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. The text was submitted by the authors in English.     

Estimate of the fraction of primary photons in the cosmic-ray flux at energies ∼1017 eV from the EAS-MSU experiment data

We reanalyze archival EAS-MSU data in order to search for events with an anomalously low content of muons with energies E _μ > 10 GeV in extensive air showers with the number of particles N _e ? 2 × 10⁷. We confirm the first evidence for a nonzero flux of primary cosmic gamma rays at energies E ～ 10¹⁷ eV. The estimated fraction of primary gamma rays in the flux of cosmic particles with energies E ? 5.4 × 10¹⁶ eV is ε_γ = (0.43 _?0.11 ^+0.12 )%, which corresponds to the intensity I _γ = (1.2 _?0.3 ^+0.4 ) × 10^?16 cm^?2 s^?1 sr^?1. The study of arrival directions does not favor any particular mechanism of the origin of the photon-like events.     

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.     

Correlations between the arrival directions of ultrahigh energy cosmic rays and the large-scale structure of the universe

We present the results of analyzing the arrival directions of cosmic rays with energies E ₀≥ 4×10¹⁷ eV and zenith angles θ≤45° detected at the Yakutsk setup during 1974–2000. It is shown that increased particle fluxes exceeding the anticipated random distribution levels by 4–5)σ arrive from the galactic plane at E ₀≈(2–4)×10¹⁸ eV and from the supergalactic plane at E ₀≥8×10¹⁸ eV.     

Estimate of the mass composition of cosmic rays above 4×10<Superscript>19</Superscript> eV

The arrival directions of extensive air showers are analyzed on the basis of world data. It is found that the zenith-angle distributions for energies E>10¹⁹ eV and E>4×10¹⁹ eV differ from each other. According to our estimates, the SUGAR array detects no showers above 10²⁰ eV. The mass composition of ultrahigh-energy cosmic rays is estimated. Cosmic rays with E>4×10¹⁹ eV most likely consist of superheavy nuclei.     

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.     

Estimation of the energy of the electron-photon component of cosmic rays on the basis of data for Cherenkov light from ultrahigh-energy extensive air showers

The energy fraction E _em/E ₀ dissipated to the electron-photon component of extensive air showers (EASs) for E ₀=10¹⁵?10¹⁹ eV is estimated using data on Cherenkov radiation and charged particles from the Yakutsk EAS array. The results are compared with models with different dissipations to the electron-photon component and with calculations for various primary nuclei. In the energy range 10¹⁵?10¹⁶ eV and 10¹⁸?10¹⁹ eV, the ratio E _em/E ₀ is equal to 77 ± 2 and 88 ± 2, respectively, in agreement with the mixed and proton contents of primary cosmic rays in the former and latter energy ranges, respectively.     

Muons in extensive air showers of energies E 0=1016.6–1019.8 eV

Data on muons with the threshold energy E _μ≈1.0×secθ GeV in extensive air showers of energies E ₀≥4×10¹⁶ eV measured on the Yakutsk and Akeno arrays are jointly analyzed. The results are compared with the calculations by the quark-gluon-string model with jets. It is shown that this model does not contradict the data measured for energies E ₀≤10¹⁸ eV on both arrays under the assumption that the primary particle composition differs from the composition where heavy nuclei dominate over protons. Experimental data for energies E ₀≥3×10¹⁸ eV indicate that the shower development differs from that predicted by the quark-gluon-string model with jets.     

On the origin of <Emphasis Type="Italic">E</Emphasis><Subscript>0</Subscript> ≥ 10<Superscript>17</Superscript> eV cosmic rays according to data from the Yakutsk array for studying extensive air showers

Results are presented that were obtained by analyzing the energy spectrum and anisotropy of E ₀ ≥ 10¹⁷ eV primary cosmic rays on the basis of data accumulated at the Yakutsk array for studying extensive air showers over the period between 1974 and 2004. It is shown that spectra corresponding to different regions of the sky differ in shape. Particle fluxes going from the Galaxy and Supergalaxy (a local supercluster of galaxies) disks are enhanced for E ₀ ≥ 5 × 10¹⁸ eV and are reduced for E ₀ ≤ (2–3) × 10¹⁸ eV. This observation is interpreted as a manifestation of the possible interaction between extragalactic primary cosmic rays and matter of the above structures of space.     

On the anisotropy of <Emphasis Type="Italic">E</Emphasis><Subscript>0</Subscript> ⩾ 5.5× 10<Superscript>19</Superscript> eV cosmic rays according to data of the Pierre Auger Collaboration

The Pierre Auger Collaboration discovered, in a solid angle of radius about 18°, a local group of cosmic rays having energies in the region E ₀ ≥ 5.5 × 10¹⁹ eV and coming from the region of the Gen A radio galaxy, whose galactic coordinates are l _G = 309.5° and b _G = 19.4°. Near it, there is the Centaur supercluster of galaxies, its galactic coordinates being l _G = 302.4° and b _G = 21.6°. It is noteworthy that the Great Attractor, which may have a direct bearing on the observed picture, is also there.     

Diffuse extragalactic very-high-energy γ rays

γ Rays with energies E > 4 × 10¹⁹ eV, which are produced upon interactions of cosmic rays with the background radiation in intergalactic space, have been analyzed. The intensity of such γ rays is calculated under different assumptions about the cosmic-ray sources and using different estimates of the nonthermal background radio emission.     

Identification of extragalactic cosmic-ray sources using data from various detection facilities

Showers with energies E>3.2×10¹⁹ eV and E≥10²⁰ eV detected at the AGASA (Akeno, Japan), Haverah Park, and Yakutsk arrays are investigated. The question of how the identification of sources depends on the error in determining the shower arrival directions is analyzed. Confirmation is obtained for the conclusion in the author’s earlier work, that the principal sources of shower-driven particles are Seyfert galaxies with red shifts z≤0.0092, which are weak emitters in the x-ray and radio ranges. Zh. éksp. Teor. Fiz. 116, 1121–1130 (October 1999)     

The GZK horizon and constraints on the cosmic ray source spectrum from observations in the GZK regime

We discuss the GZK horizon of protons and present a method to constrain the injection spectrum of ultrahigh energy cosmic rays (UHECRs) from supposedly identified extragalactic sources. This method can be applied even when only one or two events per source are observed and is based on the analysis of the probability for a given source to populate different energy bins, depending on the actual cosmic ray injection spectral index. In particular, we show that for a typical source density of 4 × 10⁻⁵ Mpc⁻³, a data set of 100 events above 6 × 10¹⁹ eV allows one in 97% of all cases to distinguish a source spectrum dN/dE ∝ E ^−1.1 from one with E ^–2.7 at 95% confidence level. The article is published in the original.     

Cosmic rays of energies E 0≥1019 eV and the large-scale structure of the universe

Arrival directions of cosmic rays with energies E ₀≥10¹⁹ eV are analyzed in the supergalactic coordinates. It is shown that increased particle fluxes arrive from the supergalactic plane and regions symmetrically adjoining it at angles ±b _SG≈6.5°. Relatively high densities of clusters of galaxies and quasars, which are related to the large-scale structure of the universe, are observed in these regions.     

Nonstationary phenomena in cosmic rays with <Emphasis Type="Italic">E</Emphasis><Subscript>0</Subscript> ≤ 10<Superscript>18</Superscript> eV according to the data of the Yakutsk EAS array

The energy spectrum of cosmic rays and the fraction of muons with the threshold 1.0secθ GeV in the total number of charged particles in extensive air showers with energy E ₀ ≥ 10¹⁷ eV according to Yakutsk array data collected during 35 years of its continuous operation in 1978–2012 have been analyzed. It has been shown that these characteristics are noticeably different in different time periods. Before 1996, the integral intensity of the spectrum at E ₀ = 10¹⁷ eV varied near one stable position and then began to increase. It increased by (45 ± 5)% in seven years and, then, began to decrease. This phenomenon was accompanied a similar change in the fraction of muons and was caused by a significant increase in the average weight of the chemical composition of cosmic rays after 1996 as compared to preceding years.     

Atmospheric gamma rays in the energy region 40–1000 GeV

A large stack of lead-emulsion sandwich detector assembly was flown over Hyderabad, India. High energy gamma rays at the float altitude were unambiguously identified from the cascades they induced, and their energies reliably determined by improved methods. From an analysis of 163 gamma rays of energy ≳ 30 GeV, it is found that the differential energy spectrum is represented by the power lawJ _r (E)= 129·4E ^{−2·62±0·12} photons m⁻² sr⁻¹sec⁻¹ GeV⁻¹ at an effective atmospheric depth of 14·3 g cm⁻²; this is the first reliable balloon measurement of atmospheric gamma rays in the energy range 40–1000 GeV. After correcting for the gamma rays radiated by the primary cosmic ray electrons, the production spectrum of gamma rays, resulting from the collisions of cosmic ray nuclei with air nuclei, at the top of the atmosphere isP _r (E, 0)=8·2 × 10⁻⁴ E^2.60±0.09 photons g⁻¹sr⁻¹sec⁻¹ GeV⁻¹. The atmospheric propagation of the electromagnetic component due to the cascade process is also derived from the gamma ray production spectrum.     

Radio signals from extensive air showers with the energies <Emphasis Type="Italic">E</Emphasis> <Subscript>0</Subscript> ≥ 10<Superscript>19</Superscript> eV according to data from the Yakutsk extensive air shower array

A radio instrument and results obtained from the measurements of the 32-MHz radio signal from particles of extensive air showers (EASs) with energies E₀ ≥ 1×10¹⁹ eV are reported in brief. The data were obtained at the Yakutsk EAS array in 1987–1989 (the first series of measurements) and in 2009–2014 (new series of measurements). The radio signal from EASs with energies above 10²⁰eV was detected at the Yakutsk EAS array for the first time, including the shower with the record energy of ~2×10²⁰ eV for the Yakutsk EAS array.     

Mass content of ultrahigh-energy cosmic rays within different time periods

Estimates obtained for the average atomic number 〈lnA〉 of nuclei of primary particles with energies in the region of E ₀ ? 10¹⁵ eV over the past 36 years at the Yakutsk array and other arrays worldwide for studying extensive air showers are presented. It is shown that these estimates are markedly different with in different time periods. Earlier than 1996, the composition of cosmic rays in the energy range of 5 × 10¹⁵–10¹⁸ eV was markedly lighter than in later years. After 2008, there appeared a trend toward a decrease in 〈lnA〉. This is likely to be a manifestation of some explosive process in the Milky Way Galaxy after 1996.     

Energy spectrum and anisotropy of cosmic rays with <Emphasis Type="Italic">E</Emphasis><Subscript>0</Subscript>≥10<Superscript>17</Superscript> eV from Yakutsk EAS array data

Data from the Yakutsk extensive air shower array for the period 1974–2004 are used to analyze the energy spectrum and anisotropy of primary cosmic rays (PCRs) with energy E₀≥10¹⁷ eV. The spectra from different regions of the sky are shown to differ in shape. Enhanced and reduced particle fluxes come from the disks of the Galaxy and the Supergalaxy (the Local Supercluster of galaxies) at E₀≥5×10¹⁸ eV and E₀≤ (2?3)×10¹⁸ eV, respectively. This is interpreted as a manifestation of the possible interaction between extragalactic PCRs and the matter of these spatial structures.     

Fraction of protons in primary cosmic rays according to data from the PAMIR experiment in consideration of the response of X-ray emulsion chambers

Adjusted data on the fraction of protons in the mass composition of primary cosmic rays (PCRs) in the energy range of 10¹⁵–10¹⁷ eV are presented. Adjustments are made according to detailed calculations of the response of the X-ray emulsion chamber in the PAMIR experiment. It is demonstrated that the fraction of protons in a PCR is 16–18% for E ₀ ≈ 10¹⁵–10¹⁶ eV and does not change within the error for E ₀ ≈ 10¹⁶–10¹⁷ eV.