On the Significance of the Observed Clustering of Ultra–High-Energy Cosmic Rays

Three pairs of possibly correlated ultra-high energy cosmic ray events werereported by Hayashida et al (1996). Here we calculate the propagation of thecorresponding particles through both the intergalactic and galactic magneticfields. The large scale disc and halo magnetic components are approximated bythe models of Stanev (1997). The intergalactic magnetic field intensity ismodulated by the actual density of luminous matter along the correspondinglines of sight, calculated from the CfA redshift catalogue (Huchra et al,1995). The results indicate that, if the events of each pair had a commonsource and were simultaneously produced, they either originated inside thegalactic halo or otherwise very unlikely events were observed. On the otherhand, an estimate of the arrival probability of ultra-high energy cosmic rays,under the assumption that the distribution of luminous matter in the nearbyuniverse traces the distribution of the sources of the particles and intensityof the intergalactic magnetic field, suggests that the pairs are chanceclusterings.Comment: Ap. J. Letters Accepted - 13 pages + 4 figure