University of Montpellier

About: University of Montpellier is a education organization based out in Montpellier, Languedoc-Roussillon, France. It is known for research contribution in the topics: Population & Context (language use). The organization has 26816 authors who have published 53843 publications receiving 1646905 citations. The organization is also known as: Université de Montpellier.

Fermi observations of GRB 090510 : A short-hard gamma-ray burst with an additional, hard power-law component from 10 keV to GeV energies

Abstract: We present detailed observations of the bright short-hard gamma-ray burst GRB 090510 made with the Gammaray Burst Monitor (GBM) and Large Area Telescope (LAT) on board the Fermi observatory. GRB 090510 is the first burst detected by the LAT that shows strong evidence for a deviation from a Band spectral fitting function during the prompt emission phase. The time-integrated spectrum is fit by the sum of a Band function with E-peak = 3.9 +/- 0.3 MeV, which is the highest yet measured, and a hard power-law component with photon index -1.62 +/- 0.03 that dominates the emission below approximate to 20 keV and above approximate to 100 MeV. The onset of the high-energy spectral component appears to be delayed by similar to 0.1 s with respect to the onset of a component well fit with a single Band function. A faint GBM pulse and a LAT photon are detected 0.5 s before the main pulse. During the prompt phase, the LAT detected a photon with energy 30.5(-2.6)(+5.8) GeV, the highest ever measured from a short GRB. Observation of this photon sets a minimum bulk outflow Lorentz factor, Gamma greater than or similar to 1200, using simple.. opacity arguments for this GRB at redshift z = 0.903 and a variability timescale on the order of tens of ms for the approximate to 100 keV-few MeV flux. Stricter high confidence estimates imply Gamma greater than or similar to 1000 and still require that the outflows powering short GRBs are at least as highly relativistic as those of long-duration GRBs. Implications of the temporal behavior and power-law shape of the additional component on synchrotron/synchrotron self-Compton, external-shock synchrotron, and hadronic models are considered.

First principles reaction modeling of the electrochemical interface: Consideration and calculation of a tunable surface potential from atomic and electronic structure

Abstract: A method for calculating and subsequently tuning the electrochemical potential of a half cell using periodic plane-wave density functional theory and a homogenous counter-charge is presented and evaluated by comparison to simulations which explicitly model the countercharge by a plane of ions. The method involves the establishment of two reference potentials, one related to the potential of the free electron in vacuo, and the other related to the potential of ${\mathrm{H}}_{2}\mathrm{O}$ species far from the electrode. The surface potential can be specifically adjusted by the explicit introduction of excess or deficit surface charges in the simulation cell and the application of periodic boundary conditions. We demonstrate the absence of field emission from the electrode over the range of realistic electrochemical potentials covered and confirm that the method can explicitly determine reaction energies and adsorption geometries as a function of electrochemical potential. This latter point is most useful as it asserts the viability of this method to model electrochemical and electrocatalytical systems of academic as well as applied interest. We present two case studies. The first examines the changes in the structure of water at the metal interface as a function of potential over $\mathrm{Cu}(111)$. At cathodic potential, we observe the repulsion of ${\mathrm{H}}_{2}\mathrm{O}$ from the interface and the rotation of the water dipole toward the interface. The second study follows the initial pathways for the electrocatalytical activation of methanol over $\mathrm{Pt}(111)$ and the corresponding potential dependent reaction energetics for these paths. The results demonstrate that changes in the electrochemical potential can significantly alter the reaction energetics as well as the overall reaction selectivity. While the case studies presented herein described equilibrium geometries (i.e., the ideal forms at zero kelvin), the method is also suitable for application to ensembles of thermally activated systems.