Results from observations of AGNs with the H.E.S.S. telescope system and Future plans

Springer-Verlag series "ESO Astrophysics Symposia" International audience The H.E.S.S. (High Energy Stereoscopic System) phase I is comprised of four Imaging Atmospheric Cherenkov telescopes, for observation of galactic and cosmic sources of very high energy gamma rays. Its installation in the Khomas highlands, Namibia has been completed in December, 2003. The first of these telescopes was installed in June 2003, and data-taking has proceeded since that time. The HESS telescope system provides a significant improvement in sensitivity and a threshold for detection below that of previous Imaging Atmospheric Cherenkov Telescopes. The characteristics for the phase-I will be presented, together with plans for phase-II of the experiment, comprised of a large telescope in the centre of the current phase-I providing a lowered threshold and increased sensitivity. We can observe AGNs up to redshift 0.5 with HESS and 2-3 with HESS Phase-2, which provides a unique capability for study of spectral and temporal characteristics on timescales of several hours or even less than 1 h (depending on the strength of flares). We will present the first results from a number of southern AGN observed during the installation of the phase-I, in particular concerning the detection and spectral properties of the AGN PKS2155$-$305.