Gamma - Rays 1 Observation of gamma - rays greater than 10 TeV from

We have observed Markarian 421 during an extraordinarily high state in January and March 2001 with the CANGAROO-II imaging Cherenkov telescope. From 14 hours observations at a large zenith angle of ∼70◦, a signal of 298± 52 gamma-ray–like events was detected at E > 10 TeV with 5.7 σ significance. Under the assumption of power-law spectrum, we derived a steep energy spectrum of photon index ∼4.0, and supports the evidence for a cutoff in the spectrum of Markarian 421. However, the 4σ excess at energies greater than 20 TeV in our data favors a cutoff energy of ∼8 TeV, at the upper end of the range previously reported from measurements at TeV energies.