S. Ohara

Network observation of air showers as cosmic ray interferometer

Abstract The Large Area Air Shower (LAAS) group has been performing a network observation of extensive air showers (EAS) since 1996. Ten compact EAS arrays (stations) are scattered over a large part of Japan and operated independently and simultaneously. Each station has 4–12 scintillation counters and a Global Positioning System (GPS), which provides time stamps of EAS arrivals with an accuracy of 1 μs. The primary purpose of the network observation is to study large-scale correlations in ultra-high-energy cosmic rays, which are attracting much interest of the astroparticle physicists community in recent years. The present state of the network and some results from computer simulations are reported here.

The LAAS network observation for studying time correlations in extensive air showers

The Large Area Air Shower (LAAS) group has been performing a network observation of extensive air showers (EAS) since 1996 in Japan. Ten compact EAS arrays are operating simultaneously at distant stations (up to ≈1000 km) and detecting EAS with mean energy of ≈1015 eV. Each station has 4--12 scintillation counters and a Global Positioning System (GPS), which provides time stamps of EAS triggers with an accuracy of 1μs. As a consequence of the comparable time stamps, uniformly-adjusted detectors and a standardized data format among all stations, we can treat the independent observations as a gigantic EAS detector system as a whole. The primary purpose of the network observation is to study large-scale correlations in ultra-high-energy cosmic rays. On the other hand, three nearby stations within 1~km distance at Okayama area have a possibility to detect extremely-high-energy EAS (≈1019 eV) as coincident triggers of the three stations. The present status of the network and some results from computer simulations are reported here.

Simultaneous, parallel and multiple EAS events due to Gerasimova‐Zatsepin effect in LAAS experiments

To study the ultra‐high energy cosmic ray composition by making use of the GZ scenario, Large Area Air Shower (LAAS) experiments had been started in 1996 by the collaboration of 7 institutes in Japan. We present here some results of LAAS observations on the GZ event search during the data period from 1996 to January 2007. A brief profile of the potential of LAAS EAS array is also presented.

Observation of coincident air showers over a very large area

Abstract A network observation of air showers ( E 0 ≈ 10 15 eV) has been performed since 1996 in Japan. Ten compact air shower arrays, scattered over a large part of Japan, are operated independently by each institute. To explore large-scale correlations in primary cosmic rays, air shower data from five arrays of the network are analyzed in terms of arriving phases of coincident air showers, hypothetically induced by ultra-high-energy γ-rays from point sources or by secondary particles from interactions of extremely-high-energy cosmic rays with interstellar matter. Four coincident event candidates with extremely small time differences are found out. Two of these events are observed in the direction of the Crab Nebula, a well-known ultra-high-energy γ-ray source. However, significances of the events are not enough to invoke the claim for the detection of large-scale correlations.