The cosmic-ray energy spectrum above 10 18.5 eV is reported using the updated data set of the Akeno Giant Air Shower Array (AGASA) from February 1990 to October 1997. The energy spectrum extends beyond 10 20 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.PACS numbers: 98.70. Sa, 96.40.Pq, 96.40.De Typeset using REVT E X

Anisotropy in the arrival directions of cosmic rays with energies above 10 17 eV is studied using data from the Akeno 20 km 2 array and the Akeno Giant Air Shower Array (AGASA), using a total of about 114,000 showers observed over 11 years. In the first harmonic analysis, we have found strong anisotropy of ∼ 4% around 10 18 eV, corresponding to a chance probability of ∼ 0.2% after taking the number of independent trials into account. With two dimensional analysis in right ascension and declination, this anisotropy is interpreted as an excess of showers near the directions of the Galactic Center and the Cygnus region.

With the Akeno Giant Air Shower Array, 581 cosmic rays above 1019 eV, 47 above 4 ] 1019 eV, and seven above 1020 eV were observed until 1998 August. The arrival direction distribution of these extremely high energy cosmic rays has been studied. While no signiÐcant large-scale anisotropy is found on the celestial sphere, some interesting clusters of cosmic rays are observed. Above 4 ] 1019 eV, there are one triplet and three doublets within a separation angle of and the probability of observing 2¡ .5, these clusters by a chance coincidence under an isotropic distribution is smaller than 1%. The triplet is especially observed against expected 0.05 events. The distribution expected from the dark cos (h GC ) matter halo model Ðts the data as well as an isotropic distribution above 2 ] 1019 and 4 ] 1019 eV, but the Ðt with the dark matter halo model is poorer than the isotropic distribution above 1019 eV. The arrival direction distribution of seven 1020 eV cosmic rays is consistent with that of lower energy cosmic rays and is uniform. Three of the seven are members of doublets above about 4 ] 1019 eV.

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Using data from more than ten-years of observations with the Akeno Giant Air Shower Array (AGASA), we published a result that the energy spectrum of ultra-high energy cosmic rays extends beyond the cutoff energy predicted by Greisen, and Zatsepin and Kuzmin. In this paper, we reevaluate the energy determination method used for AGASA events with respect to the lateral distribution of shower particles, their attenuation with zenith angle, shower front structure, delayed particles observed far from the core and other factors. The currently assigned energies of AGASA events have an accuracy of $\pm$25% in event-reconstruction resolution and $\pm$18% in systematic errors around 10$^{20}$eV. This systematic uncertainty is independent of primary energy above 10$^{19}$eV. Based on the energy spectrum from 10$^{14.5}$eV to a few times 10$^{20}$eV determined at Akeno, there are surely events above 10$^{20}$eV and the energy spectrum extends up to a few times 10$^{20}$eV without a GZK-cutoff.Comment: accepted to Astroparticle Physics, 29pages, 17+1 figures, elsart.cls The AGASA exposure is 5.1x10^16 m2 sr s until July 200