Temporal and spectral gamma-ray properties of Mkn 421 above 250 GeV from CAT observations between 1996 and 2000

The gamma -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, mak ing it the second extragalactic source detected by CAT. The time-averaged d ifferential spectrum has been measured from 0.3 to 5 TeV,which is well fitt ed with a power law: d phi /dE proportional to E-TeV(-2.88 +/- 0.12stat +/- 0.06syst). In 2000, the source showed an unprecedented activity, with vari ability time-scales as short as one hour, as for instance observed during t he night between 4 and 5 February. The 2000 time-averaged spectrum measured is compatible with that of 1998, but some indication of a spectral curvatu re is found between 0.3 and 5 TeV. The possibility of TeV spectral hardenin g during flares is also discussed, and the results are compared to those ob tained on the other TeV BL Lac, Markarian 501.