Anisotropic susceptibility of the geometrically frustrated spin-chain compound Ca3Co2O6

Ca3Co2O6 is a system exhibiting a series of fascinating properties, including magnetization plateaus and remarkably slow dynamics at low-T. These properties are intimately related to the geometrical frustration, which results from a particular combination of features: (i) the chains are arranged on a triangular lattice; (ii) there is a large uniaxial anisotropy; (iii) the intrachain and interchain couplings are ferromagnetic and antiferromagnetic, respectively. The uniaxial anisotropy is thus an issue of crucial importance for the analysis of the physical properties of Ca3Co2O6. However, it turns out that no precise investigation of this magnetic anisotropy has been performed so far. On the basis of susceptibility data directly recorded on single crystals, the present study reports on quantitative information about the anisotropy of Ca3Co2O6.Comment: 10 pages, 5 figure

Study of the X-ray activity of Sgr A* during the 2011 XMM-Newton campaign

In Spring 2011 we observed Sgr A*, the supermassive black hole at the center of our Galaxy, with XMM-Newton with a total exposure of ~226 ks in coordination with the 1.3 mm VLBI. We have performed timing analysis of the X-ray emission from Sgr A* using Bayesian blocks algorithm to detect X-ray flares observed with XMM-Newton. Furthermore, we computed X-ray smoothed light curves observed in this campaign in order to have better accuracy on the position and the amplitude of the flares. We detected 2 X-ray flares on the 2011 March 30 and April 3 which have for comparison a peak detection level of 6.8 and 5.9 sigma in the XMM-Newton/EPIC light curve in the 2-10 keV energy range with a 300 s bin. The former is characterized by 2 sub-flares: the first one is very short (~458 s) with a peak luminosity of ~9.4E34 erg/s whereas the second one is longer (~1542 s) with a lower peak luminosity of ~6.8E34 erg/s. The comparison with the sample of X-ray flares detected during the 2012 Chandra XVP campaign favors the hypothesis that the 2011 March 30 flare is a single flare rather than 2 distinct sub-flares. We model the light curve of this flare with the gravitational lensing of a simple hotspot-like structure but we can not satisfactorily reproduce the large decay of the light curve between the 2 sub-flares with this model. From magnetic energy heating during the rise phase of the first sub-flare and assuming an X-ray photons production efficiency of 1 and a magnetic field of 100 G at 2 r_g, we derive an upper limit to the radial distance of the first sub-flare of 100 r_g. We estimate using the decay phase of the first sub-flare a lower limit to the radial distance of 4 r_g from synchrotron cooling in the infrared. The X-ray emitting region of the first sub-flare is located at a radial position of 4-100 and has a corresponding radius of 1.8-2.87 in r_g unit for a magnetic field of 100 G at 2 r_g.Comment: Version published in A&A + corrigendum published in A&

Torsions and intersection forms of 4-manifolds from trisection diagrams

Gay and Kirby introduced trisections which describe any closed oriented smooth 4-manifold $X$ as a union of three four-dimensional handlebodies. A trisection is encoded in a diagram, namely three collections of curves in a closed oriented surface $\Sigma$, guiding the gluing of the handlebodies. Any morphism $\varphi$ from $\pi_1(X)$ to a finitely generated free abelian group induces a morphism on $\pi_1(\Sigma)$. We express the twisted homology and Reidemeister torsion of $(X;\varphi)$ in terms of the first homology of $(\Sigma;\varphi)$ and the three subspaces generated by the collections of curves. We also express the intersection form of $(X;\varphi)$ in terms of the intersection form of $(\Sigma;\varphi)$.Comment: Comments are welcom

Renforcer l’analyse réflexive des tuteurs en formation

Comprend des références bibliographiquesNous présentons dans cette communication l’étude de l’engagement des élèves d’une école d’ingénieur française dans un cours en fonction de sa modalité : cours en présentiel ou cours-vidéo. Sur la base de l’analyse de diverses données prises auprès de ces élèves sur deux ans - deux questionnaires écrits et 12 interviews - nous pouvons conclure que l’engagement est soutenu, indépendant de la modalité. Nous attribuons ce résultat entre autres aux caractéristiques des apprenants qui sont fortement motivés et possèdent une bonne connaissance de soi ce qui leur permet de choisir et de créer des conditions de travail adaptées afin de réussir leurs études

Entre situations convenues et richesse comique : Les mousquetaires au couvent de Louis Varney

Louis Varney fut un auteur à succès d’opérettes, qui est tombé de nos jours dans l’oubli. Cet article étudie les procédés de comique musical employés dans son chef-d’œuvre, Les mousquetaires au couvent (1880). En s’appuyant sur les écrits théoriques de Baudelaire et de Bergson, cet essai souligne la variété du comique dans cette opérette : verbal, gestuel, mécanique, hypertrophie musicale, détournement musical, jeu avec des situations musicales topiques, superposition de niveaux de lecture. La richesse des réponses musicales proposées par Varney est remarquable et rappelle la valeur d’un artiste qui souffre de la mauvaise réputation de l’opérette française – dépolitisée et passant pour un pur divertissement – des années 1880