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 10^19eV,\n47 above 4 x 10^19eV, and 7 above 10^20eV are observed until August 1998.\nArrival direction distribution of these extremely high energy cosmic rays has\nbeen studied. While no significant large-scale anisotropy is found on the\ncelestial sphere, some interesting clusters of cosmic rays are observed. Above\n4 x 10^19eV, there are one triplet and three doublets within separation angle\nof 2.5^o and the probability of observing these clusters by a chance\ncoincidence under an isotropic distribution is smaller than 1 %. Especially the\ntriplet is observed against expected 0.05 events. The cos(\\theta_GC)\ndistribution expected from the Dark Matter Halo model fits the data as well as\nan isotropic distribution above 2 x 10^19eV and 4 x 10^19eV, but is a poorer\nfit than isotropy above 10^19eV. Arrival direction distribution of seven\n10^20eV cosmic rays is consistent with that of lower energy cosmic rays and is\nuniform. Three of seven are members of doublets above about 4 x 10^19eV.\n