Small-scale anisotropy of cosmic rays above 10^19eV observed with the Akeno Giant Air Shower Array

摘要

With the Akeno Giant Air Shower Array (AGASA), 581 cosmic rays above 10^19eV,47 above 4 x 10^19eV, and 7 above 10^20eV are observed until August 1998.Arrival direction distribution of these extremely high energy cosmic rays hasbeen studied. While no significant large-scale anisotropy is found on thecelestial sphere, some interesting clusters of cosmic rays are observed. Above4 x 10^19eV, there are one triplet and three doublets within separation angleof 2.5^o and the probability of observing these clusters by a chancecoincidence under an isotropic distribution is smaller than 1 %. Especially thetriplet is observed against expected 0.05 events. The cos(\theta_GC)distribution expected from the Dark Matter Halo model fits the data as well asan isotropic distribution above 2 x 10^19eV and 4 x 10^19eV, but is a poorerfit than isotropy above 10^19eV. Arrival direction distribution of seven10^20eV cosmic rays is consistent with that of lower energy cosmic rays and isuniform. Three of seven are members of doublets above about 4 x 10^19eV.