Abstract. The γ-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 differential spectrum has been measured from 0.3 to 5 TeV, which is well fitted with a power law: dφ dE _ E −2.88±0.12 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.

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.