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Study of small-scale anisotropy of ultra-high-energy cosmic rays observed in stereo by the high resolution fly’s eye detector

Abstract:

The High Resolution Fly's Eye (HiRes) experiment is an air fluorescence detector which, operating in stereo mode, has a typical angular resolution of 0 fdg 6 and is sensitive to cosmic rays with energies above 10 18 eV. The HiRes cosmic-ray detector is thus an excellent instrument for the study of the arrival directions of ultra-high-energy cosmic rays. We present the results of a search for anisotropies in the distribution of arrival directions on small scales (< 5) and at the highest energies (> 10 19 eV). The search is based on data recorded between 1999 December and 2004 January, with a total of 271 events above 10 19 eV. No small-scale anisotropy is found, and the strongest clustering found in the HiRes stereo data is consistent at the 52% level with the null hypothesis of isotropically distributed arrival directions.