We discuss the lateral distribution of charged particles in extensive air showers with energy above $10^{17}$ eV measured by surface scintillation detectors of Yakutsk EAS array. The analysis covers the data obtained during the period from 1977 to 20
13. Experimental values are compared to theoretical predictions obtained with the use of CORSIKA code within frameworks of different hadron interaction models. The best agreement between theory and experiment is observed for QGSJet01 and QGSJet-II-04 models. A change in the cosmic ray mass composition towards proton is observed in the energy range $(1-20) times 10^{17}$ eV.
Responses of surface and underground scintillation detectors of Yakutsk array are calculated for showers initiated by primary particles with energy E0>=1.0E17 eV within the frameworks of QGSJet01 QGSIIJet-II-04, SIBYLL-2.1 and EPOS-LHC hadron interac
tion models. A new estimation of E0 is obtained with the use of various methods. The resulting energy is lower compared to the obtained with earlier method by factor ~1.33.
Several energy spectra of cosmic rays with energies E_0 geq 10^17 eV measured at the Yakutsk EAS, AGASA, Haverah Park, HiRes, Auger, and SUGAR arrays are considered. It is shown that the fairly good mutual agreement of the spectrum shapes can be achi
eved if the energy of each spectrum is multiplied by a factor K specific for each spectrum. These factors exhibit a pronounced dependence on the latitude of the above-mentioned arrays.
Results are presented that were obtained by analysing the arrival directions of E0 > 8x10**18 eV primary cosmic rays recorded at the Yakutsk array over the period between 1974 and 2003 and at the SUGAR array (Australia). The greatest primary cosmic r
ay flux is shown to arrive from the region of visible intersection of the planes of the Galaxy and the Supergalaxy (local supercluster of galaxies) at a galactic longitude of about 137 degres. On a global scale, the lowest temperature of the cosmic microwave background is typical of this region.
The Pierre Auger Collaboration discovered, in a solid angle of radius about 18degree, a local group of cosmic rays having energies in the region E0 geq 5.5times1019 eV and coming from the region of the Gen A radio galaxy, whose galactic coordinates a
re lG = 309.5degree and bG = 19.4degree. Near it, there is the Centaur supercluster of galaxies, its galactic coordinates being lG = 302.4degree and bG = 21.6degree. It is noteworthy that the Great Attractor, which may have a direct bearing on the observed picture, is also there.
The fluxes of electrons, positrons, gammas, Cherenkov photons and muons in individual extensive air showers induced by the primary protons and helium, oxygen and iron nuclei at the level of observation have been estimated with help of the code CORSIC
A 6.616. The comparison show that the values of the function Xi**2 per one degree of freedom changes from 1.1 for iron nuclei to 0.9 for primary protons. As this difference is small all readings of detectors of the Vavilov-Cherenkov radiation have been used. At last, readings of underground detectors of muons with energies above 1 GeV have been exploited to make definite conclusion about chemical composition.
The energies of the most energetic extensive air showers observed at the Yakutsk array have been estimated with help of the all detectors readings instead of using of the standard procedure with a parameter s(600). The energy of the most energetic ex
tensive air shower observed at the Yakutsk array happened to be 200, 200, 180 and 165 EeV with the values of the Xi**2 function per one degree of freedom 0.9, 1., 0.9 and 1.1 for the primary protons and helium, oxygen and iron nuclei accordingly.
Comparing the signals measured by the surface and underground scintillator detectors of the Yakutsk Extensive Air Shower Array, we place upper limits on the integral flux and the fraction of primary cosmic-ray photons with energies E > 10^18 eV, E >
2*10^18 eV and E > 4*10^18 eV. The large collected statistics of the showers measured by large-area muon detectors provides a sensitivity to photon fractions < 10^-2, thus achieving precision previously unreachable at ultra-high energies.
We analyse a sample of 33 extensive air showers (EAS) with estimated primary energies above 2cdot 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 sim 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.
