The 320 EeV Fly's Eye event: a key messenger or a statistical oddity ?

Abstract

With only a small fraction of the exposure of more recent and larger experiments, the Fly's Eye detector recorded the most energetic cosmic ray event ever observed. At 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 stroke of luck, given that subsequent observatories with up to 60 times more exposure, 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 photons, protons or heavy nuclei, and therefore its source must be relatively close. Using numerical simulations for the propagation of protons and nuclei, we reexamine the problem of its origin by testing different hypothesis about the nature and location of the source as well as the injection spectrum. Thus, we show that the most feasible scenario is a nearby (\(\sim 2-3\) Mpc) bursting source which injected into the intergalactic medium a heavy composition with a hard spectrum (\(\gamma \le 1.5\)) and cut-off energy between the \(300\) EeV and \(1000\) EeV. Such a scenario produces a natural observation scale at around 300 EeV which maximizes at \(\sim 15\%\) the probability of simultaneously verifying the observation of one event by Fly's Eye, while obtaining a null result from Telescope array for the same portion of the sky.