids, such as
water, and in models of patchy colloidal particles. Differently from these
models, theoretical treatments devised for spherical potentials, e.g. integral
equations and ideal mode coupling theory for the glass transition can be
applied in the present case, opening the way for a deeper understanding of the
thermodynamic and dynamic behavior of low valence molecules and particles.Comment: 11 pages, 11 figure
The Assembly and Merging History of Supermassive Black Holes in Hierarchical Models of Galaxy Formation
We assess models for the assembly of supermassive black holes (SMBHs) at the
center of galaxies that trace their hierarchical build-up far up in the dark
halo `merger tree'. We assume that the first `seed' black holes (BHs) formed in
(mini)halos collapsing at z=20 from high-sigma density fluctuations. As these
pregalactic holes become incorporated through a series of mergers into larger
and larger halos, they sink to the center owing to dynamical friction, accrete
a fraction of the gas in the merger remnant to become supermassive, form a
binary system, and eventually coalesce. The merger history of dark matter halos
and associated BHs is followed by cosmological Monte Carlo realizations of the
merger hierarchy. A simple model, where quasar activity is driven by major
mergers and SMBHs accrete at the Eddington rate a mass that scales with the
fifth power of the velocity dispersion, is shown to reproduce the optical LF of
quasars in the redshift range 1<z<4. Binary and triple BH interactions are
followed in our merger tree. The assumptions underlying our scenario lead to
the prediction of a population of massive BHs wandering in galaxy halos and the
intergalactic medium at the present epoch, and contributing <10% to the total
BH mass density. At all epochs the fraction of binary SMBHs in galaxy nuclei is
of order 10%, while the fraction of binary quasars is less than 0.3%Comment: revised version, accepted for publication in the ApJ, emulateapj, 15
pages, 16 figure
Shilnikov Lemma for a nondegenerate critical manifold of a Hamiltonian system
We prove an analog of Shilnikov Lemma for a normally hyperbolic symplectic
critical manifold of a Hamiltonian system. Using this
result, trajectories with small energy shadowing chains of homoclinic
orbits to are represented as extremals of a discrete variational problem,
and their existence is proved. This paper is motivated by applications to the
Poincar\'e second species solutions of the 3 body problem with 2 masses small
of order . As , double collisions of small bodies correspond to
a symplectic critical manifold of the regularized Hamiltonian system
Cosmic antiprotons as a probe for supersymmetric dark matter?
The flux of cosmic ray antiprotons from neutralino annihilations in the
galactic halo is computed for a large sample of models in the MSSM (the Minimal
Supersymmetric extension of the Standard Model). We also revisit the problem of
estimating the background of low-energy cosmic ray induced secondary
antiprotons, taking into account their subsequent interactions (and energy
loss) and the presence of nuclei in the interstellar matter.
We consider a two-zone diffusion model, with and without a galactic wind. We
find that, given the uncertainties in the background predictions, there is no
need for a primary (exotic) component to explain present data. However,
allowing for a signal by playing with the uncertainties in the background
estimate, we discuss the characteristic features of the supersymmetric models
which give a satisfactory description of the data. We point out that in some
cases the optimal kinetic energy to search for a signal from supersymmetric
dark matter is above several GeV, rather than the traditional sub-GeV region.
The large astrophysical uncertainties involved do not, one the other hand,
allow the exclusion of any of the MSSM models we consider, on the basis of
data.
We present besides numerical results also convenient parameterizations of the
antiproton yields of all `basic' two-body final states. We also give examples
of the yield and differential energy spectrum for a set of supersymmetric
models with high rates.
We also remark that it is difficult to put a limit on the antiproton lifetime
from present measurements, since the injection of antiprotons from neutralino
annihilation can compensate the loss from decay.Comment: 22 pages, 11 figures, uses emulateapj.st
Cosmological Constraints from the ROSAT Deep Cluster Survey
The ROSAT Deep Cluster Survey (RDCS) has provided a new large deep sample of
X-ray selected galaxy clusters. Observables such as the flux number counts
n(S), the redshift distribution n(z) and the X-ray luminosity function (XLF)
over a large redshift baseline (z\lesssim 0.8) are used here in order to
constrain cosmological models. Our analysis is based on the Press-Schechter
approach, whose reliability is tested against N-body simulations. Following a
phenomenological approach, no assumption is made a priori on the relation
between cluster masses and observed X-ray luminosities. As a first step, we use
the local XLF from RDCS, along with the high-luminosity extension provided by
the XLF from the BCS, in order to constrain the amplitude of the power
spectrum, \sigma_8, and the shape of the local luminosity-temperature relation.
We obtain \sigma_8=0.58 +/- 0.06 for Omega_0=1 for open models at 90%
confidence level, almost independent of the L-T shape. The density parameter
\Omega_0 and the evolution of the L-T relation are constrained by the RDCS XLF
at z>0 and the EMSS XLF at z=0.33, and by the RDCS n(S) and n(z) distributions.
By modelling the evolution for the amplitude of the L-T relation as (1+z)^A, an
\Omega_0=1 model can be accommodated for the evolution of the XLF with 1<A<3 at
90% confidence level, while \Omega_0=0.4^{+0.3}_{-0.2} and \Omega_0<0.6 are
implied by a non--evolving L-T for open and flat models, respectively.Comment: 12 pages, 9 colour figures, LateX, uses apj.sty, ApJ, in press, May
20 issu
An integrated ultra-high vacuum apparatus for growth and in situ characterization of complex materials
Here we present an integrated ultra-high vacuum apparatus \u2013 named MBE-Cluster \u2013 dedicated to the growth
and in situ structural, spectroscopic and magnetic characterization of complex materials. Molecular Beam
Epitaxy (MBE) growth of metal oxides, e.g. manganites, and deposition of patterned metallic layers can be
fabricated and in situ characterized by reflection high-energy electron diffraction (RHEED), low-energy
electron diffraction (LEED) - Auger Electron Spectroscopy, X-ray photoemission spectroscopy (PES) and
azimuthal longitudinal magneto-optic Kerr effect (MOKE). The temperature can be controlled in the range
from 5 to 580 K, with the possibility of application of magnetic fields H up to \ub17 kOe and electric fields E for
voltages up to \ub1500 V. The MBE-Cluster operates for in-house research as well as user facility in combination
with the APE beamlines at Sincrotrone-Trieste and the high harmonic generator (HHG) facility for timeresolved
spectroscopy
The potential impact of enhanced hygienic measures during the COVID-19 outbreak on hospital-acquired infections: A pragmatic study in neurological units
Objective: Hospital-acquired infections (HAIs) are frequent complications among acute patients hospitalized in neurological units, especially among those hospitalized for stroke. This study aimed to investigate if enhanced hygienic measures, including the systematic use of personal protective equipment (PPE), determined a decrease in HAI during the recent COVID-19 outbreak in “COVID-free” neurological units. Methods: Patients hospitalized in neurology and stroke units of Policlinico Umberto I Hospital in Rome from March 8, 2020 and discharged prior to May 31, 2020 were included in the study and compared with patients hospitalized during the same period in 2019. Results: A total of 319 patients were included in the study (n = 103 in 2020, n = 216 in 2019). Among patients hospitalized in 2019, the incidence of HAIs was 31.5% (95% confidence interval (CI): 0.25–0.38), compared with 23.3% (95% CI: 0.15–0.32) in 2020 (p = 0.12). Multivariable logistic regression showed that hospitalization during 2020 was independently associated with a lower risk of HAIs (odds ratio: 0.34, 95% CI:0.16–0.71, p = 0.004). Poisson regression models showed that hospitalization during 2020 was also independently associated with both a lower number of HAIs (relative risk [RR]: 0.56, 95% CI:0.38–0.81, p = 0.01) and a lower number of prescribed antibiotics per patient (RR: 0.66, 95% CI: 0.49–0.87, p = 0.02). Conclusion: Our study design provides evidence regarding the impact of stricter hygienic measures, such as increased PPE use, on HAIs. Larger studies are needed to support the extension of preventive measures even after the COVID-19 outbreak in order to limit the occurrence of HAIs
Building global capacity for conducting operational research using the SORT IT model: where and who?
This paper is an output of the Structured Operational Research and Training Initiative (SORT IT), a global partnership led by the Special Program for Research and Training in Tropical Diseases at the World Health Organization (WHO/TDR), Geneva, Switzerland.Setting. Research capacity is weakest in low and middle-income countries (LMICs) where operational research is highly relevant and needed. Structured Operational Research and Training Initiative (SORT IT) courses have been developed to train participants to conduct and publish operational research and influence policy and practice. Twenty courses were completed in Asia, Africa, Europe and the South Pacific between 2009 and 2014. Objectives. In the 20 completed SORT IT courses, to assess where the research was conducted, who was trained, who became facilitators in subsequent courses and course outcomes. Design. A cohort study of completed SORT IT courses. Results. There were 236 participants (41% female) including 64 nationalities who conducted research in 59 countries, mostly from Asia and Africa (mean course duration = 9.7 months). Most participants (68%) were from government health programs and non-governmental agencies. A total of 213(90%) participants completed all milestones successfully with 41(19%) becoming subsequent course facilitators, 88% of whom were from LMICs. Of 228 manuscripts submitted to scientific journals, 197(86%) were either published or in press; in 86%, the principal investigator (first author) was a LMIC national. Papers were published in 23 scientific journals (impact factor 0.5–4.4) and covered 21 disease categories (median publication time = 5.7 months). Published papers (186) had 94,794 cumulative article views/downloads. Article views/downloads for immediate open access articles were double those from closed access journals. Conclusion. The SORT IT model has been effective in training personnel to produce relevant operational research in LMICs. It merits continued commitment and support for further scale-up and development.Publisher PDFPeer reviewe
Levosimendan in Acute and Advanced Heart Failure : an Expert Perspective on Posology and Therapeutic Application
Levosimendan, a calcium sensitizer and potassium channel-opener, is widely appreciated by many specialist heart failure practitioners for its effects on systemic and pulmonary hemodynamics and for the relief of symptoms of acute heart failure. The drug's impact on mortality in large randomized controlled trials has been inconsistent or inconclusive but, in contrast to conventional inotropes, there have been no indications of worsened survival and some signals of improved heart failure-related quality of life. For this reason, levosimendan has been proposed as a safer inodilator option than traditional agents in settings, such as advanced heart failure. Positive effects of levosimendan on renal function have also been described. At the HEART FAILURE 2018 congress of the Heart Failure Association of the European Society of Cardiology, safe and effective use levosimendan in acute and advanced heart failure was examined in a series of expert tutorials. The proceedings of those tutorials are summarized in this review, with special reference to advanced heart failure and heart failure with concomitant renal dysfunction. Meta-analysis of clinical trials data is supportive of a renal-protective effect of levosimendan, while physiological observations suggest that this effect is exerted at least in part via organ-specific effects that may include selective vasodilation of glomerular afferent arterioles and increased renal blood flow, with no compromise of renal oxygenation. These lines of evidence require further investigation and their clinical significance needs to be evaluated in specifically designed prospective trials.Peer reviewe
- …