Yakutsk EAS array data on the variations in intensities of the highest energy cosmic rays

The results of analysis of arrival frequency of cosmic rays with energies E ₀≥4×10¹⁷ eV are presented based on the data collected on the Yakutsk array during its 24 years of continuous operation (1977–2000). It is shown that the intensity of cosmic rays is variable. At E ₀≤(3?5)×10¹⁸ eV, the (2–3)-month data show many deviations by (3–4)σ from the mean level. At E ₀≥10¹⁹ eV, the intensities steadily decrease, on the average, by 1.5 times during the time period considered.     

Analysis of anisotropy of cosmic rays with the energy of about 1017 eV by Yakutsk EAS array data

A harmonic analysis of the directions of arrival of cosmic-ray particles with an energy of about 10¹⁷ eV in the vicinity of the registration threshold of the Yakutsk extensive air showers (EAS) array is given. A method for determining the contribution of inhomogeneous observation conditions and seasonal variations of the frequency of extensive air showers to the observed anisotropy is suggested. Taking into account these factors results in a considerable decrease of the amplitude characterizing the degree of anisotropy of cosmic-ray primaries. The amplitude of the first harmonic with respect to the right ascension is (0.45 ± 0.55)%, which shows that no probably significant anisotropy of the primary radiation is observed at 10¹⁷ eV.     

Clusters in the arrival directions of 2×1017-eV cosmic rays detected at the Yakutsk EAS array

Arrival directions of (1.3–4)×10¹⁷-eV cosmic rays detected for zenith angles θ≤53° at the Yakutsk array from 1974 to 2001 are analyzed. These directions exhibit numerous clusters correlating with the supergalactic plane.     

Portion of muons with a threshold energy above 1 GeV in extensive air showers with ultra-high energies,according to the Yakutsk EAS array data

Characteristics of the muon component in extensive air showers and the fluctuations of muons are considered. In this work, we compare experimental data with computations performed for various models of the hadron interactions of protons and iron nuclei. The aim of this analysis is to obtain information on the mass composition of cosmic rays in an ultra-high energy region.     

On the composition of cosmic rays with E 0 ≥ 1017 eV according to the Yakutsk EAS array data

The energy spectrum and anisotropy of primary cosmic rays, as well as the lateral distribution functions of electrons and muons in extensive air showers (EASs) with E ₀ ≥ 10¹⁷ eV, are presented according to the Yakutsk EAS array data. It has been shown that the spectrum and lateral distribution functions in some energy ranges have different shapes for the particles that arrive from the disc of the Supergalaxy (Local Supercluster of galaxies) and from the other part of the celestial sphere. This is interpreted as the manifestation of the interaction of extragalactic primary cosmic rays with the gas of the Supergalaxy that possibly leads to the production of new ultra-high-energy particles.     

Determining the primary cosmic ray energy from the total flux of Cherenkov light measured at the Yakutsk EAS array

We present a method for determining the energy of the primary particle that generates an extensive air shower (EAS) of comic rays based on measuring the total flux of Cherenkov light from the shower. Applying this method to Cherenkov light measurements at the Yakutsk EAS array has allowed us to construct the cosmic ray energy spectrum in the range 10¹⁵ ? 3 × 10¹⁹ eV.     

The light nuclei energy spectrum of primary cosmic rays using EAS MSU array data

The change in the energy spectrum index Δγ in the knee region was determined by means of analysis of extensive air showers (EAS) spectra using the number of the particles measured by the EAS MSU array. The value was equal to 1.00 ± 0.12. A comparison of lightweight nuclei energy spectra collected with different installations was carried out.     

A new method for observing the shower cascade profile at the Yakutsk EAS array

An empirical approach is presented for reconstruction of the extensive air shower (EAS) cascade curve according to data of the Cherenkov track detector. The proposed method, unlike conventional ones, is based on experimentally measured parameters and on calculated values which weakly depend on the model of shower development.     

Delayed gamma rays from muon capture on 207Pb

We have measured the energies and intensities of delayed γ-rays from ^207,201Tl following muon capture on ²⁰⁷Pb. Probabilities for the emission of 0–6 neutrons have been deduced. Measured and predicted neutron multiplicities (based on an impulse approximation model) are in reasonable agreement for multiplicities of 2, 3, 4 and 5. Experimental values for high and low multiplicities are not accounted for by the theory. High angular momentum states involving the $\text{h}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ proton hole are observed in several of the thallium daughters. Using a simple shell-model approach, we have correlated μ-capture on the $\text{h}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ proton shell with the energy spectrum of directly emitted neutrons.     

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 E ₀ ≥ 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 E ₀ is lower by a factor of about 1.41 than that previously obtained within the calorimetric method for EASs.     

Astrophysical and structural peculiarities of extensive air showers with energy E 0 ≥ 1017 eV from Yakutsk EAS array data

The astrophysical characteristics of primary cosmic rays (PCRs) and the structure of extensive air showers (EASs) with energy E ₀ ≥ 10¹⁷ eV are simultaneously analyzed using the Yakutsk EAS array data acquired in the period 1974–2005. Enhanced and reduced particle fluxes are shown to come from the disk of the Supergalaxy (the Local Supercluster of galaxies) at E ₀ ≥ 5 × 10¹⁸ eV and E ₀ ≤ (2?3) × 10¹⁸, respectively. The development of air showers with E ₀ ≥ (3?5) × 10¹⁸ eV differs significantly from that at lower energies. This is interpreted as a manifestation of the possible interaction between extragalactic PCRs and the matter of this spatial structure.     

Derivation of muon spectrum at high energies from the recent JACEE primary cosmic-ray spectrum using Fermilab data on meson production

Summary The energy spectrum of sea level muons in the range (10²÷10⁴) GeV has been calculated from the latest directly measured JACEE primary spectrum using Fermilab results on pp→π^± X and pp→K^± X inclusive reactions. The conventional pion atmospheric diffusion equation after Bugaevet al. has been used in this analysis to account the flux of muons emerged from the multiple generation of mesons in air. The derived muon spectrum has been compared with the earlier magnetic spectrograph data of Durham and Kiel groups. The latest BAKSAN scintillator telescope data is well in agreement with the calculated integral spectrum originated from the meson decays in the range (1÷10⁴) GeV.     

Cosmic ray energy spectrum (>1017 eV) of the Yakutsk EAS array

The energy spectrum obtained on the Yakutsk array for the parameter S ₆₀₀ is compared with the results obtained on the AGASA and HiRes arrays and the latest Pierre Auger Observatory data. The discrepancy in the intensity of the energy spectra obtained in different experiments can be explained by the presence of systematic errors in shower energy estimations.     

Integral energy spectrum of primary cosmic rays at high energies

The integral energy spectrum of primary cosmic rays has been obtained. In the energy range (2.4×10³−1.1×10⁵ GeV), the spectrum of all nuclei is consistent with a power law of indexγ=1.55±0.06 and the flux of all nuclei is:N(⩾E ₀)⋍(5.1±1.8)×10⁻¹×E ₀ ^−1.55 particles/cm² sterad. sec., whereE ₀ is in GeV. The spectrum of primaryα-particles in the energy range (4.4×10³−4.8×10⁴) GeV is also consistent with a power law of indexγ=1.71±0.12 and the flux is:N(⩾E ₀)=(4.2±1.4)×10⁻¹×E ₀ ^−1.71 , particles per cm² sterad. sec, whereE ₀ is in GeV.     

Derivation of relativistic solar cosmic-ray spectra from the Baksan EAS array data

Fifteen increases in the intensity of solar cosmic rays (SCRs) from 30 investigated ground level enhancement (GLE) events have detected by Baksan EAS arrays in the period from 1981 until now. These data make it possible to refine the high-energy part of the SCR spectra calculated previously according to the data of the world-wide network of neutron monitors. A method for obtaining SCR spectra and calculating the specific yield functions for the Andyrchi, Carpet, and BMD Baksan arrays is briefly described. The SCR spectra for different instants in the GLE event of January 20, 2005 are presented.     

Searching for preferential arrival directions of ultrahigh-energy cosmic rays on the Moscow State University EAS array

There is evidence for possible clustering in the arrival directions of extensive air showers (EASs). A program for analyzing EAS arrival directions has been developed to investigate this problem and the database of the Moscow State University EAS array for 1982–1989 has been analyzed. The distributions in equatorial coordinates have been derived for cosmic rays with energies E ≥ 2 × 10¹⁵ eV and zenith angles <40°. The uniformity of the derived distributions is verified using various statistical tests.     

Upper limit on the photon fraction in highest-energy cosmic rays from AGASA data

A new method to derive an upper limit on photon primaries from small data sets of air showers is developed which accounts for shower properties varying with the primary energy and arrival direction. Applying this method to the highest-energy showers recorded by the AGASA experiment, an upper limit on the photon fraction of 51% (67%) at a confidence level of 90% (95%) for primary energies above 1.25 x 10(20) eV is set. This new limit on the photon fraction above the Greisen-Zatsepin-Kuzmin cutoff energy constrains the -burst model of the origin of highest-energy cosmic rays.