C. Renault

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.

GigaHertz Analogue Memories in ground-based gamma-ray astronomy

The ARS0 (“Analogue Ring Sampler”) GigaHertz Analogue Memory was developed for the ANTARES experiment by DAPNIA-SEI. Its application for an imaging Cherenkov camera is considered. This chip contains five analogue ring buffers with 128 cells each, with sampling on the nanosecond time-scale. After the arrival of a trigger signal, the analogue data may be read out at a slower rate, up to several hundred microseconds later. These characteristics allow a fully-integrated, compact imaging camera to be built based on this circuit. Results of tests on prototype circuit boards, for a possible application to the HESS project, are presented.

Constraints on the IR background based on CAT spectrum of Mkn 501

The TeV and X-ray data obtained by the imaging Cherenkov telescope CAT and X-ray satellite BeppoSAX during the remarkable flare of Mkn 501 in April 16, 1997 are used to constrain the flux of the Cosmic Infrared Background (CIB) using different CIB models. We show that a non-negligible absorption of γ-rays due to the CIB could take place already in the low-energy (sub-TeV) domain of the spectrum of Mkn 501. The density of the near-infrared background with typical “starlight spectrum” around 1 μm should be between 5 and 35 nWu200am−2u200asr−1 (99% CL), with the most likely value around 20 nWu200am−2u200asr−1. We also argue that the CAT γ-ray data alone allow a rather robust upper limits on the CIB, λFλ⩽60u200anWu200am−2u200asr−1 at 1 μm, based on the belief that, within the conventional models of γ-ray production, the differential source spectrum of γ-rays hardly could be flatter than dN/dE∝E−1. The high flux of CIB at ⩽ few μm wavelengths, λFλ⩾10u200anWu200am−2u200asr−1 implies significant distortion of the initial (source) sub-TeV spectrum of γ...

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.