Ultra-high-energy cosmic rays may come from clustered sources

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) similar or equal to 10 Mpc for field galaxies, H-0 = 56 km s(-1) Mpc(-1)). Future experiments will allow the determination of r(0) through the detection of anisotropies in arrival directions of similar to 10(20) eV cosmic rays over angular scales of Theta similar to r(0)/30 Mpc.