Cluster analysis of extremely high energy cosmic rays in the northern sky

The arrival directions of extremely high energy cosmic rays (EHECR) above 4 x 10(19) eV, observed by four surface array experiments in the northern he misphere, are examined for coincidences from similar directions in the sky. The total number of cosmic rays is 92, A significant number of double coin cidences (doublet) and triple coincidences (triplet) is observed on the sup ergalactic plane within the experimental angular resolution. The chance pro bability of such multiplets from a uniform distribution is less than 1% if we consider a restricted region within +/- 10 degrees of the supergalactic plane. Though there is still a possibility of chance coincidence, the prese nt results on small angle clustering along the supergalactic plane may be i mportant in interpreting EHECR enigma. An independent set of data is requir ed to check our claims. (C) 2000 Elsevier Science B.V, All rights reserved.