M. I. Pravdin

The spectrum features of UHECRs below and surrounding GZK

The energy spectrum of ultra-high energy cosmic rays (UHECRs) is discussed on the basis of the Yakutsk array database analysis. In the region E 0 = 10 17 to 3 × 10 19 eV the showers are detected under trigger-500, while at energies above 3 × 10 19 eV the whole acceptance area for trigger-1000 is used in order to utilize all the data available in the region of Greisen-Zatsepin-Kuzmin (GZK) cutoff. Updated relations are adapted to estimate the primary energy having detected densities of charged particles at the distance 300/600 m from the shower axis.

Muon content of ultrahigh-energy air showers: Yakutsk data versus simulations

A sample of 33 extensive air showers (EASs) with estimated primary energies above 2 × 1019 eV and high-quality muon data recorded by the Yakutsk EAS array is analyzed. The observed muon density is compared event-by-event to that expected from CORSIKA 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 good agreement with the expected composition in which the light component corresponds to protons and the heavy component to iron-like nuclei. With SIBYLL, 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 a two-component proton-iron composition and the EPOS model, the fraction of protons with energies E > 1019 eV is 0.52−0.20+0.19 at the 95% C.L.

Muon component of EAS with energies above 1017 eV

Abstract Experimental data on muons (with threshold energy of 1 GeV) in extensive air showers (EAS) obtained at the Yakutsk EAS array from 1974 to 1992 are presented. Periods of reliable detector operation have been selected. Muon detector readings near the threshold have been carefully taken into account. Mean lateral distribution functions (LDF) of muons in the energy range of 1017–5 × 1019 eV and the zenith angle range of 0–60 ° have been obtained. The dependence of LDF on E0 and θ at energies above 5 × 1018 eV is shown to be considerably different in comparison with the region of lower energies.

Constraints on the Fraction of Primary Gamma Rays at Ultra-High Energies from the Muon Data of the Yakutsk EAS Array ¶

By using data on the total signal and on the muon component of air showers detected at the Yakutsk array, the possible upper limits on the fraction of primary gamma rays at ultra-high energies is analyzed in the framework of the recently suggested event-by-event approach. Upper limits on the photon fraction in the integral flux of primary cosmic rays are derived. At 95% C.L., these limits are 22% for primary energies E0 > 4 × 1019 eV and 12% for E0 > 2 × 1019 eV. Despite the presence of muonless events, the data are consistent with the complete absence of photons at least at 95% C.L. The sensitivity of the results to systematic uncertainties, in particular to those of the energy determination for nonphoton primaries, is discussed.

The composition of the primary particles at energies 3 ×10 17 -3 ×10 19 eV observed at the Yakutsk array

Signals in the surface and underground scintillation detectors of the Yakutsk array from particles of extensive air showers and a fraction of muons at 600 m from the vertical shower axis have been simulated. Calculations have been carried out in terms of QGSJET-II and Gheisha-2002d models with the help of the CORSIKA 6.616 and GEANT4 codes at energies 3×1017–3×1019 eV. A comparison of the results of these simulations with data observed at the Yakutsk array shows rather heavy composition of the primary radiation in this energy region. But it was shown that the number of muons is underestimated by ~15% if the QGSJET-II model was used at energies above 100 GeV and by ~ 10% if the Gheisha-2002d model was used at energies below 100 GeV. Besides, signals in the surface and underground detectors from a muon differ by 3%. A comparison of the results with these corrections taken into account with data favors rather a light (maybe a proton-dominated) composition of the primary radiation in the energy region of 2.3×1018–1.14×1019 eV. In the energy interval of 3×1017–2.3×1018 eV, the composition is heavier. At energies above 1.14×1019 eV, a composition may also be heavier as illustrated by a trend of data.

Main results of the study of ultrahigh-energy cosmic rays in the Yakutsk Extensive Air Shower Array

This paper describes the current state of the Yakutsk Extensive Air Shower Array (EASA) and the prospects for its modernization. The main results of the study of the energy spectrum, mass composition, and anisotropy of the ultra-high energy cosmic rays (UHECRs) that were obtained recently by the array are given.

Wide field-of-view Cherenkov telescope for the detection of cosmic rays in coincidence with the Yakutsk extensive air shower array

The Yakutsk array group is developing a wide field-of-view Cherenkov telescope to be operated in coincidence with the surface detectors of the extensive air shower array. Currently, the engineering prototype of the reflecting telescope with the front-end electronics is designed, assembled, and tested to demonstrate the feasibility of the conceived instrument. The status and specifications of the prototype telescope are presented, as well as the modernization program of the already existing Cherenkov light detectors subset of the array measuring ultra-high energy cosmic rays.

Reestimation of the energy of extensive air showers at the Yakutsk EAS array with the CORSIKA package

The responses of ground and underground muon scintillation detectors of the Yakutsk extensive air shower (EAS) array from primary particles with the energy E0 ≥ 1017 eV have been calculated within the QGSJET-01-d, QGSJET-II-04, SIBILL, and EPOS-LHC models with the CORSIKA package. A new estimate obtained for E0 is lower by a factor of about 1.41 than that previously obtained within the calorimetric method for EASs.

Enhancement of the Yakutsk array by atmospheric Cherenkov telescopes to study cosmic rays above

The aim of the Yakutsk array enhancement project is to create an instrument to study the highest-energy galac- tic cosmic rays (CRs) - their sources, energy spectrum, and mass composition. Additionally, there will be unique ca- pabilities for investigations in the transition region between galactic and extragalactic components of CRs. Using the well-developed imaging atmospheric Cherenkov telescope technique adapted to the energy region E>10 15 eV, we plan to measure the longitudinal structure parameters of the shower, e.g. angular and tempo- ral distributions of the Cherenkov signal related to Xmax and the mass composition of CRs. The main advantages of the Yakutsk array, such as its multi-component measurements of extensive air showers, and model-independent CR energy estimation based on Cherenkov light measurements, will be inherited by the in- strument to be created. Some theoretical predictions may soon be verified exper- imentally. In particular, it is shown in nonlinear kinetic the- ory of CR acceleration in SNRs that the maximum energy of nuclei is proportional to their charge ( Berezhko and V¨ olk,

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The energy spectrum of primary cosmic rays with ultra-high energies based on the Yakutsk EAS Array data is presented. For the largest events values of S600 and axis coordinates have been obtained using revised lateral distribution function. The effect of the arrival time distribution at several axis distance on estimated density for Yakutsk and AGASA is considered.