N. Kawasumi

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.

Upper Limit on Gamma-Ray Flux above 1019 eV Estimated by the Akeno Giant Air Shower Array Experiment

The origin of the highest energy cosmic rays (≥1020 eV) is not well understood. Interesting models called top-down scenarios have been proposed to explain the origin. The γ-ray flux in ultra-high-energy cosmic rays is a key parameter for giving constraints on such models. To study the properties of γ-ray showers, we carry out simulation studies that take into account both the Landau-Pomeranchuk-Migdal effect and electromagnetic interactions in the geomagnetic field. Based on an analysis of muons in air showers observed by the Akeno Giant Air Shower Array, the upper limits on the γ-ray flux are estimated to be 28% above 1019 eV and 67% above 1019.5 eV in the observed air showers at a confidence level of 95%. Above 1020 eV, the primary composition is in agreement with an extrapolation from lower energies, and there is no indication that the observed events are mostly γ-ray showers. These results provide observational constraints for origin models up to the highest energies.

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.

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.

Energy spectrum of TeV gamma rays from Mrk 501 obtained by stereoscopic analysis

Abstract We have observed flares of TeV-gamma rays from Mrk501 in 1997 using three telescopes of the Utah Seven Telescope Array at Dugway, Utah. Determination of the energy spectrum from such Active Galactic Nuclei (AGN) is very important, because the gamma-ray spectrum is expected to steepen around 10 TeV from objects like Mrk501 by the interaction of the infrared photons. We have developed the method to estimate energies of the gamma rays by stereoscopic analysis using multiple telescopes. The differential index of the energy spectrum obtained is well expressed by −2.5 between 700 GeV and 3 TeV. This spectrum seems to become steeper above several TeV.