Extension of the Cosmic-Ray Energy Spectrum beyond the Predicted Greisen-Zatsepin-Kuz’min Cutoff

M. Takeda,1 N. Hayashida,1 K. Honda,2 N. Inoue,3 K. Kadota,4 F. Kakimoto,4 K. Kamata,5 S. Kawaguchi,6 Y. Kawasaki,7 N. Kawasumi,8 H. Kitamura,9 E. Kusano,3 Y. Matsubara,10 K. Murakami,11 M. Nagano,12 D. Nishikawa,1 H. Ohoka,1 N. Sakaki,1 M. Sasaki,1 K. Shinozaki,3 N. Souma,3 M. Teshima,1 R. Torii,1 I. Tsushima,8 Y. Uchihori,13 T. Yamamoto,1 S. Yoshida,1 and H. Yoshii14 1Institute for Cosmic Ray Research, University of Tokyo, Tokyo 188-8502, Japan 2Faculty of Engineering, Yamanashi University, Kofu 400-8511, Japan 3Department of Physics, Saitama University, Urawa 338-8570, Japan 4Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan 5Nishina Memorial Foundation, Komagome, Tokyo 113, Japan 6Faculty of General Education, Hirosaki University, Hirosaki 036-8560, Japan 7Department of Physics, Osaka City University, Osaka 558-8585, Japan 8Faculty of Education, Yamanashi University, Kofu 400-8510, Japan 9Department of Physics, Kobe University, Kobe 657-8501, Japan 10Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya 464-8601, Japan 11Nagoya University of Foreign Studies, Nissin, Aichi 470-0131, Japan 1211-401, 5-8 Higashi, Hasuda, Saitama 394-0111, Japan 13National Institute of Radiological Sciences, Chiba 263-8555, Japan 14Department of Physics, Ehime University, Matsuyama 790-8577, Japan (Received 26 May 1998) The cosmic-ray energy spectrum above 1018.5 eV is reported using the updated data set of the Akeno Giant Air Shower Array from February 1990 to October 1997. The energy spectrum extends beyond 1020 eV and the energy gap between the highest energy event and the others is being filled up with recently observed events. The spectral shape suggests the absence of the 2.7 K cutoff in the energy spectrum or a possible presence of a new component beyond the 2.7 K cutoff. [S0031-9007(98)06893-8]