Angular Correlation Estimates for Ultrahigh Energy Cosmic Rays

Identifying the sources of ultrahigh energy cosmic rays remains one of the major challenges in astrophysics. If these cosmic rays are not too severely deflected by magnetic fields while in transit, then their arrival directions will provide crucial information about their origins. The High Resolution Fly’s Eye (HiRes) experiment, consisting of two air fluorescence detector sites, has been making stereo observations of ultrahigh energy cosmic rays since 1999. The angular resolution (∼ 0.6◦) achieved by this experiment sets the stage for the most precise measurements yet of small-angle correlation among the highest energy cosmic rays. In this paper we analyze HiRes events with energies above 10 eV observed between 1999 December and 2004 January. More details about this data set are given in Ref. 1. We also analyze a combination of events from HiRes and from the Akeno Giant Air Shower Array (AGASA) with energies above 4× 10 eV.