H. Yoshii

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.

SMALL-SCALE ANISOTROPY OF COSMIC RAYS ABOVE 1019 EV OBSERVED WITH THE AKENO GIANT AIR SHOWER ARRAY

With the Akeno Giant Air Shower Array, 581 cosmic rays above 1019 eV, 47 above 4 ) 1019 eV, and seven above 1020 eV were observed until 1998 August. The arrival direction distribution of these extremely high energy cosmic rays has been studied. While no signi—cant large-scale anisotropy is found on the celestial sphere, some interesting clusters of cosmic rays are observed. Above 4 ) 1019 eV, there are one triplet and three doublets within a separation angle of and the probability of observing 2i.5, these clusters by a chance coincidence under an isotropic distribution is smaller than 1%. The triplet is especially observed against expected 0.05 events. The distribution expected from the dark cos (h GC ) matter halo model —ts the data as well as an isotropic distribution above 2 ) 1019 and 4 ) 1019 eV, but the —t with the dark matter halo model is poorer than the isotropic distribution above 1019 eV. The arrival direction distribution of seven 1020 eV cosmic rays is consistent with that of lower energy cosmic rays and is uniform. Three of the seven are members of doublets above about 4 ) 1019 eV. Subject headings: cosmic raysgalaxies: generalGalaxy: halolarge-scale structure of universe

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.

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.

Observations of TeV Gamma-Ray Flares from Markarian 501with the Telescope Array Prototype

We will report the observations of TeV gamma ray flares from Markarian 501 using Telescope Array Prototype. The observation were carried out continuously from the end of March to the end of July in 1997. The energy spectrum, and the time variation of the gamma ray intensities are shown. The intensity has been changed by the order of magnitude in this period and the possible quasi periodic oscillation of 12.7days were discovered.We report the observations of TeV gamma-ray flares from Markarian 501 using the Telescope Array Prototype. The observations were carried out continuously from the end of March to the end of July of 1997. The energy spectrum and the time variation of the gamma-ray intensities are shown. The intensity has been changed by an order of magnitude in this period, and the possible quasi-periodic oscillation of 12.7 days were discovered.

The Energy Spectrum and the Chemical Composition of Primary Cosmic Rays with Energies from 1014 to 1016 eV

We have measured extensive air showers with primary energies above 6 TeV at Mount Chacaltaya in Bolivia. The data were collected by an air shower array called the Minimum Air Shower (MAS) array starting in 2000 March. We applied an equi-intensity analysis method to the extensive air showers extended over the region of their maximum development. We varied the mixture of protons and iron in our simulations and compared these to the data to determine the mixing ratio of protons as a function of the primary energy. Using this, we derived the primary energy spectrum from 1014 to 5 × 1016 eV. Consequently, we conclude that the power-law index of the spectrum changes gradually around 1015.5 eV and that the obtained proton ratio decreases with increasing energy. We directly measured the longitudinal development of air showers generated by primaries with energies around the knee. We found that the average mass number of primary cosmic rays shows a steady increase with energy above 1014.5 eV and that the dominant component around the knee is not protons.

CHARGE WAVEFORM OF A NEW TWO-DIMENSIONAL POSITION-SENSITIVE SILICON DETECTOR.

The operation principles of the two-dimensional position-sensitive silicon detector newly developed by Doke et al. were studied using a simple model. This model treats the detector as an area of continuously distributed capacitance C and resistance Rs of position surface layer. A linear relationship can then be obtained between the position of the incident particle and change collected at the contacts of the detector. The kinetics of charge collected at corner contacts, ballistic deficit and noise were calculated. Rise time of the charge pulse (10–90%) was found to vary with the position of incidence up to about RsC/8. It was found that a shaping time constant longer than RsC/3 is required for pulse shaping with single CR-differentiation and single CR-integration in order to obtain a ballistic deficit of less than 1%.

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.