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Characteristics of air-shower-triggered atmospheric families detected in the hybrid experiments together with emulsion chamber and AS-array at high mountains are compared with those of simulations. The analysis of families accompanied by air-showers with N e ≥ 10 7 shows that the experimental family-energy distribution favors a proton-dominant chemical composition of primary cosmic-rays but the lateral distribution favors a heavy-dominant composition. Thus no simulations can describe the overall characteristics of the data of hybrid experiments.

Longitudinal and lateral characteristics of the families detected by emulsion chambers in the hybrid experiment with AS-array at Mt. Chacaltaya are studied in detail. Although many groups discuss about an increase of the heavy component in primary cosmic-rays beyond the knee region, it is shown that the observed characteristics of the families accompanied by large shower size, N e > 10 6 , can not be explained by an increase of heavy primaries alone. It is necessary to assume some changes of nuclear interaction in order to explain the observed characteristics of the air-showers accompanied by families.

An experiment using an air shower array, a hadron calorimeter and an emulsion chamber is under way at Mt. Chacaltaya (5200 m, Bolivia). One of the highest energy events, having a halo (∼ 1 cm) in the centre of the family together with many γ -ray and hadron showers, is analyzed in detail. Available data for the event are on the halo ( E h a l o = 750 TeV) and on the high energy particles of electron/photon components by the emulsion chamber, and on the characteristics of the accompanied air shower ( N e = 7.0 × 10 7 , s = 0.59 ) by the air shower array. The diagram of the air shower size and the total energy of electron/photon components in the family, which shows discrepancy between the experimental data and simulated events (QGSJET code for nuclear collisions) in our previous reports, is discussed including the present event.

A hybrid experiment to operate simultaneously an air shower array, a hadron calorimeter and an emulsion chamber is under way at Mt Chacaltaya (5200 m, Bolivia). An event with a halo, a blackened area of ~1 cm on x-ray film of the emulsion chamber, was observed with the experiment. Information about the halo (Ehalo = 850 TeV) and on high energy particles of electromagnetic and hadronic components outside the halo (?E? = 632.5 TeV and ?E(?)h = 278.8 TeV with a detection threshold of 2 TeV) was obtained from emulsion chamber data. Details about low energy hadrons were determined from the hadron calorimeter data, and characteristics of the accompanying air shower (Ne = 7.0 ? 107, s = 0.59) were determined by the air shower array. We reconstruct the event, based on the observed data. The event is compared with simulated events, which supports the conclusion that nuclear interactions change their characteristics in the high energy region so as to result in stronger energy subdivision.

An experiment of an air shower array, a hadron calorimeter and an emulsion chamber is under way at Mt. Chacaltaya (5,200 m). Analysis of a halo event, detected by hybrid detectors, is presented. The data available for the event are on the halo ( E halo = 750 TeV), on the height of the interaction, the energy of the electron/photon and hadronic components from the emulsion chamber, on low energy hadrons from the hadron calorimeter, and on the characteristics of the accompanied air shower ( Ne = 7.0 × 10 7 , s = 0.59) using the air shower array. The structure and origin of the event is discussed based on the observed data of various components obtained by the hybrid detectors. The event is compared to simulated events, which support the conclusion that nuclear interactions change their characteristics in the high energy region resulting in a stronger energy subdivision.

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.

An experiment of an air shower array, a hadron calorimeter (8 m 2 ) and an emulsion chamber (8 m 2 , 15 cm Pb) is under way at Mt. Chacaltaya (5200 m above sea level, Bolivia), in order to study the hadron interactions and the primary cosmic rays in the energy region exceeding 10 15 eV. The number of particles in the hadronic component in the air shower, which is detected by the hadron calorimeter, is not compatible with that obtained by simulations, indicating that violation of the Feynman scaling law is stronger at 10 16 eV than the one assumed in the simulations. The average mass number of the primary cosmic rays, estimated from the distribution of the number of hadrons in the air shower, is A >= 2.8 ± 0.5 at 10 16 eV.

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.

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., 16 pages, 4 figures

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.

The observation of families and air showers is carried out at Mt. Chacaltaya (5200m, 540 g/cm2 atmospheric depth, Bolivia) by operating the emulsion chamber and the air shower array simultaneously. We summarize in this paper the results of the experiment, leaving the description of experimental apparatus, experimental procedure, etc. elsewhere (Kawasumi N. et al., AIP Conf. Proc.,276 (1992) 88;Proc. XXIV ICRC, Rome,1 (1995) 201). The analysis relies on the data of 47 families of ΣE γ≥10 TeV with the accompaniment of air showers.

Simultaneous observations of families and accompanied air showers with emulsion chambers and the air shower array of electronic equipment at Mt. Chacaltaya (5200 m, 540 g/${\mathrm{cm}}^{2}$) reveal that families bear the data of nuclear interactions generated deep in the atmosphere. 47 outstanding families with \ensuremath{\Sigma}${\mathit{E}}_{\ensuremath{\gamma}}$\ensuremath{\ge}10 TeV are correlated with the accompanied air showers of the size ${10}^{5}$--${10}^{8}$. A scatter plot of the average family energy versus the size of the relative air shower requires further energy fractionizing process(es) in the propagation of high energy cosmic rays in the atmosphere, such as a larger dissipative mechanism in nuclear interaction, heavier chemical composition of the primary cosmic rays, etc. We reach the conclusion that nuclear interaction changes its features in the energy region ${\mathit{E}}_{0}$\ensuremath{\gtrsim}${10}^{16}$ eV, because the heavier composition, proposed so far, is not sufficient for the required dissipative process. A comparison with the data from the HADRON experiment at a similar altitude with a similar technique shows that no larger deviations are present between both experiments. \textcopyright{} 1996 The American Physical Society.

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 (C'mu /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

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.

A very energetic cosmic ray of energy about (1.7-2.6) \ifmmode\times\else\texttimes\fi{} ${10}^{20}$ eV was observed by the Akeno Giant Air Shower Array on 3 December 1993 from the direction of galactic longitude $l=131\ifmmode^\circ\else\textdegree\fi{}$ and galactic latitude $b=\ensuremath{-}41\ifmmode^\circ\else\textdegree\fi{}$ within an error circle of 1.0\ifmmode^\circ\else\textdegree\fi{} radius. 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.

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.

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.

At mountain altitudes, high energy γ -ray bundles ( γ -rays in parallel trajectories and with the same direction of an air shower) can be observed in the central part of air showers. These γ -ray bundles (≥ 1 TeV ) and their associated air showers are observed simultaneously at Mt. Chacaltaya (5200 m ) by our detectors. The γ -ray bundle in the air shower can give information on the primary composition and the characteristics of proton interaction. In this paper, we report on experimental results, simulations calculated for the present experiments, and also problems in simulations.

Observation to detect VHE gamma-ray emissions has been carried out at Mt.Chacaltaya with the SYS air shower array since Feb.1986. A significant excess of hadron-less air showers with energies above 1014eV from the direction of Scorpius X-1 has been found during the last fifteen days in May 1986. However, no excess as the continuous emissions could be observed for several X-ray binary objects in the southern hemisphere. Small abundance of hadron-less air showers could be seen from the diffused region of galactic centre.

Simultaneous observation of families and accompanied air showers is carried out at Mt. Chacaltaya with emulsion chambers and a air shower array. The present analysis is based on the data of experiment from 1969 to 1986. 47 families with ΣEγ ≥ 10 TeV are coupled to the accompanied air showers of size 105 to 108. There is discrepancy between the experimental data and the simulation calculation, which indicate a need of more rapid energy dissipating process in the atmosphere in high energy region of E0 > 1016 eV.

Using data from more than ten-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, and Zatsepin and Kuzmin. 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 $\pm$25% in event-reconstruction resolution and $\pm$18% in systematic errors around 10$^{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., accepted to Astroparticle Physics, 29pages, 17+1 figures, elsart.cls The AGASA exposure is 5.1x10^16 m2 sr s until July 2002

An experimental setup of an air shower array, hadron calorimeter, and emulsion chamber is being carried out at Mt. Chacaltaya (5200 m, Bolivia), in order to study the hadron interaction and the primary cosmic rays in the energy region exceeding ${10}^{15}$ eV. The number of hadrons in the air shower, detected by the hadron calorimeter, is discussed mainly in this paper. A comparison with the simulation shows that the number of hadrons in the air shower is not compatible with that of the simulation, indicating that the Feynman scaling law is violated more strongly than the one assumed in the simulation at ${10}^{16}$ eV. The average mass number of the primary cosmic rays, estimated from the distribution of the number of hadrons, is $〈\mathrm{ln}A〉=2.8\ifmmode\pm\else\textpm\fi{}0.5$ at ${10}^{16}$ eV.

With the Akeno Giant Air Shower Array (AGASA), 581 cosmic rays above 10^19eV, 47 above 4 x 10^19eV, and 7 above 10^20eV are observed until August 1998. Arrival direction distribution of these extremely high energy cosmic rays has been studied. While no significant large-scale anisotropy is found on the celestial sphere, some interesting clusters of cosmic rays are observed. Above 4 x 10^19eV, there are one triplet and three doublets within separation angle of 2.5^o and the probability of observing these clusters by a chance coincidence under an isotropic distribution is smaller than 1 %. Especially the triplet is observed against expected 0.05 events. The cos(\theta_GC) distribution expected from the Dark Matter Halo model fits the data as well as an isotropic distribution above 2 x 10^19eV and 4 x 10^19eV, but is a poorer fit than isotropy above 10^19eV. Arrival direction distribution of seven 10^20eV cosmic rays is consistent with that of lower energy cosmic rays and is uniform. Three of seven are members of doublets above about 4 x 10^19eV., Comment: 40 pages, 12 figure, AASTeX *** Authors found a typo on Table 2 -- Energy of event 94/07/06 ***

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., to be published in PRL, 3 figures, REVTEX format

We have observed high energy γ families and associated air showers simultaneously at Mt. Chacaltaya. Monte Carlo simulations were calculated for the present experimental setup. From comparisons with both results, mechanism of γ family production, change of interaction model, study of the chemical composition are discussed.

Accumulated data of the Akeno Giant Air Shower Array (AGASA) indicate that arrival directions of a significant fraction of extremely high energy cosmic rays (EHECR) are uniformly distributed over the observable sky. However, three pairs of showers with angular separation of less than 2.5\ifmmode^\circ\else\textdegree\fi{} within the pair are observed among the 36 events above 40 EeV ( $4\ifmmode\times\else\texttimes\fi{}{10}^{19}\mathrm{eV}$), corresponding to a chance probability of 2.9% from uniform distribution. It should be noted that two pairs of them are observed to be within 2.0\ifmmode^\circ\else\textdegree\fi{} of the supergalactic plane.

We have studied the lateral distribution of charged particles associated with giant air showers and the attenuation of the local particle density at 600 m from the core, S600, with atmospheric depth using data collected with the Akeno Giant Air Shower Array (AGASA). The lateral distribution at distances of more than 1 km from the core has been observed to be much steeper than that suggested by some of the earlier measurements. The shape of the lateral distribution function has been observed to depend on the zenith angle of showers but there is no significant evidence for the dependence on the primary energy, within the resolution of the array. The systematic errors in energy estimation due to the uncertainties in the lateral distribution and the attenuation length of S600 are smaller than statistical errors. These errors have been estimated to be approximately 5% and approximately 3% respectively for near vertical showers with sec theta =1.1, and approximately 10% and approximately 12% respectively for showers with sec theta =1.4.

We have observed optical images of scintillation induced by shower particles traversing a plastic scintillator by means of an image intensifier system. These shower particles are produced by the electromagnetic cascade shower initiated with electrons and photons in the central part of air showers. We have also taken photographs of Cherenkov light emitted by particles of air showers in the atmosphere. The air shower is a phenomenon produced through successive nuclear interactions of high energy cosmic rays with air nuclei. The number of shower particles produced and their energies change according to the stage of development of air showers through the nuclear cascade in the atmosphere. In order to study the characteristics of the nuclear interaction in the energy regions above accelerator energies, it is important to observe the lateral distribution and the energy spectrum of shower particles in the air shower core and its longitudinal development in the atmosphere.