Small-Scale Anisotropy of Cosmic Rays above 1019 eV Observed with the Akeno Giant Air Shower Array

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 significant 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 2.°5, and the probability of observing 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 cos(θGC) distribution expected from the dark matter halo model fits the data as well as an isotropic distribution above 2 × 1019 and 4 × 1019 eV, but the fit 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.