Ultra-high energy cosmic rays may come from clustered sources

TL;DR

This paper explores how clustering of cosmic-ray sources influences observed fluxes above 10^{20} eV, suggesting that future anisotropy measurements can determine source clustering scales.

Contribution

It introduces a model linking source clustering to flux variations and proposes using anisotropy detection to measure clustering scale length r_0.

Findings

Clustering affects flux predictions beyond the GZK cutoff.

Standard deviation in event counts depends on source correlation scale.

Future experiments can measure source clustering through anisotropy detection.

Abstract

Clustering of cosmic-ray sources affects the flux observed beyond the cutoff imposed by the cosmic microwave background and may be important in interpreting the AGASA, Fly's Eye, and HiRes data. The standard deviation, sigma, in the predicted number, N, of events above 10^{20} eV is sigma/N = 0.9(r_0/10 Mpc)^{0.9}, where r_0 is the unknown scale length of the correlation function (r_0 = 10 Mpc for field galaxies). Future experiments will allow the determination of r_0 through the detection of anisotropies in arrival directions of ~ 10^{20} eV cosmic-rays over angular scales of Theta ~ r_0/30 Mpc.