M. Nagano

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.

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.

Energy spectrum of primary cosmic rays above 1017.0 eV determined from extensive air shower experiments at Akeno

The energy spectrum of primary cosmic rays above 1017.0 eV has been updated from data collected with two extensive air shower arrays operating at Akeno, one with area 1 km2 and the other with area 20 km2. Along with the authors previous results in the lower energy region, the energy spectrum has been determined over about five decades of energy from 1014.5 eV to 1019.8 eV. A change in the index of the power-law energy spectrum is observed around 1017.8 eV, as well as the usual features, namely the knee around 1015.7 eV and the ankle around 1019.0 eV. The indices of the differential power-law energy spectrum are: (2.62+or-0.12) below 1015.7 eV, (3.02+or-0.03) for 1015.7 approximately 1017.8 eV and (3.16+or-0.08) for 1017.8 approximately 1018.8 eV. There is an indication of a flattening of the spectrum above approximately 1018.8 eV with an index of (2.8+or-0.3). The flux above 1018 eV is (1.5 approximately 2.4)*10-12 m-2s-1sr-1 and is in good agreement with other experiments. The number of showers above 1019.5 eV is seven for an exposure of 80 km2 yr sr and further investigation by a new Akeno giant air shower array (AGASA), whose operation has started, is necessary to determine a cutoff energy in the spectrum, if any.

Energy Spectrum of Primary Cosmic Rays Between 10**14.5-ev and 10**18-ev

Size spectra of electrons (Ne) and muons (Nmu ) are obtained from the Akeno extensive air-shower experiment. The primary spectrum estimated from each spectrum is the same and is expressed by J(E0)dE0(4.0-5.0)*10-23(E0/1015.67)- gamma dE0 m-2 s-1 sr-1 where gamma =2.62+or-0.12 below and gamma =3.02+or-0.05 above 1015.67 eV. There is no other significant change of slope in either the electron or the muon size spectrum beyond the corresponding energy 1015.67 eV.

Akeno Giant Air Shower Array (AGASA) covering 100 km2 area

A very large surface array has been constructed recently at Akeno, 120 km west of Tokyo, to study the spectral features of the primary cosmic ray energy spectrum at the highest energies and to search for discrete sources emitting cosmic rays at energies above 1017 eV. The new array, AGASA (Akeno Giant Air Shower Array) spread over an area of about 100 km2, consists of 111 scintillation detectors, each 2.2 m2 in area and 27 muon detectors of six different sizes. The distance between the detectors is about 1 km. The data acquisition network developed for AGASA links detectors with each other and with the four branch controllers with two optical-fiber cables for all communications. We present here the salient design features of AGASA and discuss the estimates of the accuracy in the determination of arrival direction and primary energy of showers expected to be achieved with AGASA.

Muons (>or=1 GeV) in large extensive air showers of energies between 1016.5 eV and 1019.5 eV observed at Akeno

The properties of muons (>or=1 GeV) in giant air showers between 1016.5 eV and 1019.5 eV are measured by the Akeno 1 km2, 20 km2 and 100 km2 air shower arrays. The lateral distribution of muons is well fitted by the formula given by Greisen (1960) with parameters beta =2.52+or-0.02 and log(R0)=(0.58+or-0.04) (sec theta -1)+(2.39+or-0.05) between 1016.5 eV and 1019.0 eV within 800 m of the air shower axis, where theta is the zenith angle of the shower arrival direction. The relation between the total number of muons (Nmu ) and the total number of electrons (Ne) derived using these parameters is expressed on average as Nmu =(2:6+or-1.3)*105.0+ alpha (Ne/107)b where alpha =(1.07+or-0.13) (sec theta -1) and b=(0.77+or-0.02)-(0.17+or-0.02) (sec theta -1). The lateral distribution of muons becomes steeper than expected from extrapolation of the above formula for core distances >800 m and is expressed as rho mu =Nmu (Cmu /R02)r-0.75 (1+r)- beta (1+(R/800 m)3)- delta (r=R/R0, where R is a core distance). No deviation from this formula has been observed for sec theta <2.0 up to 1019.0 eV. The relation between the muon density (S(600)) and the charged-particle density on the ground at 600 m from the core (S(600)) is studied between 1016.5 eV and 1019.0 eV. A systematic change in the chemical composition of cosmic rays from a predominantly heavy to a predominantly light composition above 1017.5 eV claimed by the Flys Eye group has not been detected beyond the present experimental uncertainties. The present experiment suggests a much smaller rate of change of composition between 1017.5 eV and 1018.5 eV than that from the Flys Eye experiment.

Telescope array for advanced studies of cosmic rays at the highest and TEV energies

Abstract For advancement of the studies of cosmic rays, a telescope array project is now under consideration.

An upper limit on the muon flux at energies above 100 TeV determined from horizontal air showers observed at Akeno

Determination of the cosmic-ray muon energy spectrum above 100 TeV by observing extensive air showers from the horizontal direction (HAS) has been carried out at Akeno for four years. No definite shower of muon origin with size above 105 has been observed at zenith angles above 60 degrees . The upper limit of HAS intensity is 5*10-12 m-2 s-1 sr-1 (90% confidence level) for sizes greater than 105. This value indicates that the upper limit of the muon flux above 100 TeV is about 1.3*10-8 m-2 s-1 sr-1, which is in agreement with that expected from the primary spectrum with a knee, assuming scaling in the fragmentation region and 40% protons in the primary beam. It also implies that the critical energy at which a muon flux from prompt processes (decay of charmed particles) takes over from that due to conventional process (decay of pions and kaons) is higher than 100 TeV in the horizontal direction.

Anisotropy of the arrival direction of extensive air showers observed at Akeno

The anisotropy of the arrival direction of the primary cosmic ray of energy 1015 to 1018 eV is studied using approximately 300000 showers observed in 1981 and 1982 at the EAS array at Akeno. The anisotropy of the second harmonics appears to be (1.1+or-0.4)% in the region 1016 to 1017 eV. The showers with rich muon content seem to come preferentially from a direction of about 230 degrees in the right ascension. No statistically meaningful anisotropy is found for muon-poor showers.