ABSTRACT. 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 25, 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(θ_GC) 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.

Subject headings: cosmic rays; galaxies: general; Galaxy: halo; large-scale structure of universe

Print publication: Issue 1 (1999 September 1)
Received 1999 February 17, accepted for publication 1999 April 12