Observation of gamma-rays greater than 10 TeV from Markarian 421

摘要

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 ~70deg,a signal of 298 +-52 gamma-ray-like events was detected at E > 10 TeV with 5.7sigma 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 4sigma 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.