AGASA (Akeno Giant Air Shower Array)

Research Subjects

The Equipment

Here is a list of Collaborators

Scientific Results

Detection of an extremely high energy particle

We detected a very energetic cosmic ray of energy about 2x10^20eV on December 3, 1993. This is the second highest energy particles traveling in the Universe which has ever been observed. The above figure is a map of the particle density distribution of this superbig event. The radius of each circle represents the logarithm of the density at each detector location. You can see that the associated particles spread over a 4km x 4km area. If this cosmic ray were a proton, its origin could be extragalactic. However, the distance of the source cannot be much more than a few times 10 Mpc due to the energy loss during its travel from interactions with universal background radiation. There is no known active object within 50 Mpc near the arrival direction of this shower. Here is our paper to report this interesting event.

Energy Spectrum of the highest energy cosmic rays

Measurement of the energy spectrum constrains the origin of the energetic particles due to modifications in the energy spectrum as a result of the interaction between the energetic particles and the universal cosmic background radiation. The shape of the spectrum should yield an information about the possible distribution of sources and how long these particles have been traveled. The above figure is the measured energy spectrum. The solid curve shows the spectrum expected from a model of many sources distributed homogeneously and uniformly in the Universe. This model looks in good agreement with the measurement. Active radio galaxies with hot spots are the best candidate to satisfy these results, but the hypothesis that the extremely energetic particles are produced by decay of monopoles and/or cosmic strings predicted by GUTs may produce is also acceptable, taking into consideration the existence of the biggest event with an energy larger than the spectrum cutoff. Here is our published paper to report our spectrum measurement. Updated energy spectrum by AGASA is also available.

Mass composition of the highest energy cosmic rays

We observed muon components in the detected air showers and studied their characteristics. Generally speaking, more muons in a shower cascade favors heavier primary hadrons and measurement of muons is one of the methods used to infer the chemical composition of the energetic cosmic rays. Our recent measurement indicates no systematic change in the mass composition from a predominantly heavy to a light composition above 0.3 EeV claimed by the Fly's Eye group. Here is our preprint of the muon stuff.