[en] The CELESTE experiment uses the heliostats of an old solar farm in the French Pyrenees to detect gamma ray air showers by the atmospheric Cerenkov technique. CELESTE has been operating since November 1999 with an array of 40 heliostats fully instrumented with 1 GHz flash ADCs. Significant advances have been made in the detector simulations and in the data analysis techniques. We report here on results from recent observations of the Crab nebula above an energy threshold of 50 GeV. The results and simulations illustrate the current sensitivity of the experiment

[en] The CELESTE solar farm gamma ray telescope detected the Crab Nebula near 80 GeV in early 1998, with 18 heliostats. CELESTE is now in its final configuration with 40 heliostats, with a trigger threshold below 30 GeV (7x10²⁴ Hz) and a rate above 20 Hz. Overall, the detector is calibrated and aligned. Delays during construction combined with remarkably bad weather resulted in small data sets, and evidence for a Crab signal obtained when 25 heliostats were operational is weak. We describe the detector, present the current state of the data analysis and discuss our observations

[en] Two new analysis methods for imaging atmospheric Cherenkov telescopes have been developed in the framework of the H.E.S.S. experiment. Both methods make use of the full stereoscopic information, beyond the Hillas parameters of each image. The first method is a simple 3D-modeling of an electromagnetic shower; the second is based on an analytical model of the light profile in the images. Both methods have confirmed the standard H.E.S.S. analysis for the currently observed sources. They further provide some improvement in angular resolution and are well-suited to background subtraction for extended sources

[en] Following the discovery of the cosmic rays by Victor Hess in 1912, more than 70 years and numerous technological developments were needed before an unambiguous detection of the first very-high-energy gamma-ray source in 1989 was made. Since this discovery, the field on very-high-energy gamma-ray astronomy experienced a true revolution: a second, then a third generation of instruments were built, observing the atmospheric cascades from the ground, either through the atmospheric Cherenkov light they comprise, or via the direct detection of the charged particles they carry. Present arrays, 100 times more sensitive than the pioneering experiments, have detected a large number of astrophysical sources of various types, thus opening a new window on the non-thermal Universe. New, even more sensitive instruments are currently being built; these will allow us to explore further this fascinating domain. In this article we describe the detection techniques, the history of the field and the prospects for the future of ground-based very-high-energy gamma-ray astronomy. (authors)

[en] The analysis method initially developed for the single telescope C.A.T. (Cerenkov At Themis) has been recently improved for stereoscopic observations with H.E.S.S. (High Energy Stereoscopic System). This method is based on an analytical model giving the light distribution resulting from a gamma-ray initiated shower in the atmosphere in the focal plane of the cameras. It simultaneously reconstructs the energy and source location of the initiating gamma-ray. Results of this method applied to H.E.S.S. Crab Nebula data are shown together with its spectrum. The potential of the H.E.S.S. experiment to analyse off-axis or extended source with this method is highlighted

[en] The shell-type supernova remnant (SNR) RX J1713.7-3946 (G347.3-0.5) was discovered with ROSAT in X-rays and later claimed as source of TeV γ-rays. This object, together with several other southern hemisphere SNRs, is a prime target for observations with the High Energy Stereoscopic System (H.E.S.S.), a new system of imaging atmospheric Cherenkov telescopes which was completed at the end of 2003 in Namibia and is now in full operation. Here we report on observations of the SNR RX J1713.7-3946 which have been performed during the construction and commissioning of the H.E.S.S. system (data originally published here). We confirm TeV emission from this source and present the first ever γ-ray image of an astronomical object resolved on arc minute scales. This image shows shell morphology similar to that seen in X-rays, however at photon energies some nine orders of magnitude higher. The characteristics of the energy spectrum imply efficient acceleration of charged particles to energies beyond 100 TeV, consistent with current ideas of particle acceleration in young SNR shocks

[en] We have converted the THEMIS solar array (French Pyrenees) into an atmospheric Cherenkov telescope, called CELESTE, sensitive to astrophysical gamma rays above 30 GeV (7x10²⁴ 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

[en] The detection of an unidentified extended TeV γ-ray source in the Southern Cross region close to the galactic plane being named HESS J1303-631 is reported. The observations have been performed between February and June 2004 with the new stereoscopic system of four Cherenkov telescopes operated by the H.E.S.S. collaboration in Namibia. The telescopes were initially pointed to the binary system PSR B1259-63/SS 2883 which was for the first time detected at TeV energies within this observation campaign (see parallel paper). In the same dataset the unidentified TeV source HESS J1303-631 has been discovered serendipitously roughly 0.6 deg. north of the PSR B1259-63 position leading --for the first time in TeV γ-ray astronomy - to the detection of two sources within the same field of view. The new source is extended on the 0.2 deg. level and - up to now - no counterpart in other wavelengths has been identified. The measured flux is compatible with constant emission on the 10% flux level of the Crab nebula and shows a hard energy spectrum which can be described by a power-law with an index of Γ = 2.2 ± 0.2stat. In this paper various consistency checks which confirm the celestial origin of the observed excess are presented and preliminary results on the source extension and energy spectrum of the source are reported

[en] Mkn 421 was observed during a high flux state for nine nights in April and May 2004 with the fully operational High Energy Stereoscopic System (H.E.S.S.) in Namibia. The observations were carried out at zenith angle distances of 61-64 deg., which result in an increased average threshold of 1.5 TeV. Observations of the Crab nebula at similar zenith angles are used to check on the reliability of the large zenith angle observation technique. Roughly 7000 photons from Mkn421 were accumulated with an average gamma-ray rate of 11 gammas/min. The overall significance of the detection exceeds 80 standard deviations. The light curve shows night-by-night variations by a factor of 2. The energy spectrum is curved and may be fit with a power law with an exponential cut-off at 2.1 TeV

[en] The HESS experiment is now fully operational with the four telescopes installed by the end of December, 2003. Many galactic and extragalactic objects have been observed since operation began and the detection of various sources has proven the performance of the detector and validated the technical options chosen. The collaboration is currently studying the next phase of the HESS project. The detector system currently in operation has a threshold around 100 GeV. Many sources such as pulsars, micro-quasars, or neutralino annihilation are expected to emit gamma radiation at lower energy. The second phase of the HESS experiment consists of an additional larger telescope positioned in the centre of the existing four-telescope array. The new system may reach a threshold as low as 10-20 GeV in single telescope mode and about 50 GeV in coincidence with the four other telescopes. It will also improve the sensitivity of the existing system above 100 GeV. The construction should start next year and the installation is expected to take place in 2008, less than one year after the launch of the GLAST satellite. After a brief overview of the HESS phase I experiment, we will describe the upgraded parameters of the HESS camera. Then the set-up and expected performance are presented