Observation of multi-TeV gamma rays from the Crab Nebula using the Tibet air shower array

The Tibet experiment, operating at Yangbajing (4300 m above sea level), is the lowest energy air shower array, and thr new high-density array constructed in 1996 is sensitive to gamma-ray air showers at energies as low as 3 TeV. With this new array, the Crab Nebula was observed in multi-TeV gamma-rays and a signal was detected at the 5.5 sigma level. We also obtained the energy spectrum of gamma-rays in the energy region above 3 TeV which partially overlaps those observed with imaging atmospheric Cerenkov telescopes. The Crab spectrum observed in this energy region can be represented by the power-law fit dJ(E)/dE = (4.61 +/- 0.90) x 10(-12)(E/3 TeV)(-2.62+/-0.17) cm(-2) s(-1) TeV-1. This is the first observation of gamma-ray signals from point sources with a conventional air shower array using scintillation detectors.