Hints of Energy Dependences in AGASA EHECR Arrival Directions
Abstract
A correlation and probability analysis of the distribution of arrival directions for a sample of AGASA events reported to have energies above 4x1019 eV shows the small scale clustering to remain significant at the 99.5 - 99.9% CL and to be consistent with previous results. For the sample taken as a whole, there are no departures from either homogeneity or isotropy on angular scales greater than 5 degrees. The sample of events with E >= 6x1019 eV possesses no small scale clustering. Cross correlating subsamples partitioned by energy reveals three uncorrelated distributions in the intervals 4 - 5x1019 eV, 5 - (8-10)x1019 eV, and greater than (8-10)x1019 eV. The partition with 5 <= E < 8x1019 eV is correlated with the supergalactic equatorial plane while the other two groups are statistically consistent with isotropic distributions. The presence of three distinct energy-partitioned groups of events could reflect possible changes in primary composition, different source distributions, differing levels of GZK losses, or deflection effects of magnetic fields.
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