K. Kamata

Extension of the cosmic ray energy spectrum beyond the predicted Greisen-Zatsepin-Kuz'min cutoff

The cosmic-ray energy spectrum above 10^{18.5} eV is reported using the updated data set of the Akeno Giant Air Shower Array (AGASA) from February 1990 to October 1997. The energy spectrum extends beyond 10^{20} eV and the energy gap between the highest energy event and the others is being filled up with recently observed events. The spectral shape suggests the absence of the 2.7 K cutoff in the energy spectrum or a possible presence of a new component beyond the 2.7 K cutoff.

Energy determination in the Akeno Giant Air Shower Array experiment

Abstract Using data from more than 10 years of observations with the Akeno Giant Air Shower Array (AGASA), we published a result that the energy spectrum of ultra-high energy cosmic rays extends beyond the cutoff energy predicted by Greisen [Rhys. Rev. Lett. 16 (1966) 748] and Zatsepin and Kuzmin [Zh. Eksp. Teor. Fiz. 4 (1966) 114]. In this paper, we reevaluate the energy determination method used for AGASA events with respect to the lateral distribution of shower particles, their attenuation with zenith angle, shower front structure, delayed particles observed far from the core and other factors. The currently assigned energies of AGASA events have an accuracy of ±25% in event-reconstruction resolution and ±18% in systematic errors around 10 20 eV. This systematic uncertainty is independent of primary energy above 10 19 eV. Based on the energy spectrum from 10 14.5 eV to a few times 10 20 eV determined at Akeno, there are surely events above 10 20 eV and the energy spectrum extends up to a few times 10 20 eV without a GZK cutoff.

The anisotropy of cosmic ray arrival directions around 10 18 eV

Abstract Anisotropy in the arrival directions of cosmic rays with energies above 1017 eV is studied using data from the Akeno 20 km2 array and the Akeno Giant Air Shower Array (AGASA), using a total of about 114 000 showers observed over 11 years. In the first harmonic analysis, we have found a strong anisotropy of ∼ 4% around 1018 eV, corresponding to a chance probability of ∼ 0.2% after taking the number of independent trials into account. with two-dimensional analysis in right ascension and declination, this anisotropy is interpreted as an excess of showers near the directions of the Galactic Center and the Cygnus region.

The cosmic ray energy spectrum above 3 × 1018 eV measured by the Akeno Giant Air Shower Array

Abstract We report the first result on the cosmic ray energy spectrum above 3 × 1018 eV measured by the Akeno Giant Air Shower Array (AGASA) from July 1990 to February 1994. The analysis method and the energy resolution of the AGASA experiment are described in some detail. The flattening of the spectrum around 1019 eV (ankle) is observed with a significance of 2.9σ. If we express the differential energy spectrum of cosmic rays of energy E (in eV) with an ankle energy Ea as J(E) = κ( E E a ) −γ m −2 s −1 sr −1 eV −1 , γ for 1018.5 eV ≤ E ≤ Ea is in good agreement with that from the previous experiment and is 3.2 ± 0.1. The slope γ above Ea depends strongly on the value Ea. For the case Ea = 1019 eV, κ = (2.3−0.2+0.1) × 10−33 and γ = 2.3−0.3+0.5 for 1019 eV ≤ E ≤ 1020 eV. If Ea = 1018.8 eV, then κ = (1.0 ± 0.1) × 10−32 and γ = 2.7−0.4+0.2 for 1018.8 eV ≤ E ≤1020 eV, after correcting for both the statistical error and the energy resolution of the present experiment. If we interpret the present results assuming an extragalactic origin for cosmic rays above 1019 eV, the observed data is consistent with either a homogeneous and isotropic distribution of sources or with localized sources at redshift of greater than ∼ 0.1. A (1.7–2.6) × 1020 eV event was observed on December 3, 1993 from the direction of l = 131° and b = −41°. This shower energy is a factor 3 larger than the second highest energy event.

Expanded Array for Giant Air Shower Observation at Akeno

Abstract As the first stage of a future huge air shower array, the Akeno array was expanded to about 20 km2 by adding 19 scintillation counters of 2.25 m2 area outside the present 1 km2 array and installing a new data collection system. These detectors are connected successively by two optical fiber cables. The total number of electrons and the arrival direction of extensive air showers of 1010 GeV can be determined with accuracies of 25% and 3° respectively with this array. The present recording system is applicable to other experiments in which many sensors are connected in sequence.

Further development of data acquisition system of the Akeno Giant Air Shower Array

The data acquisition system of the Akeno Giant Air Shower Array (AGASA) is described. The AGASA array covers an area of about 100 km2 and has been operated since 1990 to study the origin of extremely high energy cosmic rays. In the early stage of our experiment, AGASA was divided into four sub-arrays called branches for topographical reasons so that air showers were observed independently at each branch. In December 1995, we have improved the data acquisition system and unified the four branches into a single detection system. By this unification, the effective detection area of the AGASA increases by about 1.7 times in the early stage.

Possible evidence for ⩾ 10 GeV neutrons associated with the solar flare of 4 June 1991

Abstract Neutrons associated with the solar flare of 4 June 1991 were observed by scintillation detectors of the Akeno Giant Air Shower Array (AGASA), which is about 900 m above sea level (920 g/cm2). The total area of plastic scintillators used for the present analysis is 182.6 m 2 and the excess counting rate is (0.35 ± 0.1) per m 2 s. From attenuation of counting rates observed by scintillation detectors at Mt. Norikura and at Akeno, the excess signals can be interpreted as muons produced in the upper atmosphere by solar neutrons of energies above 10 GeV. The flux of solar neutrons above 10 GeV is about 1 ∼ 2 per m 2 s at the top of the atmosphere and lasted for more than 20 minutes after the solar flare.