Search for coincident air showers in the network observation

Here we report results of the coincident event analysis, using air shower data taken at six stations of the Large Area Air Shower (LAAS) group in Japan. In this analysis we search for pairs of coincident air showers between stations, which are hypothetically induced by bursts of ultra-high-energy γ-ray sources or by secondary particles from interactions of extremely-high-energy cosmic rays with interstellar matter. From four years data (2.3 x 106 air showers) we find a pair of air showers with a very small time difference of 195 microseconds and an angular distance of 5.3 degrees, which is within the angular accuracy of our arrays, between two stations separated by 152 km. The chance probability of this event is 0.16. The Crab Nebula, a well-known ultra-high-energy γ-ray source, is within the angular accuracy from the arrival direction of this event. We expect that this event was induced by the ultra-high-energy γ-rays emitted in a burst of the Crab Nebula