Small-scale anisotropy of cosmic rays above 1019eV observed with the Akeno Giant Air Shower Array
Abstract
With the Akeno Giant Air Shower Array (AGASA), 581 cosmic rays above 1019eV, 47 above 4 x 1019eV, and 7 above 1020eV are observed until August 1998. 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 1019eV, there are one triplet and three doublets within separation angle of 2.5o and the probability of observing these clusters by a chance coincidence under an isotropic distribution is smaller than 1 %. Especially the triplet is 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 x 1019eV and 4 x 1019eV, but is a poorer fit than isotropy above 1019eV. Arrival direction distribution of seven 1020eV cosmic rays is consistent with that of lower energy cosmic rays and is uniform. Three of seven are members of doublets above about 4 x 1019eV.
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