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The Universe Viewed in Gamma-Rays 1 Observation of gamma-rays greater than 10 TeV from

By Kimihiro Okumura


for CANGAROO collaboration 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 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. 1

Year: 2012
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