he Pierre Auger Observatory, located on a vast, high plain in western\ud
Argentina, is the world's largest cosmic ray observatory. The objectives\ud
of the Observatory are to probe the origin and characteristics of cosmic\ud
rays above 10(17) eV and to study the interactions of these, the most\ud
energetic particles observed in nature. The Auger design features an\ud
array of 1660 water Cherenkov particle detector stations spread over\ud
3000 km(2) overlooked by 24 air fluorescence telescopes. In addition,\ud
three high elevation fluorescence telescopes overlook a 23.5 km(2),\ud
61-detector infilled array with 750 in spacing. The Observatory has been\ud
in successful operation since completion in 2008 and has recorded data\ud
from an exposure exceeding 40,000 km(2) sr yr. This paper describes the\ud
design and performance of the detectors, related subsystems and\ud
infrastructure that make up the Observatory
a b s t r a c tThe surface detector array of the Pierre Auger Observatory consists of 1600 water-Cherenkov detectors, for the study of extensive air showers (EAS) generated by ultra-high-energy cosmic rays. We describe the trigger hierarchy, from the identification of candidate showers at the level of a single detector, amongst a large background (mainly random single cosmic ray muons), up to the selection of real events and the rejection of random coincidences. Such trigger makes the surface detector array fully efficient for the detection of EAS with energy above 3 Â 10 18 eV, for all zenith angles between 0 3 and 60 3 , independently of the position of the impact point and of the mass of the primary particle. In these range of energies and angles, the exposure of the surface array can be determined purely on the basis of the geometrical acceptance.
Data collected by the Pierre Auger Observatory provide evidence for anisotropy in the arrival directions of the cosmic rays with the
highest-energies, which are correlated with the positions of relatively nearby active galactic nuclei (AGN) [Pierre Auger Collaboration,
Science 318 (2007) 938]. The correlation has maximum significance for cosmic rays with energy greater than 6x10^19 eV and AGN at a
distance less than ~75 Mpc. We have confirmed the anisotropy at a confidence level of more than 99% through a test with parameters
specified a priori, using an independent data set. The observed correlation is compatible with the hypothesis that cosmic rays with the
highest-energies originate from extra-galactic sources close enough so that their flux is not significantly attenuated by interaction with the
cosmic background radiation (the Greisen–Zatsepin–Kuz’min effect). The angular scale of the correlation observed is a few degrees,
which suggests a predominantly light composition unless the magnetic fields are very weak outside the thin disk of our galaxy. Our pres-
ent data do not identify AGN as the sources of cosmic rays unambiguously, and other candidate sources which are distributed as nearby AGN are not ruled out. We discuss the prospect of unequivocal identification of individual sources of the highest-energy cosmic rays
within a few years of continued operation of the Pierre Auger Observatory
Abstract. The model dependence of the development of extensive air showers generated by high-energy cosmic-ray particles in the atmosphere is studied. The increase of proton-proton and proton-air inelastic cross-sections and values for the elasticity are varied in the hadronic interaction model QGSJET. Using the CORSIKA simulation program, the impact of these changes is investigated on air shower observables like the average depth of the shower maximum Xmax and the number of muons and electrons at ground level. Calculating the mean logarithmic mass from experimental Xmax values, it is found that a moderate logarithmic increase of the proton-proton inelastic cross-section from σ inel pp = 51 mb at E 0 = 10 6 GeV to σ inel pp = 64 mb at E 0 = 10 8 GeV and an elasticity, additionally increased by 10% to 15%, describes the data best. Using these parameters, the mean logarithmic mass ln A derived from Xmax measurements is compatible with the extrapolations of the results of direct measurements to high energies using the poly-gonato model.
Simultaneous measurements of air showers with the fluorescence and surface detectors of the Pierre Auger Observatory allow a sensitive search for EeV photon point sources. Several Galactic and extragalactic candidate objects are grouped in classes to reduce the statistical penalty of many trials from that of a blind search and are analyzed for a significant excess above the background expectation. The presented search does not find any evidence for photon emission at candidate sources, and combined p-values for every class are reported. Particle and energy flux upper limits are given for selected candidate sources. These limits significantly constrain predictions of EeV proton emission models from non-transient Galactic and nearby extragalactic sources, as illustrated for the particular case of the Galactic center region.
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