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 4x10^{19} 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 >= 6x10^{19} eV possesses no small scale clustering. Cross correlating subsamples partitioned by energy reveals three uncorrelated distributions in the intervals 4 - 5x10^{19} eV, 5 - (8-10)x10^{19} eV, and greater than (8-10)x10^{19} eV. The partition with 5 <= E < 8x10^{19} 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.