Context. Some intra-day variable, compact extra-galactic radio sources show brightness temperatures severely exceeding 10 12 K, the limit set by catastrophic inverse-Compton (IC) cooling in sources of incoherent synchrotron radiation. The violation of the IC limit, actually possible under non-stationary conditions, would lead to IC avalanches in the soft-γ-ray energy band during transient periods. Aims. For the first time, broadband signatures of possible IC catastrophes were searched for in a prototypical source, S5 0716+71. Methods. A multifrequency observing campaign targetting S5 0716+71 was carried out during November 06−20, 2003. The observations, organized under the framework of the European Network for the Investigation of Galactic nuclei through Multifrequency Analysis (ENIGMA) together with a campaign by the Whole Earth Blazar Telescope (WEBT), involved a pointing by the soft-γ-ray satellite INTEGRAL, optical, near-infrared, sub-millimeter, millimeter, radio, as well as Very Long Baseline Array (VLBA) monitoring. Results. S5 0716+71 was very bright at radio frequencies and in a rather faint optical state (R = 14.17−13.64) during the INTEGRAL pointing; significant inter-day and low intra-day variability was recorded in the radio regime, while typical fast variability features were observed in the optical band. No obvious correlation was found between the radio and optical emission. The source was not detected by INTEGRAL, neither by the X-ray monitor JEM-X nor by the γ-ray imager ISGRI, but upper limits to the source emission in the 3−200 keV energy band were estimated. A brightness temperature T b > 2.1 × 10 14 K (violating the IC limit) was inferred from the variability observed in the radio regime, but no corresponding signatures of IC avalanches were recorded at higher energies. Conclusions. In the most plausible scenario of negligible contribution of the interstellar scintillation to the observed radio variability, the absence of the signatures of IC catastrophes provides either a lower limit δ > ∼ 8 to the Doppler factor affecting the radio emission or strong constraints for modelling of the Compton-catastrophe scenario in S5 0716+71.Key words. galaxies: active -galaxies: BL Lacertae objects: general -galaxies: BL Lacertae objects: individual: S5 0716+71 -galaxies: quasars: general -gamma-rays: observations -radiation mechanisms: non-thermal Partially based on observations obtained with INTEGRAL, an ESA project with instruments and science data centre funded by ESA member states (especially the PI countries:

We have converted the former solar electrical plant THEMIS (French Pyrenees) into an atmosphericČerenkov detector called CELESTE, which records gamma rays above 30 GeV (7 × 10 24 Hz). Here we present the first sub-100 GeV -2detection by a ground based telescope of a gamma ray source, the Crab nebula, in the energy region between satellite measurements and imaging atmospherič Cerenkov telescopes. At our analysis threshold energy of 60 ± 20 GeV we measure a gamma ray rate of 6.1 ± 0.8 per minute. Allowing for 30% systematic uncertainties and a 30% error on the energy scale yields an integral gamma ray flux of I(E > 60 GeV) = 6.2 +5.3 −2.3 × 10 −6 photons m −2 s −1 . The analysis methods used to obtain the gamma ray signal from the raw data are detailed. In addition, we determine the upper limit for pulsed emission to be ¡12% of the Crab flux at the 99% confidence level, in the same energy range. Our result indicates that if the power law observed by EGRET is attenuated by a cutoff of form e −E/E 0 then E 0 < 26 GeV. This is the lowest energy probed by ǎ Cerenkov detector and leaves only a narrow range unexplored beyond the energy range studied by EGRET.

The CAT (Cherenkov Array at Thémis) imaging telescope, equipped with a veryhigh-definition camera (546 fast phototubes with 0.12 • spacing surrounded by 54 larger tubes in two guard rings) started operation in Autumn 1996 on the site of the former solar plant Thémis (France). Using the atmospheric Cherenkov technique, it detects and identifies very high energy γ-rays in the range 250 GeV to a few tens of TeV. The instrument, which has detected three sources (Crab nebula, Markarian 421 and Markarian 501), is described in detail.