B. Fabre

Temporal and spectral gamma-ray properties of Mkn 421 above 250 GeV from CAT observations between 1996 and 2000

The -ray emission above 250 GeV from the BL Lac object Markarian 421 was observed by the CAT Cherenkov imaging telescope between December, 1996, and June, 2000. In 1998, the source produced a series of small flares, making it the second extragalactic source detected by CAT. The time-averaged dierential spectrum has been measured from 0: 3t o 5 TeV, which is welltted with ap ower law: dE E 2:880:12 stat 0:06 syst TeV . In 2000, the source showed an unprecedented activity, with variability time-scales as short as one hour, as for instance observed during the night between 4 and 5 February. The 2000 time-averaged spectrum measured is compatible with that of 1998, but some indication of a spectral curvature is found between 0:3 and 5 TeV. The possibility of TeV spectral hardening during flares is also discussed, and the results are compared to those obtained on the other TeV BL Lac, Markarian 501.

A NEW ANALYSIS METHOD FOR VERY HIGH DEFINITION IMAGING ATMOSPHERIC CHERENKOV TELESCOPES AS APPLIED TO THE CAT TELESCOPE

Abstract A new method of shower-image analysis is presented which appears very powerful as applied to those Cherenkov Imaging Telescopes with very high definition imaging capability. It provides hadron rejection on the basis of a single cut on the image shape, and simultaneously determines the energy of the electromagnetic shower and the position of the shower axis with respect to the detector. The source location is also reconstructed for each individual γ-ray shower, even with one single telescope, so for a point source the hadron rejection can be further improved. As an example, this new method is applied to data from the CAT (Cherenkov Array at Themis) imaging telescope, which has been operational since Autumn, 1996.

CELESTE: an atmospheric Cherenkov telescope for high energy gamma astrophysics

Abstract CELESTE is an atmospheric Cherenkov telescope based on the sampling method which makes use of the de-commissioned THEMIS solar electrical plant in the French Pyrenees. A large (2000 m 2 ) mirror surface area from 40 independent heliostats followed by a secondary optic, a trigger system using analog summing techniques and signal digitization with 1 GHz flash ADCs make possible the detection of cosmic γ-rays down to 30 GeV . This paper provides a detailed technical description of the CELESTE installation.

First detection of gamma rays from the crab nebula with the CELESTE “solar farm” Cherenkov detector

Abstract We have converted the THEMIS solar array (French Pyrenees) into an atmospheric Cherenkov telescope, called CELESTE, sensitive to astrophysical gamma rays above 30 GeV (7×10 24 Hz). In early 1998 the Crab nebula was detected at 80 GeV with a preliminary 18 heliostat setup. The full 40 heliostat array has since been commissioned. The STACEE experiment using the same technique in New Mexico is also analysing their first data. Thus, the window between the EGRET instrument and the Cherenkov imagers has been opened. We describe the CELESTE detector and the data analysis, and discuss the prospects for studying AGN (specifically, blazars) and galactic sources in this energy range.

Search for diffuse gamma rays from the Galactic anticentre region above 100 GeV with CELESTE

Context. Located in the French Pyrenees, CELESTE was the first ground-based γ-ray telescope with an energy threshold below 100 GeV. It acquired data from 1999 to 2004, and allowed flux measurements of the Crab nebula and the blazars Mrk 421 and Mrk 501. Aims. We search for Galactic diffuse γ-ray emission, which is most significant around the Galactic plane, for b = [−5 ◦ , +5 ◦ ]. Methods. By using the significant data set available for the Crab nebula, we selected Crab OFF-source data at various Galactic latitudes, in order to analyse the diffuse emission. Selection criteria were applied to the sky position, atmospheric conditions, and detector stability. Results. We obtained 108 mn of data in the Galactic anticentre region, providing the first upper limits of around 100 GeV to the diffuse γ-ray emission with atmospheric Cherenkov detectors. These limits are φ UL (E > 140 GeV) = 9.4 × 10 −3 m −2 s −1 sr −1 and φ UL (E > 120 GeV) = 1.2 × 10 −2 m −2 s −1 sr −1 .

Observation of supernova remnants with the CAT Cherenkov imaging telescope

Supernova remnants (SNR) can be broadly classified into two types: plerionic or plasma-filled SNR, like the Crab Nebula, and shell-type SNR, like Cassiopeia A (Cas A). VHE observations of the Crab Nebula and Cas A made with the CAT atmospheric Cherenkov imaging telescope are used to constrain models for production of gamma rays in SNR. For both plerionic and shell-type SNR, these observations serve primarily to impose a limit on the magnetic field in the region where the particles are accelerated.