Anisotropy of successive air showers

We have investigated the anisotropy of successive air shower (SAS) events, which we define as the detection of many air showers within a short time wi ndow, using data from si:x stations of the Large Area Air Shower (LAAS) gro up. On the criterion of 22 air showers within 20 minutes, five SAS events a re found against 1.4 expected from the Poisson distribution in Okayama Univ ersity station's data. From six stations' data, we find 24 SAS events in to tal. By plotting them in equatorial coordinates, it is revealed that SAS ev ents are observed more frequently when the Galactic plane is around the zen ith. This can be attributed to a hypothetical small flux of ultra-high-ener gy gamma -rays front the direction of the Galactic plane superposed on conv entional cosmic rays. If this hypothesis is true, the analytical procedure used here has potential to measure ultra-high-energy gamma -ray sources by even small air shower arrays like ours.