A nearby source of ultra-high energy cosmic rays

Abstract

Recently the Telescope Array collaboration reported an observation of cosmic ray event with very high energy 244 EeV (2.44 × 1020 eV). Importantly, the event is hard to correlate with the matter distribution in the local Universe, even after taking into account deflections in magnetic fields. This implies that the event is likely a nucleus with a large charge. An attenuation length of the nucleus of such a high energy in intergalactic space is quite small, therefore its source should be relatively close to our Galaxy. Using these arguments we derive a new upper bound on a distance to the closest ultra-high energy cosmic ray (UHECR) source and a lower bound on the UHECR source number density in general. The distance to the closest source should not exceed 5 Mpc at 95% C.L. and the 95% C.L. lower-bound on the sources number density is ρ > 1.0 × 10-4 Mpc-3. The number density of UHECR sources emitting heavy nuclei is constrained for the first time.