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 10(19) eV, 47 above 4 x 10(19) eV, and seven above 10(20) 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 x 10(19) eV, there are one triplet and three doublets within a separation angle of 2.degrees 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 (theta(GC)) distribution expected from the dark matter halo model fits the data as well as an isotropic distribution above 2 x 10(19) and 4 x 10(19) eV, but the fit with the dark matter halo model is poorer than the isotropic distribution above 10(19) eV. The arrival direction distribution of seven 10(20) 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 x 10(19) eV.