Study of Broad-Scale Anisotropy of Cosmic-Ray Arrival Directions from 2 x 1017 to 1020 Electron Volts from Fly's Eye Data

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Abstract
We report results on the broad-scale anisotropy of arrival directions of cosmic rays in the energy range from 2×1017 to 1020 eV. The data were taken by the Fly's Eye detector in both monocular and stereo modes of operation. We look for dependence on galactic latitude or supergalactic latitude by fitting the data to a Wdowczyk and Wolfendale plane enhancement function and a north-south gradient functional form. We report a small but statistically significant galactic plane enhancement in the energy range between 2×1017 and 3.2×1018 eV. The probability that this anisotropy is due to fluctuations of an isotropic distribution is less than 0.06%. The most significant galactic plane enhancement factor fE=0.104±0.036 is in the energy range (0.4-1.0)×1018 eV. No statistically significant evidence for a north-south gradient is found. There is no sign of significant deviation from isotropic background when the data are analyzed in terms of supergalactic latitude distributions.