We present the interpretation of the muon and scintillation signals of ultra-high-energy air showers observed by AGASA and Yakutsk extensive air shower array experiments. We consider case-by-case ten highest energy events with known muon content and conclude that at the 95% confidence level (C.L.) none of them was induced by a primary photon. Taking into account statistical fluctuations and differences in the energy estimation of proton and photon primaries, we derive an upper limit of 36% at 95% C.L. on the fraction of primary photons in the cosmic-ray flux above 10 20 eV. This result disfavors the Z-burst and superheavy dark-matter solutions to the GZK-cutoff problem.

We analyse a sample of 33 extensive air showers (EAS) with estimated primary energies above 2·10 19 eV and high-quality muon data recorded by the Yakutsk EAS array.We compare, event-by-event, the observed muon density to that expected from COR-SIKA simulations for primary protons and iron, using SIBYLL and EPOS hadronic interaction models. The study suggests the presence of two distinct hadronic components, "light" and "heavy". Simulations with EPOS are in a good agreement with the expected composition in which the light component corresponds to protons and the heavy component to iron-like nuclei. With SYBILL, simulated muon densities for iron primaries are a factor of ∼ 1.5 less than those observed for the heavy component, for the same electromagnetic signal. Assuming two-component proton-iron composition and the EPOS model, the fraction of protons with energies E > 10 19 eV is 0.52 +0.19 −0.20 at 95% confidence level.Number of muons in extensive air showers (EAS) is used as an estimator of the primary composition of ultra-high-energy cosmic rays detected by surface arrays. Precision of muon-based composition studies is limited by their sensitivity to hadronic interaction models which incorporate extrapolation of experimental data to kinematic regions never tested in a laboratory experiment. On the other hand, the responses of the ground-based cosmic-ray detectors to different EAS components are different, and variations in the muon content may affect the relation between the signal recorded on the ground and the inferred energy 1 of the primary particle. This is important in particular for detectors sensitive to the muon component (e.g. the surface detector of the Pierre Auger Observatory).In this Letter we compare the observed and simulated muon contents of EAS detected by the Yakutsk array [1]. This experiment is currently the only one capable of detecting air showers initiated by particles of E 10 19 eV and equipped with muon detectors. Large area and high saturation threshold of each detector station make it possible to obtain high-quality muon data [2].Recently, the Pierre Auger Observatory (PAO) collaboration reported [3] an excess of muons as compared to simulations with the QGSJET II hadronic interaction model [4].Since PAO is not equipped with muon detectors, indirect methods were used. The same result has been previously reported [5] by the Yakutsk collaboration (by making use of older hadronic models). Here, we perform a direct and detailed study of this effect for each individual shower in a high-quality sample and for the sample as a whole, in the frameworks of two different hadronic interaction models, SIBYLL [6] and EPOS [7] (the muon content of showers simulated with QGSJET II is between these two [8]). We use a precise statistical method [9] to construct the distributions of muon densities simulated for individual events, which are then compared to the observed data (see Refs. [10,11] for other applications of the method).In our study, we use a sample of 33 events with reconstructed energies ab...

By making use of the data on the total signal and on the muon component of the air showers detected by the Yakutsk array, we analyze, in the frameworks of the recently suggested event-by-event approach, how large the fraction of primary gamma-rays at ultra-high energies can be. We derive upper limits on the photon fraction in the integral flux of primary cosmic rays. At the 95% confidence level (CL), these limits are 22% for primary energies E0 > 4 · 10 19 eV and 12% for E0 > 2 · 10 19 eV. Despite the presence of muonless events, the data are consistent with complete absence of photons at least at 95% CL. The sensitivity of the results to systematic uncertainties, in particular to those of the energy determination for non-photon primaries, is discussed. PACS

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The energy spectrum of UHECRs is discussed on the basis of the Yakutsk array database analysis. In the region E=0.1 to 30 EeV the showers are detected under trigger-500, while at energies above 30 EeV the whole acceptance area for trigger-1000 is used in order to utilize all the data available in the region of GZK cutoff.Comment: Invited talk at CRIS2004: GZK and surroundings, Catania, Italy, 31.05.04. To appear in Nucl. Phys. B Proc. Supp