Observation of Gamma Rays Greater than 10 T[CLC]e[/CLC]V from Markarian 421
K. Okumura, A. Asahara, G. V. Bicknell, P. G. Edwards, R. Enomoto, S. Gunji, S. Hara, T. Hara, S. Hayashi, C. Itoh, S. Kabuki, F. Kajino
We have observed Markarian 421 in January and March 2001 with the CANGAROO-II imaging Cherenkov telescope during an extraordinarily high state at TeV energies. From 14 hours observations at very large zenith angles, ∼70^∘, a signal of 298 ± 52 gamma-ray--like events (5.7 σ) was detected at E>10 TeV, where a higher sensitivity is achieved than those of usual observations near the zenith, owing to a greatly increased collecting area. Under the assumption of an intrinsic power-law spectrum, we
... ved a differential energy spectrum dN/dE = (3.3 ± 0.9_stat.± 0.3_syst.)×10^-13 (E/10 Te V)^-(4.0 ^+0.9_-0.6_stat.± 0.3_syst.) ph./cm^2/sec/TeV, which is steeper than those previously measured around 1 TeV, and supports the evidence for a cutoff in the spectrum of Markarian 421. However, the 4 σ 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.