The Anisotropy of Cosmic Ray Arrival Direction around 1018eV
Abstract
Anisotropy in the arrival directions of cosmic rays around 1018eV is studied using data from the Akeno 20 km2 array and the Akeno Giant Air Shower Array (AGASA), using a total of about 216,000 showers observed over 15 years above 1017eV. In the first harmonic analysis, we have found significant anisotropy of 4 % around 1018eV, corresponding to a chance probability of 10-5 after taking the number of independent trials into account. With two dimensional analysis in right ascension and declination, this anisotropy is interpreted as an excess of showers near the directions of the Galactic Center and the Cygnus region. This is a clear evidence for the existence of the galactic cosmic ray up to the energy of 1018eV. Primary particle which contribute this anisotropy may be proton or neutron.
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