2,517 research outputs found
Composite mirror facets for ground based gamma ray astronomy
Composite mirrors for gamma-ray astronomy have been developed to fulfill the
specifications required for the next generation of Cherenkov telescopes
represented by CTA (Cherenkov Telescope Array). In addition to the basic
requirements on focus and reflection efficiency, the mirrors have to be stiff,
lightweight, durable and cost efficient. In this paper, the technology
developed to produce such mirrors is described, as well as some tests that have
been performed to validate them. It is shown that these mirrors comply with the
needs of CTA, making them good candidates for use on a significant part of the
array.Comment: 16 pages, 13 figures, accepted to be published on NIM
An ETKF approach for initial state and parameter estimation in ice sheet modelling
International audienceEstimating the contribution of Antarctica and Greenland to sea-level rise is a hot topic in glaciology. Good estimates rely on our ability to run a precisely calibrated ice sheet evolution model starting from a reliable initial state. Data assimilation aims to provide an answer to this problem by combining the model equations with observations. In this paper we aim to study a state-of-the-art ensemble Kalman filter (ETKF) to address this problem. This method is implemented and validated in the twin experiments framework for a shallow ice flowline model of ice dynamics. The results are very encouraging, as they show a good convergence of the ETKF (with localisation and inflation), even for small-sized ensembles
Micromegas in a Bulk
In this paper we present a novel way to manufacture the bulk Micromegas
detector. A simple process based on the PCB (Printed Circuit Board) technology
is employed to produce the entire sensitive detector. Such fabrication process
could be extended to very large area detectors made by the industry. The low
cost fabrication together with the robustness of the electrode materials will
make it extremely attractive for several applications ranging from particle
physics and astrophysics to medicineComment: 6 pages, 4 figure
Impact of mesh resolution for MISMIP and MISMIP3d experiments using Elmer/Ice
International audienceThe dynamical contribution of marine ice sheets to sea level rise is largely controlled by grounding line (GL) dynamics. Two marine ice sheet model intercomparison exercises , namely MISMIP and MISMIP3d, have been proposed to the community to test and compare the ability of models to capture the GL dynamics. Both exercises are known to present a discontinuity of the friction at the GL, which is believed to increase the model sensitivity to mesh resolution. Here, using Elmer/Ice, the only Stokes model which completed both intercomparisons, the sensitivity to the mesh resolution is studied from an extended MISMIP experiment in which the friction continuously decreases over a transition distance and equals zero at the GL. Using this MISMIP-like setup, it is shown that the sensitivity to the mesh resolution is not improved for a vanishing friction at the GL. For the original MISMIP experiment, i.e. for a discontinuous friction at the GL, we further show that the results are moreover very sensitive to the way the friction is interpolated in the close vicinity of the GL. In the light of these new insights, and thanks to increased computing resources, new results for the MISMIP3d experiments obtained for higher resolutions than previously published are made available for future comparisons as the Supplement
Modeling Northern Hemisphere ice-sheet distribution during MIS 5 and MIS 7 glacial inceptions
The present manuscript compares Marine Iso- tope Stage 5 (MIS 5, 125â115 kyr BP) and MIS 7 (236â 229 kyr BP) with the aim to investigate the origin of the difference in ice-sheet growth over the Northern Hemi- sphere high latitudes between these last two inceptions. Our approach combines a low resolution coupled atmosphereâ oceanâsea-ice general circulation model and a 3-D thermo- mechanical ice-sheet model to simulate the state of the ice sheets associated with the inception climate states of MIS 5 and MIS 7. Our results show that external forcing (orbitals and GHG) and sea-ice albedo feedbacks are the main fac- tors responsible for the difference in the land-ice initial state between MIS 5 and MIS 7 and that our cold climate model bias impacts more during a cold inception, such as MIS 7, than during a warm inception, such as MIS 5. In addition, if proper ice-elevation and albedo feedbacks are not taken into consideration, the evolution towards glacial inception is hardly simulated, especially for MIS 7. Finally, results high- light that while simulated ice volumes for MIS 5 glacial in- ception almost fit with paleo-reconstructions, the lack of pre- cipitation over high latitudes, identified as a bias of our cli- mate model, does not allow for a proper simulation of MIS 7 glacial inception
Prospects for a Dark Matter annihilation signal towards the Sagittarius dwarf galaxy with ground based Cherenkov telescopes
Dwarf galaxies are widely believed to be among the best targets for indirect
dark matter searches using high-energy gamma rays; and indeed gamma-ray
emission from these objects has long been a subject of detailed study for
ground-based atmospheric Cherenkov telescopes. Here, we update current
exclusion limits obtained on the closest dwarf, the Sagittarius dwarf galaxy,
in light of recent realistic dark matter halo models. The constraints on the
velocity-weighted annihilation cross section of the dark matter particle are of
a few 10 cms in the TeV energy range for a 50 h exposure.
The limits are extrapolated to the sensitivities of future Cherenkov Telescope
Arrays. For 200 h of observation time, the sensitivity at 95% C.L. reaches
10 cms. Possible astrophysical backgrounds from gamma-ray
sources dissembled in Sagittarius dwarf are studied. It is shown that with
long-enough observation times, gamma-ray background from millisecond pulsars in
a globular cluster contained within Sagittarius dwarf may limit the sensitivity
to dark matter annihilations.Comment: 12 pages, 5 figures, 2 tables, accepted for publication in Ap
Search for direct CP-violation in K+- --> pi+-pi0pi0 decays
A search for direct CP-violation in K+- --> pi+-pi0pi0 decays based on 47.14
million events has been performed by the NA48/2 experiment at the CERN SPS. The
asymmetry in the Dalitz plot linear slopes A_g=(g^+ - g^-)/(g^+ + g^-) is
measured to be A_g=(1.8 +- 2.6).10^{-4}. The design of the experiment and the
method of analysis provide good control of instrumental charge asymmetries in
this measurement. The precision of the result is limited by statistics and is
almost one order of magnitude better than that of previous measurements by
other experiments.Comment: 14 page
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