The 320 EeV Fly’s Eye event: a key messenger or a statistical oddity ?. (arXiv:1906.11170v1 [astro-ph.HE])

The 320 EeV Fly’s Eye event: a key messenger or a statistical oddity ?. (arXiv:1906.11170v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Fitoussi_T/0/1/0/all/0/1">Thomas Fitoussi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Medina_Tanco_G/0/1/0/all/0/1">Gustavo Medina-Tanco</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+DOlivo_J/0/1/0/all/0/1">Juan-Carlos D&#x27;Olivo</a>

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.

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.

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