Electromagnetic shielding by thin periodic structures and the Faraday cage effect

Dans cette note, nous nous intéressons à la diffraction des ondes électromagnétiques (équations de Maxwell en régime harmonique) par une nappe perforée plane constituée de petit obstacles parfaitement conducteurs placée à l’interface entre deux milieux homogènes. La taille des obstacles et la distance séparant deux obstacles consécutifs sont du même ordre de grandeur δ, δ supposé petit. En étudiant trois configurations modèles ((i) obstacles « discrets », (ii) fils parallèles, (iii) maillage constitué de deux nappes de fils parallèles), nous montrons que la limite de la solution quand δ tend vers 0 dépend de la forme des obstacles constituant la nappe périodique, le phénomène de « cage de Faraday » n’apparaissant que dans le cas du maillage de fils

The KX method for producing K-band flux-limited samples of quasars

The longstanding question of the extent to which the quasar population is affected by dust extinction, within host galaxies or galaxies along the line of sight, remains open. More generally, the spectral energy distributions of quasars vary significantly and flux-limited samples defined at different wavelengths include different quasars. Surveys employing flux measurements at widely separated wavelengths are necessary to characterise fully the spectral properties of the quasar population. The availability of panoramic near-infrared detectors on large telescopes provides the opportunity to undertake surveys capable of establishing the importance of extinction by dust on the observed population of quasars. We introduce an efficient method for selecting K-band, flux-limited samples of quasars, termed ``KX'' by analogy with the UVX method. This method exploits the difference between the power-law nature of quasar spectra and the convex spectra of stars: quasars are relatively brighter than stars at both short wavelengths (the UVX method) and long wavelengths (the KX method). We consider the feasibility of undertaking a large-area KX survey for damped Ly-alpha galaxies and gravitational lenses using the planned UKIRT wide-field near-infrared camera.Comment: 6 pages, 3 figures, to appear in MNRA

The lens and source of the optical Einstein ring gravitational lens ER 0047-2808

(Abridged) We perform a detailed analysis of the optical gravitational lens ER 0047-2808 imaged with WFPC2 on the Hubble Space Telescope. Using software specifically designed for the analysis of resolved gravitational lens systems, we focus on how the image alone can constrain the mass distribution in the lens galaxy. We find the data are of sufficient quality to strongly constrain the lens model with no a priori assumptions about the source. Using a variety of mass models, we find statistically acceptable results for elliptical isothermal-like models with an Einstein radius of 1.17''. An elliptical power-law model (Sigma \propto R^-beta) for the surface mass density favours a slope slightly steeper than isothermal with beta = 1.08 +/- 0.03. Other models including a constant M/L, pure NFW halo and (surprisingly) an isothermal sphere with external shear are ruled out by the data. We find the galaxy light profile can only be fit with a Sersic plus point source model. The resulting total M/L_B contained within the images is 4.7 h_65 +/-0.3. In addition, we find the luminous matter is aligned with the total mass distribution within a few degrees. The source, reconstructed by the software, is revealed to have two bright regions, with an unresolved component inside the caustic and a resolved component straddling a fold caustic. The angular size of the entire source is approx. 0.1'' and its (unlensed) Lyman-alpha flux is 3 x 10^-17 erg/s/cm^2.Comment: 13 pages, 5 figures. Revised version accepted for publication in MNRA

Numerical evaluation of singular integrals on non-disjoint self-similar fractal sets

We consider the numerical evaluation of a class of double integrals with respect to a pair of self-similar measures over a self-similar fractal set (the attractor of an iterated function system), with a weakly singular integrand of logarithmic or algebraic type. In a recent paper (Gibbs et al. Numer. Algorithms 92, 2071–2124 2023), it was shown that when the fractal set is “disjoint” in a certain sense (an example being the Cantor set), the self-similarity of the measures, combined with the homogeneity properties of the integrand, can be exploited to express the singular integral exactly in terms of regular integrals, which can be readily approximated numerically. In this paper, we present a methodology for extending these results to cases where the fractal is non-disjoint but non-overlapping (in the sense that the open set condition holds). Our approach applies to many well-known examples including the Sierpinski triangle, the Vicsek fractal, the Sierpinski carpet, and the Koch snowflake

Risk factors of anterior cruciate ligament tears: Neuromuscular status

Peer reviewe

The spread of primary schooling in sub-Saharan Africa: Implications for fertility change

Caldwell has hypothesized that the onset of the fertility transition would be linked with the achievement of “mass formal schooling.” In sub-Saharan Africa, a region where some countries have begun the fertility transition but many have not, the extent of progress toward mass schooling has not yet been assessed. This paper fills a gap in the literature using newly available Demographic and Health Survey data to assess schooling patterns and trends for 17 sub-Saharan African countries. As background to that assessment, the paper includes a literature review, an overview of the recent history of African education, and an evaluation of alternative sources of data on education. These data are linked to recent markers of fertility change in order to assess the potential importance of mass schooling for the fertility transition in Africa. In most of Africa, the promise embodied in early post-independence education progress whereby the next generation of Africans would be universally exposed to basic levels of formal schooling has yet to be realized. Countries such as Botswana, Ghana, Kenya, Namibia, South Africa, Tanzania, Zambia, and Zimbabwe are the exceptions rather than the rule, and most of these had achieved some form of mass schooling by the early 1980s. Since 1980, growth rates in educational participation and attainment have slowed or halted; in some countries, they have begun to decline. Most of these same countries were in the forefront of the fertility transition in Africa; in Ghana and Tanzania, fertility declines have begun more recently. A systematic empirical analysis of cross-country patterns across all 17 countries supports Caldwell’s hypothesis. Within this group, however, we find a few countries beginning to show signs of fertility transition despite limited progress in mass schooling. Côte d’Ivoire and Senegal are notable examples of this new development. In countries where population growth rates remain high and a growing proportion of the population is of school age, the achievement of mass schooling will be increasingly difficult in the future if high fertility persists. Therefore, if fertility declines are to continue, or in some cases begin, they will have to do so in the absence of mass schooling. Instead, fertility may respond to other vehicles of communication, such as the mass media, which will be increasingly pervasive, providing alternative mechanisms for the spread of cultural change

The Promising Process to Distinguish Supersymmetric Models with Large tanβ\beta from the Standard Model: B→Xsμ+μ−B\to X_s{\mu}^{+}{\mu}^{-}

It is shown that in supersymmetric models (SUSYMs) the large supersymmetric contributions to $B \to X_s{\mu}^{+}{\mu}^{-}$ come from the Feynman diagrams which consist of exchanging neutral Higgs bosons (NHBs) and the chargino-stop loop and are proportional to $m_b m_{\mu}$tan$^3\beta/m_{h}^2$ when tan$\beta$ is large and the mass of the lightest neutral Higgs boson m$_h$ is not too large (say, less than 150 Gev). Numerical results show that the branching ratios of $B \to X_s{\mu}^{+}{\mu}^{-}$ can be enhanced by more than 100% compared to the standard model (SM) and the backward-forward asymmetry of lepton is significantly different from that in SM when tan$\beta \geq 30$.Comment: 8 pages, including 2 figure

The Frequency and Radio Properties of Broad Absorption Line Quasars

A sample of 67 Broad Absorption Line quasars (BALQSOs) from the Large Bright Quasar Survey (LBQS) is used to estimate the observed and intrinsic fraction of BAL quasars in optically--selected samples at intermediate (B_J \simeq 18.5) magnitudes. The observed BALQSO fraction in the redshift range 1.5 < z < 3.0 is 15\pm3%. A well--determined, empirical, k--correction, to allow for the differences in the spectral energy distributions of non--BALQSOs and BALQSOs shortward of \simeq 2100A in the restframe, is applied to the sample. The result is an estimate of the intrinsic fraction of BALQSOs, in the redshift range 1.5 < z < 3.0, of 22+/-4%. This value is twice that commonly cited for the occurrence of BALQSOs in optically--selected samples and the figure is in reasonable agreement with that from a preliminary analysis of the SDSS Early Data Release. The fraction of BALQSOs predicted to be present in an optical survey with flux limits equivalent to that of the FIRST Bright Quasar Survey (FBQS) is shown to be \simeq 20%. The BALQSO fractions derived from the FBQS and the LBQS suggest that optically--bright BALQSOs are half as likely as non-BALQSOs to be detectable as S_1.4GHz > 1mJy radio sources.Comment: Accepted for publication in AJ, April 2003 Issu