A nearby source of ultra-high energy cosmic rays
Recently the Telescope Array collaboration reported an observation of cosmic ray event with very high energy 244 EeV ($2.44 \times 10^{20}$ 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 $\rho > 1.0 \times 10^{-4}$ Mpc$^{-3}$. The number density of UHECR sources emitting heavy nuclei is constrained for the first time.