Dielectric-sphere-couple model for noble-gas pair polarizability

A classical model of two interacting dielectric spheres (DS) in an electric field is proposed to calculate the trace and anisotropy of the polarizability tensor of a pair of isotropic molecules. The case of noble gases is treated. It is shown that the dielectric-sphere-couple anisotropy is quite close to the dipole-induced-dipole (DID) anisotropy, while the DS trace is significantly greater than the DID trace, even at long distances between the interacting spheres. These results are interpreted as being due to the non-negligible multipolar contributions to the trace. To examine the validity of the model, its predictions are compared with results of experiments (trace and anisotropy are responsible for many collision-induced optical phenomena or properties) and of self-consistent-field calculations

Monte-Carlo simulations of the background of the coded-mask camera for X- and Gamma-rays on-board the Chinese-French GRB mission SVOM

For several decades now, wide-field coded mask cameras have been used with success to localise Gamma-ray bursts (GRBs). In these instruments, the event count rate is dominated by the photon background due to their large field of view and large effective area. It is therefore essential to estimate the instrument background expected in orbit during the early phases of the instrument design in order to optimise the scientific performances of the mission. We present here a detailed study of the instrument background and sensitivity of the coded-mask camera for X- and Gamma-rays (CXG) to be used in the detection and localisation of high-redshift GRBs on-board the international GRB mission SVOM. To compute the background spectrum, a Monte-Carlo approach was used to simulate the primary and secondary interactions between particles from the main components of the space environment that SVOM will encounter along its Low Earth Orbit (LEO) (with an altitude of 600 km and an inclination of ~ 30 deg) and the body of the CXG. We consider the detailed mass model of the CXG in its latest design. According to our results, i) the design of the passive shield of the camera ensures that in the 4-50 keV imaging band the cosmic X-Gamma-ray background is dominant whilst the internal background should start to become dominant above 70-90 keV; ii) the current camera design ensures that the CXG camera will be more sensitive to high-redshift GRBs than the Swift Burst Alert Telescope thanks to a low-energy threshold of 4 keV.Comment: 16 pages, 10 figures (1 colour), accepted for publication in Nuclear Instruments and Methods in Physics Research: Section

Essais de génotoxicité in vitro et in vivo applicables à l'environnement hydrique

Cet article est une revue des essais in vitro et in vivo utilisés pour évaluer le caractère génotoxique des micropolluants des milieux environnementaux relatifs aux eaux continentales et marines, rejets liquides d'origine domestique, industrielle ou agricole, sédiments de rivières et boues de stations de traitement d'épuration.Les essais in vitro réalisés sur cellules eucaryotes ou procaryotes sont fondés sur la détection des mutations géniques et chromosomiques, ou la mesure des adduits à l'ADN. Ils constituent des systèmes d'épreuve miniaturisés qui requièrent des volumes d'échantillons faibles; ils se prêtent ainsi au dépistage à grande échelle de la génotoxicité et à l'étude des concentrats et des extraits préparés à partir des milieux contaminés. Ils sont cependant moins bien adaptés à la prédiction de l'impact des micropolluants sur l'environnement.La recherche de conditions d'essai plus proches de la réalité environnementale a conduit au développement des essais in vivo réalisés sur organismes supérieurs, mollusques, poissons ou amphibiens, qui évaluent un potentiel génotoxique à partir d'études cytogénétiques ou d'études du caryotype des organismes exposés.Les critères de génotoxicité étudiés in vitro peuvent être utilisés dans le cadre d'études écoépidémiologiques, sur le terrain, afin d'évaluer l'impact réel des micropolluants présents dans les milieux environnementaux sujets à des contaminations d'origines diverses.This review deals with in vitro and in vivo genotoxicity bioassays carried out to evaluate the genotoxic potential of polluted environmental samples : continental and marine waters, domestic and industrial wastewaters, aquatic sediments and sludges of urban or industrial wastewater treatment plants.The end-points of the in vitro and in vivo assays are : genetic alterations, i.e. reverse and forward mutations, DNA adducts or chramosomic damages, i.e. chromosomic aberrations (AC), micronuclei (MN) and sister chromatid exchanges (SCE).The in vitro assays generally detect adverse effects on DNA only alter concentration or extraction of micropollutants. They constitute miniaturized tools, rapid and easy to use, thus well-suited for large screening studies. In vitro genotoxicity bioassays requiring only small volumes of samples are therefore systems of choice for testing concentrates or extracts from environmental contaminated samples. Among the in vitro assays reviewed, the Salmonella typhimurium gene mutation test is the most often used to assess the genotoxic potential of contaminated samples. However, genotoxicity tests performed on eukaryotic cell cultures are more relevant than those using bacteria for evaluating environmental pollution. The use of fish cell fines appears superior to the use of mammalian cells for assessing an aquatic impact.In vitro bioassays, whether performed on prokaryotic or eukaryotic cells, are limited for predicting the possible impact of genotoxic pollutants on the environment. It is clear that it is difficult to extrapolate in vitro bioassay results to higher organisms in which the response obtained integrates effects of complex metabolizing systems, hormonal regulation and immunological defenses.Therefore, genotoxicity studies performed with aquatic organisms such as molluscs (Mytilus sp.), fish (Umbra pygmaea, Notobranchius rachowi) or amphibians (Pleurodeles waltl) appear more representative of environmental conditions. The genotoxicity end-points of in vivo assays are mainly cytogenetic damage such as the SCE, AC or MN but also take into account DNA adducts. Direct testing of environmental samples without preconcentration is possible with in vivo assays. This means that factors such as bioavailability and metabolism will be integrated direcrly in the response of these assays. Hence, these in vivo assays are more sensitive titan in vitro genotoxicity tests. However, in vivo tests require important volumes of sample and it will be difficult or almost impossible to apply them for testing concentrates or sample extracts, generally only available in small quantities. An interesting area of application of in vivo assays is field studies and ecoepidemioiogy. In this respect, they would constitue an a posteriori control system of pollution effects, assuming that suitable control areas are available to eliminate the influence of confounding factors.As a general conclusion, if is important to emphasize the interest of using both in vitro and in vivo bioassays for evaluating the genotoxicity of contaminated environmental samples. This rationale is based on the fact that in vitro bioassays are well adapted for genotoxicity screening or concentrates and extracts testing, white in vivo tests are interesting because of their better representativity in terms of environmental conditions of exposure to pollutants

Investigating slim disk solutions for HLX-1 in ESO 243-49

The hyper luminous X-ray source HLX-1 in the galaxy ESO 243-49, currently the best intermediate mass black hole candidate, displays spectral transitions similar to those observed in Galactic black hole binaries, but with a luminosity 100-1000 times higher. We investigated the X-ray properties of this unique source fitting multi-epoch data collected by Swift, XMM-Newton & Chandra with a disk model computing spectra for a wide range of sub- and super-Eddington accretion rates assuming a non-spinning black hole and a face-on disk (i = 0 deg). Under these assumptions we find that the black hole in HLX-1 is in the intermediate mass range (~2 x 10^4 M_odot) and the accretion flow is in the sub-Eddington regime. The disk radiation efficiency is eta = 0.11 +/-0.03. We also show that the source does follow the L_X ~ T^4 relation for our mass estimate. At the outburst peaks, the source radiates near the Eddington limit. The accretion rate then stays constant around 4 x 10^(-4) M_odot yr^(-1) for several days and then decreases exponentially. Such "plateaus" in the accretion rate could be evidence that enhanced mass transfer rate is the driving outburst mechanism in HLX-1. We also report on the new outburst observed in August 2011 by the Swift-X-ray Telescope. The time of this new outburst further strengthens the ~1 year recurrence timescale.Comment: 24 pages, 10 figures, accepted for publication in Ap

Comparison between classical potentials and ab initio for silicon under large shear

The homogeneous shear of the {111} planes along the direction of bulk silicon has been investigated using ab initio techniques, to better understand the strain properties of both shuffle and glide set planes. Similar calculations have been done with three empirical potentials, Stillinger-Weber, Tersoff and EDIP, in order to find the one giving the best results under large shear strains. The generalized stacking fault energies have also been calculated with these potentials to complement this study. It turns out that the Stillinger-Weber potential better reproduces the ab initio results, for the smoothness and the amplitude of the energy variation as well as the localization of shear in the shuffle set

Can the scoring of the walking estimated limitation calculated by history (WELCH) questionnaire be simultaneously simplified and improved?

BACKGROUND: The WELCH questionnaire includes 4 items (A, B, C and D) and estimates the maximal walking time (MWT) on treadmill in patients with claudication. Its scoring was empirically defined. We aimed to test various methods for scoring to estimate whether the scoring of the WELCH could be improved or simplified. PATIENTS AND METHODS: In 423 patients, we tested 8 methods (from H1 to H8) of weighing D or calculating alpha, beta and gamma in the equation MWT = (alphaA + betaB + gammaC) D. RESULTS: While the WELCH Pearson r was 0.639 and area under ROC curve for the ability to walk 5 minutes on treadmill was 0.795 for the reference empirical method, tested hypotheses resulted in values ranging 0.566 to 0.661 for the Pearson r values and 0.750 to 0.809 for the areas under ROC curve respectively. CONCLUSIONS: None of the tested methods simultaneously improved the correlation to MWT, remained simple enough to be scored by mental calculation and ranged between intuitive minimal and maximal values. The original empirical scoring seems a good compromise between accuracy and simplicity

Testing the standard fireball model of GRBs using late X-ray afterglows measured by Swift

We show that all X-ray decay curves of GRBs measured by Swift can be fitted using one or two components both of which have exactly the same functional form comprised of an early falling exponential phase followed by a power law decay. The 1st component contains the prompt gamma-ray emission and the initial X-ray decay. The 2nd component appears later, has a much longer duration and is present for ~80% of GRBs. It most likely arises from the external shock which eventually develops into the X-ray afterglow. In the remaining ~20% of GRBs the initial X-ray decay of the 1st component fades more slowly than the 2nd and dominates at late times to form an afterglow but it is not clear what the origin of this emission is. The temporal decay parameters and gamma/X-ray spectral indices derived for 107 GRBs are compared to the expectations of the standard fireball model including a search for possible "jet breaks". For ~50% of GRBs the observed afterglow is in accord with the model but for the rest the temporal and spectral indices do not conform to the expected closure relations and are suggestive of continued, late, energy injection. We identify a few possible jet breaks but there are many examples where such breaks are predicted but are absent. The time, T_a, at which the exponential phase of the 2nd component changes to a final powerlaw decay afterglow is correlated with the peak of the gamma-ray spectrum, E_peak. This is analogous to the Ghirlanda relation, indicating that this time is in some way related to optically observed break times measured for pre-Swift bursts.Comment: submitted to Ap