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

(Astrophysical Journal Letter, 579, L9-L12, 2002)

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, H Katagiri, J Kataoka, A Kawachi, T Kifune, H Kubo, J Kushida, S Maeda, A Maeshiro, Y Matsubara, Y Mizumoto, M Mori, M Moriya, H Muraishi, Y Muraki, T Naito, T Nakase, K Nishijima, M Ohishi, J.R Patterson, K Sakurazawa, R Suzuki, D.L Swaby, K Takano, T Takano, T Tanimori, F Tokanai, K Tsuchiya, H Tsunoo, K Uruma, A Watanabe, S Yanagita, T Yoshida, and T Yoshikoshi

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, ~70o a signal of 298+-52 gamma-ray--like events (5.7sigma) 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 derived a differential energy spectrum dN/dE = (3.3+-0.9_{stat.}+-0.3_{syst.}) 10^{-13} (E/10{TeV})^{-(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 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.