Observations of the Crab Nebula with the HEGRA system of IACTs in   convergent mode using a topological trigger
  Routine observations of the Crab Nebula for a total of about 250 hrs, performed with the HEGRA stereoscopic system of 5 imaging atmospheric Cerenkov telescopes in the standard operational mode, have proven the energy threshold of the system to be 500 GeV for small zenith angles (< 20 deg). A topological trigger applied along with the convergent observational mode allows to reduce noticeably the energy threshold of the system down to 350 GeV. Here we present the relevant Monte Carlo simulations as well as the analysis results of 15 hrs Crab Nebula data taken in such an observational mode. From the Crab Nebula data, the final energy threshold was found to be 350 GeV. The estimated gamma ray flux from the Crab Nebula above 350 GeV is (8.1 +- 0.1(stat) +- 0.2(syst)) 10^(-11) ph cm^(-2) s^(-1), which is consistent with recent measurements reported by the STACEE, CELESTE, CAT, and Whipple groups. 
F. Lucarelli (1), A. Konopelko (2), F. Aharonian (2), W. Hofmann (2),   A. Kohnle (2), H. Lampeitl (2) and V. Fonseca (1) ((...