Systematic study of high energy cosmic-ray sources

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Gaining support of a Grant-in-Aid for Scientific Research on Priority Area (712), we constructed a high sensitivity airshower array and a solar neutron telescope at Yangbajing (4300 m a.s.1) in Tibet in collaboration with Chinese scientists. The main purposes of this research project is to search for high-energy gamma rays from active celestial objects, to study very high energy Galactic cosmic rays and to observe high energy neutrons in association with solar flares. These observations are very important to clarify the origin of cosmic rays and its acceleration mechanism at the sources, both of which remain unsolved yet The airshower array (Tibet-I0 constructed before 1998 was gradually upgraded and improved from 1999 by adding new detectors and completed in 2003. The present array (Tibet-III) consists of about 800 scintillation detectors which are deployed at a grid of 7.5 m spacing covering the area of 37000 square meters. The trigger rate of airshower events is set to about 1.7kHz … More and the arrival direction of each event can be estimated with an inaccuracy of less than 0.9 degrees in the energy region over 3 TeV. The solar neutron telescope was completed in 1999 and it has been successively operated until now A few flare neutron events were observed during this solar cycle 23.The important results obtained until now are as follows.1) High energy gamma rays above about 3 TeV have been successively observed from the Crab Nebula and active galactic nudeiMrk421 and Mrk501. This is the first observation of the gamma-ray sources with air shower array in the world..2) An energy dependence of the large-scale anisotropy of the Galactic cosmic rays has been for the first time observed and also significant anisotropy of multi-TeV cosmic rays is observed in the direction of Cygnus region.3) Energy spectra of all-particles and protons around the knee have been observed. These results strongly suggest that the primary composition becomes heavy dominant with increasing primary energy at the knee region.4) A variation of the sun shadow during the solar activity cycle 23 was successively monitored with the air shower array, and a peculiar change appeared at the solar maximum, called Gnevyshev gap, was clearly observed.5) Neutrons associated with solar flares were detected with the solar neutron telescope. A comparison with the x-ray observation with GOES satellite shows that very high energy neutrons with its energy above 10 GeV may be created at some kind of solar flares.These are the first observations in the world and are related with the origin and acceleration of cosmic rays, the propagation of cosmic rays in the solar and interplanetary magnetic fields as well as in the Galaxy, and were published in the international journals. We reported these at the international conferences and/or workshops and also at the meetings of the Physical Society I of Japan. We made a summary report on the scientific results obtained during the period from 1999 through 2005. Less