Observation of gamma rays greater than 10 TeV 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, H. Katagiri, J. Kataoka, A. Kawachi, T. Kifune, H. Kubo, J. Kushida, S. Maeda, A. MaeshiroY. 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, T. Yoshikoshi

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

25 Citations (Scopus)


We have observed Markarian 421 in 2001 January and March with the CANGAROO-II imaging Cerenkov telescope during an extraordinarily high state at TeV energies. From 14 hr of 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 derived a differential energy spectrum dN/dE = (3.3 ± 0.9stat ± 0.3syst) × 10-13(E/10 TeV) -(4.0-0.6+0.9, stat±0.3syst) photons cm-2 s -1 TeV-1, which is steeper than those previously measured around 1 TeV and supports the evidence for a cutoff in the spectrum of Mrk 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.

Original languageEnglish
Pages (from-to)L9-L12
JournalAstrophysical Journal
Issue number1 II
Publication statusPublished - 2002 Nov 1
Externally publishedYes


  • BL Lacertae objects: individual (Markarian 421)
  • Gamma rays: observations

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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