Search for neutrinos from transient sources with the ANTARES telescope and optical follow-up observations

The ANTARES telescope has the opportunity to detect transient neutrino sources, such as gamma-ray bursts, core-collapse supernovae, flares of active nuclei... To enhance the sensitivity to these sources, we have developed a new detection method based on the optical follow-up of "golden" neutrino events such as neutrino doublets coincident in time and space or single neutrinos of very high energy. The ANTARES Collaboration has therefore implemented a very fast on-line reconstruction with a good angular resolution. These characteristics allow to trigger an optical telescope network; since February 2009. ANTARES is sending alert trigger one or two times per month to the two 25 cm robotic telescope of TAROT. This follow-up of such special events would not only give access to the nature of the sources but also improves the sensitivity for transient neutrino sources.Comment: 3 pages, 3 figures, Proceedings of the 31st ICRC, Lodz, Polan, July 200

Microbiological aspects of methane emission in a ricefield soil from the Camargue (France) : 2. Methanotrophy and related microflora

Les principaux facteurs qui influencent l'activité méthanotrophe du sol (épaisseur du sol, humidité, addition d'engrais et dessiccation) ont été étudiés in vitro sur un sol de rizière de Camargue (France) afin de standardiser une méthode pour estimer l'activité méthanotrophe potentielle (PMA) d'un sol. La possibilité d'énumérer les méthanotrophes du sol a été étudiée et les souches dominantes du sol incubé pendant un mois sous un mélange de 80% air/20% CH4 ont été isolées. La méthanotrophie in vitro est un phénomène quantifiable par unité de surface plutôt que par unité de poids de sol. L'activité maximale (PMA)(1.8 mole m-2 j-1) a été mesurée sur un échantillon à une humidité voisine de la capacité au champ et incubé pendant un mois sous air/CH4. Les consommations de CH4 et d'O2 ainsi que la production de CO2 sont fortement corrélées. La consommation d'O2 liée à l'activité hétérotrophe du sol est négligeable comparée à celle due à l'activité méthanotrophe. L'addition d'engrais à des concentrations utilisées en agriculture est sans effet sur la PMA qui est restée stable pendant deux ans dans le sol conservé à l'état sec. Une très forte contamination rend les étalements sur boites de Pétri inexploitables pour dénombrer les bactéries méthanotrophes du sol. Les populations estimées par MPN sont faibles dans le sol sec (2.5 x 102 g-1) et atteignent 5 x 108 g-1 dans le sol enrichi. La morphologie des souches isolées indique leur appartenance aux bactéries méthanotrophes de type II. (Résumé d'auteur

Integer Quantum Hall Effect in Trilayer Graphene

The Integer Quantum Hall Effect (IQHE) is a distinctive phase of two-dimensional electronic systems subjected to a perpendicular magnetic field. Thus far, the IQHE has been observed in semiconductor heterostructures and in mono- and bi-layer graphene. Here we report on the IQHE in a new system: trilayer graphene. Experimental data are compared with self-consistent Hartree calculations of the Landau levels for the gated trilayer. The plateau structure in the Hall resistivity determines the stacking order (ABA versus ABC). We find that the IQHE in ABC trilayer graphene is similar to that in the monolayer, except for the absence of a plateau at filling factor v=2. At very low filling factor, the Hall resistance vanishes due to the presence of mixed electron and hole carriers induced by disorder.Comment: 5 pages, 4 figure

ALMA Capabilities for Observations of Spectral Line Emission

The Atacama Large Millimeter/submillimeter Array (ALMA) combines large collecting area and location on a high dry site to provide it with unparalleled potential for sensitive millimeter/submillimeter spectral line observations. Its wide frequency coverage, superb receivers and flexible spectrometer will ensure that its potential is met. Since the 1999 meeting on ALMA Science\cite{RefA}, the ALMA team has substantially enhanced its capability for line observations. ALMA's sensitivity increased when Japan joined the project, bringing the 16 antennas of the Atacama Compcat Array (ACA), equivalent to eight additional 12m telescopes. The first four receiver cartridges for the baseline ALMA (Japan's entry has brought two additional bands to ALMA's receiver retinue) have been accepted, with performance above the already-challenging specifications. ALMA's flexibility has increased with the enhancement of the baseline correlator with additional channels and flexibility, and with the addition of a separate correlator for the ACA. As an example of the increased flexibility, ALMA is now capable of multi-spectral-region and multi-resolution modes. With the former, one might observe e.g. four separate transitions anywhere within a 2 GHz band with a high resolution bandwidth. With the latter, one might simultaneously observe with low spectral resolution over a wide bandwidth and with high spectral resolution over a narrow bandwidth; this mode could be useful for observations of pressure-broadened lines with narrow cores, for example. Several science examples illustrate ALMA's potential for transforming millimeter and submillimeter astronomy.Comment: 6 pages, for the Proceedings of Science with ALMA: a new era for Astrophysics: International Conference, 2006 held 13 - 17 November 2006 in Madrid, Spai

Approximability of Connected Factors

Finding a d-regular spanning subgraph (or d-factor) of a graph is easy by Tutte's reduction to the matching problem. By the same reduction, it is easy to find a minimal or maximal d-factor of a graph. However, if we require that the d-factor is connected, these problems become NP-hard - finding a minimal connected 2-factor is just the traveling salesman problem (TSP). Given a complete graph with edge weights that satisfy the triangle inequality, we consider the problem of finding a minimal connected $d$-factor. We give a 3-approximation for all $d$ and improve this to an (r+1)-approximation for even d, where r is the approximation ratio of the TSP. This yields a 2.5-approximation for even d. The same algorithm yields an (r+1)-approximation for the directed version of the problem, where r is the approximation ratio of the asymmetric TSP. We also show that none of these minimization problems can be approximated better than the corresponding TSP. Finally, for the decision problem of deciding whether a given graph contains a connected d-factor, we extend known hardness results.Comment: To appear in the proceedings of WAOA 201

Cosmological inference including massive neutrinos from the matter power spectrum: biases induced by uncertainties in the covariance matrix

Data analysis from upcoming large galaxy redshift surveys, such as Euclid and DESI will significantly improve constraints on cosmological parameters. To optimally extract the information from these galaxy surveys, it is important to control with a high level of confidence the uncertainty and bias arising from the estimation of the covariance that affects the inference of cosmological parameters. In this work, we are addressing two different but closely related issues: (i) the sampling noise present in a covariance matrix estimated from a finite set of simulations and (ii) the impact on cosmological constraints of the non-Gaussian contribution to the covariance matrix of the power spectrum. We focus on the parameter estimation obtained from fitting the matter power spectrum in real space, using the DEMNUni N-body simulations. Regarding the first issue, we adopt two different approaches to reduce the sampling noise in the precision matrix that propagates in the parameter space: on the one hand using an alternative estimator of the covariance matrix based on a non-linear shrinkage, NERCOME; and on the other hand employing a method of fast generation of approximate mock catalogs, COVMOS. We find that NERCOME can significantly reduce the noise induced on the posterior distribution of parameters, but at the cost of a systematic overestimation of the error bars on the cosmological parameters. We show that using a COVMOS covariance matrix estimated from a large number of realisations (10~000) results in unbiased cosmological constraints. Regarding the second issue, we quantify the impact on cosmological constraints of the non-Gaussian part of the power spectrum covariance purely coming from non-linear clustering. We find that when this term is neglected, both the errors and central values of the estimated parameters are affected for a scale cut \kmax > 0.2\ \invMpc.Comment: 21 pages, 2 appendices, 20 figures. Submitted to A&