A closer look at the spectrum and small scale anisotropies of UHECRs

Abstract

We present results of numerical simulations of the propagation of ultra high energy cosmic rays (UHECRs) over cosmological distances, aimed at quantifying the statistical significance of the highest energy data on the spectrum and small scale anisotropies as detected by the AGASA experiment. We assess the significance of the lack of a GZK feature and its compatibility with the reported small scale anisotropies. Assuming that UHECRs are protons from extragalactic sources, we find that the small scale anisotropies are incompatible with the reported spectrum at a probability level of $2 \times 10^{-5}$. Our analysis of the AGASA results shows the power of combining spectrum and small scale anisotropy data in future high statistics experiments, such as Auger.