Search for large-scale coincidences in network observation of extensive air showers

Abstract

Large-scale coincidences of extensive air showers (EAS) have been searched for with the aim of detecting signals from extreme short bursts of any point source in the universe. In this analysis, the network observation of EAS by distant stations is elaborately treated as a 'cosmic ray interferometer'. Each station is independently operating, but the Global Positioning System provides accurate time stamps to compare arrival times of EAS among the stations. Signals from burst activities can be extracted out of a sea of background cosmic rays in two steps: (i) picking out EAS pairs with very small arrival time differences and angular distances, (ii) examining the interference between the EAS pair based on the arrival time difference and the station distance. Datasets collected simultaneously by five stations of the Large Area Air Shower (LAAS) group during 1996--2002 were analyzed in this paper. We found three EAS pairs (E≈10 14-16 eV) with extremely small arrival time differences (~100μs) and very small angular distances (≤10°). Arrival directions of two EAS pairs out of the three pointed to the Crab Nebula, suggesting that they were induced by a bundle of ultra-high-energy γ-rays from extreme short bursts of the object. However, the significances of them are not enough yet.