Initiation à une expérience de diffusion d'électrons : la diffusion Compton virtuelle

Stage Janus 05 A1-Collaboration, Institut für Kernphysik, Universität Mainz, D-55099 Mainz, German

Precipitation on the lee side of the Vosges Mountains: Multi-instrumental study of one case from the COPS campaign

In this article, we focus on one case study from the Convective and Orographically-induced Precipitation Study (COPS), which took place in north-eastern France and south-western Germany during the summer of 2007, in a low mountain area. We investigate lee side precipitation due to shallow and deep convection during one Intensive Observation Period (IOP) of COPS which have been well documented by all the instruments. For that aim, we use a set of observations from radars, radiosoundings, satellite, and a network of Global Positioning System (GPS) receivers, as well as meteorological analyses and dedicated model simulation results. The combination of these measurements with GPS tomography results suggests the role of low level water vapour accumulation and convergence as a precursor to the convective initiation. The origin of this moistening and wind convergence seems to be linked to a slight change in the wind direction in the north-west part of the COPS domain. Using a high resolution X band radar, we also describe how small scale orography affects the precipitation locations, and we show the role of hills near the mouths of the valleys in convective enhancement. This observation is confirmed by model simulation showing that convection is no longer enhanced when the hills are suppressed. The further intensification of one convective cell over the Rhine Valley, which is climatologically frequent, is also discussed

Simultaneous multi-frequency observation of the unknown redshift blazar PG 1553+113 in March-April 2008

The blazar PG 1553+113 is a well known TeV gamma-ray emitter. In this paper, we determine its spectral energy distribution using simultaneous multi-frequency data in order to study its emission processes. An extensive campaign was carried out between March and April 2008, where optical, X-ray, high-energy (HE) gamma-ray, and very-high-energy (VHE) gamma-ray data were obtained with the KVA, Abastumani, REM, RossiXTE/ASM, AGILE and MAGIC telescopes, respectively. This is the first simultaneous broad-band (i.e., HE+VHE) gamma-ray observation, though AGILE did not detect the source. We combine data to derive source's spectral energy distribution and interpret its double peaked shape within the framework of a synchrotron self compton modelComment: 5 pages, 2 figures, publishe

MAGIC Upper Limits for two Milagro-detected, Bright Fermi Sources in the Region of SNR G65.1+0.6

We report on the observation of the region around supernova remnant G65.1+0.6 with the stand-alone MAGIC-I telescope. This region hosts the two bright GeV gamma-ray sources 1FGL J1954.3+2836 and 1FGL J1958.6+2845. They are identified as GeV pulsars and both have a possible counterpart detected at about 35 TeV by the Milagro observatory. MAGIC collected 25.5 hours of good quality data, and found no significant emission in the range around 1 TeV. We therefore report differential flux upper limits, assuming the emission to be point-like (<0.1 deg) or within a radius of 0.3 deg. In the point-like scenario, the flux limits around 1 TeV are at the level of 3 % and 2 % of the Crab Nebula flux, for the two sources respectively. This implies that the Milagro emission is either extended over a much larger area than our point spread function, or it must be peaked at energies beyond 1 TeV, resulting in a photon index harder than 2.2 in the TeV band.Comment: 8 pages, 3 figures, 1 tabl

How to estimate total differential attenuation due to hydrometeors with ground-based multi-frequency radars?

Abstract. At millimeter wavelengths, attenuation by hydrometeors, such as liquid droplets or large snowflakes, is generally not negligible. When using multi-frequency ground-based radar measurements, it is common practice to use the Rayleigh targets at cloud top as a reference in order to derive attenuation-corrected reflectivities and meaningful dual-frequency ratios (DFR). By capitalizing on this idea, this study describes a new quality-controlled approach aiming at identifying regions of the cloud where particle growth is negligible. The core of the method is the identification of a Rayleigh plateau, i.e. a large enough region near cloud top where the vertical gradient of DFR remains small. By analyzing collocated Ka-W band radar and microwave radiometer (MWR) observations taken at two European sites under various meteorological conditions, it is shown how the resulting estimates of differential path-integrated attenuation (DeltaPIA) can be used to characterize hydrometeor properties. When the DeltaPIA is predominantly produced by cloud liquid droplets, this technique alone can provide accurate estimates of the liquid water path. When combined with MWR observations, this methodology paves the way towards profiling the cloud liquid water and/or quality flagging the MWR retrieval for rain/drizzle contamination and/or estimating the snow differential attenuation