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H. Krawczynski (Max-Planck-Institut f\"ur Kernphysik, Heidelberg, Germany), HEGRA Collaboration
The HEGRA System of 4 (since fall 1998: 5) Imaging Atmospheric Cherenkov Telescopes, with an energy threshold of about 500 GeV, angular resolution of 0.1\circ, energy resolution of 20% and an energy flux sensitivity \nu f\nu at 1 TeV of approximately 10-11 ergs/cm2 sec for 1 hour observation time, has been used since January 1997 for a comprehensive study of the TeV gamma-ray emission from the two BL Lac objects Mkn 501 and Mkn 421. While in 1997 Mkn 501 was in a very high state with an average flux of about 3 Crab and showed dramatic flux variations on one day time scale, Mkn 421 was at a rather modest flux level, one order of magnitude below Mkn 501. Based on a large 1997/1998 data base of approximately 150 hours observation time per source, we report detailed spectral information for both sources and compare the temporal and spectral characteristics of these two BL Lac objects. We briefly discuss the implications of these results on the physics of relativistic jets in Active Galactic Nuclei.
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