Very High-energy Gamma-ray Sources as Seen by the Cat Imaging Telescope

The CAT (Cerenkov Array at Thémis) γ-ray detector 1 operates above 250 GeV since Autumn 1996. Its observation program has been largely devoted to the study of blazars. These objects are radio-loud active galactic nuclei (AGN’s) with a relativistic jet which is pointed directly to the observer. The jet emission dominates that of the accretion disk (and of the host galaxy) over a large energy domain. Thus, blazar observations offer the possibility of investigating the physics of jets more deeply, including particle acceleration and energy extraction in the vicinity of the AGN’s central “engine”. Until now, however, only two blazars have been clearly detected a at very high-energies (VHE) from the Northern hemisphere, these are Markarian 501 (Mkn 501) and Markarian 421 (Mkn 421). Since both are variable sources, their study by an atmospheric Cherenkov experiment requires a kind of “test beam” to check that the detector response is well under control. This is further mandatory if one wishes to compare the temporal and spectral properties of these blazars between different observation epochs. In VHE γ-ray astronomy, the Crab nebula is a strong source with a steady flux which can be used as a standard candle. In the following, we review briefly the results obtained by CAT on this source over four years, before dealing with the observations of Mkn 501 and Mkn 421.