OBSERVATIONS OF THE γ-RAY EMISSION ABOVE 250 GeV FROM THE BLAZARS MARKARIAN 501 AND MARKARIAN 421 BY THE CAT CHERENKOV ATMOSPHERIC IMAGING TELESCOPE

The Very High Energy (VHE) γ-ray emission of the closest BL Lacertæ objects Markarian 501 and Markarian 421 has been observed by the CAT telescope in 1997 and 1998. In 1997 Mrk 501 exhibited a remarkable series of flares, with a VHE emission peaking above 250 GeV. The source showed correlated emissions in the X-ray and VHE γ-ray bands, together with intensityspectral hardness correlation in the latter energy range. During small flares in 1998, Mrk 421 became the second extragalactic source detected by CAT. Its spectral properties are compared to those of Mrk 501. Theoretical implications for jet astrophysics are briefly discussed.