Legacy from Fly’s Eye: Making sense of the Highest Energy Cosmic Ray Ever Observed
2019 July 22
The Fly's Eye detector recorded the most energetic cosmic ray event ever observed. With an energy of 320 EeV, it lays far beyond the suppression of the ultra-high energy cosmic ray (UHECR) energy spectrum. If its energy is indeed well determined, as the data strongly suggests, then it remains either a great mystery or an unbelievable chance, given the very small exposure of Fly's Eye when compared to those of subsequent observatories, which have never observed a remotely comparable event. At energies as high as those of the Fly's Eye event, the Universe is very opaque to electromagnetic interacting particles, whether protons or heavy nuclei, and therefore its source must be relatively close. Using numerical simulations for the propagations of protons and nuclei, we reexamine different hypothesis about the nature and location of the source both for the full-sky spectrum observed by Telescope Array only and with a superimposed secondary component which only becomes dominant at energies beyond 100 EeV. We show that the latter scenario, inducing a hardening of the spectrum at the highest energies, is more likely to reconcile the fact that Fly's Eye was able to observe such event but no particles at lower energies (e.g., at 100 EeV) while still being compatible with the non-observation of equivalent events by neither HiRes or Telescope Array, with higher exposure in the Northern Hemisphere.