The emission of radio waves from air showers has been attributed to the so-called geomagnetic emission process. At frequencies around 50 MHz this process leads to coherent radiation which can be observed with rather simple setups. The direction of the electric field induced by this emission process depends only on the local magnetic field vector and on the incoming direction of the air shower. We report on measurements of the electric field vector where, in addition to this geomagnetic component, another component has been observed that cannot be described by the geomagnetic emission process. The data provide strong evidence that the other electric field component is polarized radially with respect to the shower axis, in agreement with predictions made by Askaryan who described radio emission from particle showers due to a negative charge excess in the front of the shower. Our results are compared to calculations which include the radiation mechanism induced by this charge-excess process.

The Pierre Auger Observatory is making significant contributions towards understanding the nature and origin of ultra-high energy cosmic rays. One of its main challenges is the monitoring of the atmosphere, both in terms of its state variables and its optical properties. The aim of this work is to analyze aerosol optical depth τa(z) values measured from 2004 to 2012 at the observatory, which is located in a remote and relatively unstudied area of the Pampa Amarilla, Argentina. The aerosol optical depth is in average quite low – annual mean τa(3.5 km) ∼ 0.04 – and shows a seasonal trend with a winter minimum – τa(3.5 km) ∼ 0.03 –, and a summer maximum – τa(3.5 km) ∼ 0.06 –, and an unexpected increase from August to September – τa(3.5 km) ∼ 0.055). We computed backward trajectories for the years 2005 to 2012 to interpret the air mass origin. Winter nights with low aerosol concentrations show air masses originating from the Pacific Ocean. Average concentrations are affected by continental sources (wind-blown dust and urban pollution), while the peak observed in September and October could be linked to biomass burning in the northern part of Argentina or air pollution coming from surrounding urban areas.

We describe a new method of identifying night-time clouds over the Pierre Auger Observatory using infrared data from the Imager instruments on the GOES-12 and GOES-13 satellites. We compare cloud identifications resulting from our method to those obtained by the Central Laser Facility of the Auger Observatory. Using our new method we can now develop cloud probability maps for the 3000 km^2 of the Pierre Auger Observatory twice per hour with a spatial resolution of ~2.4 km by ~5.5 km. Our method could also be applied to monitor cloud cover for other ground-based observatories and for space-based observatories.

The Pierre Auger Collaboration has reported evidence for anisotropies in the arrival directions of cosmic rays with energies larger thanEth = 55 EeV. There is a correlation above the isotropic expectation with nearby active galaxies and the largest excess is in a celestial region around the position of the radio galaxy Cen A. If these anisotropies are due to nuclei of charge Z, the protons accelerated in those sources are expected, under reasonable assumptions, to lead to excesses in the same regions of the sky at energies above Eth/Z. We here report the lack of anisotropies at these lower energies for illustrative values of Z = 6, 13 and 26. These observations set stringent constraints on the allowed proton fraction at the sources.

The Pierre Auger Observatory in Malargüe, Argentina, is designed to study the properties of ultra-high energy cosmic rays with energies above 1018 eV. It is a hybrid facility that employs a Fluorescence Detector to perform nearly calorimetric measurements of Extensive Air Shower energies. To obtain reliable calorimetric information from the FD, the atmospheric conditions at the observatory need to be continuously monitored during data acquisition. In particular, light attenuation due to aerosols is an important atmospheric correction. The aerosol concentration is highly variable, so that the aerosol attenuation needs to be evaluated hourly. We use light from the Central Laser Facility, located near the center of the observatory site, having an optical signature comparable to that of the highest energy showers detected by the FD. This paper presents two procedures developed to retrieve the aerosol attenuation of fluorescence light from CLF laser shots. Cross checks between the two methods demonstrate that results from both analyses are compatible, and that the uncertainties are well understood. The measurements of the aerosol attenuation provided by the two procedures are currently used at the Pierre Auger Observatory to reconstruct air shower data.

A thorough search for large scale anisotropies in the distribution of arrival directions of cosmic rays detected above $10^{18}$ eV at the Pierre Auger Observatory is reported. For the first time, these large scale anisotropy searches are performed as a function of both the right ascension and the declination and expressed in terms of dipole and quadrupole moments. Within the systematic uncertainties, no significant deviation from isotropy is revealed. Upper limits on dipole and quadrupole amplitudes are derived under the hypothesis that any cosmic ray anisotropy is dominated by such moments in this energy range. These upper limits provide constraints on the production of cosmic rays above $10^{18}$ eV, since they allow us to challenge an origin from stationary galactic sources densely distributed in the galactic disk and emitting predominantly light particles in all directions.

The air fluorescence detector of the Pierre Auger Observatory is designed to perform calorimetric measurements of extensive air showers created by cosmic rays of above 10 eV. To correct these measurements for the effects introduced by atmospheric fluctuations, the Observatory contains a group of monitoring instruments to record atmospheric conditions across the detector site, an area exceeding 3,000 km. The atmospheric data are used extensively in the reconstruction of air showers, and are particularly important for the correct determination of shower energies and the depths of shower maxima. This paper contains a summary of the molecular and aerosol conditions measured at the Pierre Auger Observatory since the start of regular operations in 2004, and includes a discussion of the impact of these measurements on air shower reconstructions. Between 10 and 10 eV, the systematic uncertainties due to all atmospheric effects increase from 4% to 8% in measurements of shower energy, and 4 g cm to 8 g cm in measurements of the shower maximum.

Contributions of the Pierre Auger Collaboration to the 33rd International Cosmic Ray Conference, Rio de Janeiro, Brazil, July 2013

We derive lower bounds on the density of sources of ultra-high energy cosmic rays from the lack of significant clustering in the arrival directions of the highest energy events detected at the Pierre Auger Observatory. The density of uniformly distributed sources of equal intrinsic intensity was found to be larger than similar to (0.06 - 5) x 10(-4) Mpc(-3) at 95% CL, depending on the magnitude of the magnetic defections. Similar bounds, in the range (0.2 - 7) x 10(-4) Mpc(-3), were obtained for sources following the local matter distribution.

The surface detector array of the Pierre Auger Observatory can detect neutrinos with energy Eν between 1017 eV and 1020 eV from point-like sources across the sky south of +55◦ and north of −65◦ declinations. A search has been performed for highly inclined extensive air showers produced by the interaction of neutrinos of all flavors in the atmosphere (downward-going neutrinos), and by the decay of tau leptons originating from tau neutrino interactions in Earth’s crust (Earth-skimming neutrinos). No candidate neutrinos have been found in data up to 2010 May 31. This corresponds to an equivalent exposure of ∼3.5 years of a full surface detector array for the Earth-skimming channel and ∼2 years for the downward-going channel. An improved upper limit on the diffuse flux of tau neutrinos has been derived. Upper limits on the neutrino flux from point-like sources have been derived as a function of the source declination. Assuming a differential neutrino flux kPS · E−2 ν from a point-like source, 90% confidence level upper limits for kPS at the level of ≈5 × 10−7 and 2.5 × 10−6 GeV cm−2 s−1 have been obtained over a broad range of declinations from the searches for Earth-skimming and downward-going neutrinos, respectively.

Atmospheric conditions at the site of a cosmic ray observatory must be known for reconstructing observed extensive air showers. The Global Data Assimilation System (GDAS) is a global atmospheric model predicated on meteorological measurements and numerical weather predictions. GDAS provides altitude-dependent profiles of the main state variables of the atmosphere like temperature, pressure, and humidity. The original data and their application to the air shower reconstruction of the Pierre Auger Observatory are described. By comparisons with radiosonde and weather station measurements obtained on-site in Malargue and averaged monthly models, the utility of the GDAS data is shown. (C) 2012 Elsevier B.V. All rights reserved.

P. Abreu, M. Aglietta, E. J. Ahn, I. F.M. Albuquerque, D. Allard, I. Allekotte, J. Allen, P. Allison, A. Almeda, J. Alvarez Castillo, J. Alvarez-Muñiz, M. Ambrosio, A. Aminaei, L. Anchordoqui, S. Andringa, T. Antičić, C. Aramo, E. Arganda, F. Arqueros, H. Asorey, P. Assis, J. Aublin, M. Ave, M. Avenier, G. Avila, T. Bäcker, M. Balzer, K. B. Barber, A. F. Barbosa, R. Bardenet, S. L. C. Barroso, B. Baughman, J. Bäuml, J. J. Beatty, B. R. Becker, K.H. Becker, A. Bellétoile, J. A. Bellido, S. BenZvi, C. Berat, X. Bertou, P. L. Biermann, P. Billoir, F. Blanco, M. Blanco, C. Bleve, H. Blümer, M. Boháčová, D. Boncioli, C. Bonifazi, R. Bonino, N. Borodai, J. Brack, P. Brogueira, W.C. Brown, R. Bruijn, P. Buchholz, A. Bueno, R. E. Burton, K. S. Caballero-Mora, L. Caramete, R. Caruso, A. Castellina, O. Catalano, G. Cataldi, L. Cazon, R. Cester, J. Chauvin, S. H. Cheng, A. Chiavassa, J. A. Chinellato, J. Chirinos Diaz, J. Chudoba, R.W. Clay, M.R. Coluccia, R. Conceição, F. Contreras, H. Cook, M. J. Cooper, J. Coppens, A. Cordier, S. Coutu, C. E. Covault, A. Creusot, A. Criss, J. Cronin, A. Curutiu, S. Dagoret-Campagne, R. Dallier, S. Dasso, K. Daumiller, B. R. Dawson, R.M. de Almeida, M. De Domenico, C. De Donato, S. J. de Jong, G. De La Vega, W. J.M. de Mello Junior, J. R. T. de Mello Neto, I. De Mitri, V. de Souza, K. D. de Vries, G. Decerprit, L. del Peral, M. del Rı́o, O. Deligny, H. Dembinski, N. Dhital, C. Di Giulio, M. L. Dı́az Castro, P. N. Diep, C. Dobrigkeit, W. Docters, J. C. D’Olivo, P. N. Dong, A. Dorofeev, J. C. dos Anjos, M. T. Dova, D. D’Urso, I. Dutan, J. Ebr, R. Engel, M. Erdmann, C.O. Escobar, J. Espadanal, A. Etchegoyen, P. Facal San Luis, I. Fajardo Tapia, H. Falcke, G. Farrar, A. C. Fauth, N. Fazzini, A. P. Ferguson, A. Ferrero, B. Fick, A. Filevich, A. Filipčič, S. Fliescher, C. E. Fracchiolla, E. D. Fraenkel, U. Fröhlich, B. Fuchs, R. Gaior, R. F. Gamarra, S. Gambetta, B. Garcı́a, D. Garcia-Gamez, D. Garcia-Pinto, A. Gascon, H. Gemmeke, K. Gesterling, P. L. Ghia, U. Giaccari, M. Giller, H. Glass, M. S. Gold, G. Golup, F. Gomez Albarracin, M. Gómez Berisso, P. Gonçalves, D. Gonzalez, J. G. Gonzalez, B. Gookin, D. Góra, A. Gorgi, P. Gouffon, S. R. Gozzini, E. Grashorn, S. Grebe, N. Griffith, M. Grigat, A. F. Grillo, Y. Guardincerri, F. Guarino, G. P. Guedes, A. Guzman, J. D. Hague, P. Hansen, D. Harari, S. Harmsma, T.A. Harrison, J. L. Harton, A. Haungs, T. Hebbeker, D. Heck, A. E. Herve, C. Hojvat, N. Hollon, V. C. Holmes, P. Homola, J. R. Hörandel, A. Horneffer, P. Horvath, M. Hrabovský, T. Huege, A. Insolia, F. Ionita, A. Italiano, C. Jarne, S. Jiraskova, M. Josebachuili, K. Kadija, K. H. Kampert, P. Karhan, P. Kasper, B. Kégl, B. Keilhauer, A. Keivani, J. L. Kelley, E. Kemp, R.M. Kieckhafer, H.O. Klages, M. Kleifges, J. Kleinfeller, J. Knapp, D.-H. Koang, K. Kotera, N. Krohm, O. Krömer, D. Kruppke-Hansen, F. Kuehn, D. Kuempel, J. K. Kulbartz, N. Kunka, G. La Rosa, C. Lachaud, R. Lauer, P. Lautridou, S. Le Coz, M. S. A. B. Leão, D. Lebrun, P. Lebrun, M.A. Leigui de Oliveira, A. Lemiere, A. Letessier-Selvon, I. Lhenry-Yvon, K. Link, R. López, A. Lopez Agüera, K. Louedec, J. Lozano Bahilo, L. Lu, A. Lucero, M. Ludwig, H. Lyberis, C. Macolino, S. Maldera, D. Mandat, P. Mantsch, A. G. Mariazzi, J. Marin, V. Marin, I. C. Maris, H. R. Marquez Falcon, G. Marsella, D. Martello, L. Martin, H. Martinez, O. Martı́nez Bravo, H. J. Mathes, J. Matthews, J. A. J. Matthews, G. Matthiae, D. Maurizio, P. O. Mazur, G. Medina-Tanco, M. Melissas, D. Melo, E. Menichetti, A. Menshikov, P. Mertsch, C. Meurer, S. Mićanović, M. I. Micheletti, W. Miller, L. Miramonti, L. Molina-Bueno, S. Mollerach, M. Monasor, D. Monnier Ragaigne, F. Montanet, B. Morales, C. Morello, E. Moreno, J. C. Moreno, C. Morris, M. Mostafá, C. A. Moura, S. Mueller, M.A. Muller, G. Müller, M. Münchmeyer, R. Mussa, G. Navarra, J. L. Navarro, S. Navas, P. Necesal, L. Nellen, A. Nelles, J. Neuser, P. T. Nhung, L. Niemietz, N. Nierstenhoefer, D. Nitz, D. Nosek, L. Nožka, M. Nyklicek, J. Oehlschläger, A. Olinto, V.M. Olmos-Gilbaja, M. Ortiz, N. Pacheco, D. Pakk Selmi-Dei, M. Palatka, J. Pallotta, N. Palmieri, G. Parente, E. Parizot, A. Parra, R. D. Parsons, S. Pastor, T. Paul, M. Pech, J. Pȩkala, R. Pelayo, I.M. Pepe, L. Perrone, R. Pesce, E. Petermann, S. Petrera, P. Petrinca, A. Petrolini, Y. Petrov, J. Petrovic, C. Pfendner, N. Phan, R. Piegaia, T. Pierog, P. Pieroni, M. Pimenta, V. Pirronello, M. Platino, V.H. Ponce, M. Pontz, P. Privitera, M. Prouza, E. J. Quel, S. Querchfeld, J. Rautenberg, O. Ravel, D. Ravignani, B. Revenu, J. Ridky, S. Riggi, M. Risse, P. Ristori, H. Rivera, V. Rizi, J. Roberts, C. Robledo, W. Rodrigues de Carvalho, G. Rodriguez, J. Rodriguez Martino, J. Rodriguez Rojo, I. Rodriguez-Cabo, PRL 109, 062002 (2012) P HY S I CA L R EV I EW LE T T E R S week ending 10 AUGUST 2012

The Pierre Auger Observatory is a facility built to detect air showers produced by cosmic rays above 10^17 eV. During clear nights with a low illuminated moon fraction, the UV fluorescence light produced by air showers is recorded by optical telescopes at the Observatory. To correct the observations for variations in atmospheric conditions, atmospheric monitoring is performed at regular intervals ranging from several minutes (for cloud identification) to several hours (for aerosol conditions) to several days (for vertical profiles of temperature, pressure, and humidity). In 2009, the monitoring program was upgraded to allow for additional targeted measurements of atmospheric conditions shortly after the detection of air showers of special interest, e.g., showers produced by very high-energy cosmic rays or showers with atypical longitudinal profiles. The former events are of particular importance for the determination of the energy scale of the Observatory, and the latter are characteristic of unusual air shower physics or exotic primary particle types. The purpose of targeted (or "rapid") monitoring is to improve the resolution of the atmospheric measurements for such events. In this paper, we report on the implementation of the rapid monitoring program and its current status. The rapid monitoring data have been analyzed and applied to the reconstruction of air showers of high interest, and indicate that the air fluorescence measurements affected by clouds and aerosols are effectively corrected using measurements from the regular atmospheric monitoring program. We find that the rapid monitoring program has potential for supporting dedicated physics analyses beyond the standard event reconstruction.

Using measured events from the fluorescence detector of the Pierre Auger Observatory, an unbiased distribution of the atmospheric slant depths where showers reach their maxima has been obtained. Analyzing the tail of this distribution the proton-air cross-section for particle production at center-of-mass energies per nucleon of 57 TeV is determined to be [505$\pm$22(stat)+28$-$36(syst)] mb. Systematic uncertainties in the analysis arise from the limited knowledge of the primary mass composition, the need to use shower simulations and the selection of events. For the purpose of making comparisons with accelerator data we also calculate the inelastic and total proton-proton cross-sections using an extended Glauber model.

Technical reports on operations and monitoring of the Pierre Auger Observatory

The Surface Detector of the Pierre Auger Observatory is sensitive to neutrinos of all flavors above 0.1 EeV. These interact through charged and neutral currents in the atmosphere giving rise to extensive air showers. When interacting deeply in the atmosphere at nearly horizontal incidence, neutrinos can be distinguished from regular hadronic cosmic rays by the broad time structure of their shower signals in the water-Cherenkov detectors. In this paper we present for the first time an analysis based on down-going neutrinos. We describe the search procedure, the possible sources of background, the method to compute the exposure and the associated systematic uncertainties. No candidate neutrinos have been found in data collected from 1 January 2004 to 31 May 2010. Assuming an E-2 differential energy spectrum the limit on the single-flavor neutrino is E(2)dN/dE < 1.74 x 10(-7)GeVcm(-2)s(-1)sr(-1) at 90% C.L. in the energy range 1 x 10(17) eV < E < 1 x 10(20)eV.

We present a comprehensive study of the influence of the geomagnetic field on the energy estimation of extensive air showers with a zenith angle smaller than 60 degrees, detected at the Pierre Auger Observatory. the geomagnetic field induces an azimuthal modulation of the estimated energy of cosmic rays up to the similar to 2% level at large zenith angles. We present a method to account for this modulation of the reconstructed energy. We analyse the effect of the modulation on large scale anisotropy searches in the arrival direction distributions of cosmic rays. At a given energy, the geomagnetic effect is shown to induce a pseudo-dipolar pattern at the percent level in the declination distribution that needs to be accounted for.

The advent of the Auger Engineering Radio Array (AERA) necessitates the development of a powerful framework for the analysis of radio measurements of cosmic ray air showers. As AERA performs “radio-hybrid” measurements of air shower radio emission in coincidence with the surface particle detectors and fluorescence telescopes of the Pierre Auger Observatory, the radio analysis functionality had to be incorporated in the existing hybrid analysis solutions for fluorescence and surface detector data. This goal has been achieved in a natural way by extending the existing Auger Offline software framework with radio functionality. In this article, we lay out the design, highlights and features of the radio extension implemented in the Auger Offline framework. Its functionality has achieved a high degree of sophistication and offers advanced features such as vectorial reconstruction of the electric field, advanced signal processing algorithms, a transparent and efficient handling of FFTs, a very detailed simulation of detector effects, and the read-in of multiple data formats including data from various radio simulation codes. The source code of this radio functionality can be made available to interested parties on request.

We present the results of searches for dipolar-type anisotropies in different energy ranges above $2.5\times 10^{17}$ eV with the surface detector array of the Pierre Auger Observatory, reporting on both the phase and the amplitude measurements of the first harmonic modulation in the right-ascension distribution. Upper limits on the amplitudes are obtained, which provide the most stringent bounds at present, being below 2% at 99% $C.L.$ for EeV energies. We also compare our results to those of previous experiments as well as with some theoretical expectations.

The Pierre Auger Observatory is a detector for ultra-high energy cosmic rays. It consists of a surface array to measure secondary particles at ground level and a fluorescence detector to measure the development of air showers in the atmosphere above the array. The “hybrid” detection mode combines the information from the two subsystems. We describe the determination of the hybrid exposure for events observed by the fluorescence telescopes in coincidence with at least one water-Cherenkov detector of the surface array. A detailed knowledge of the time dependence of the detection operations is crucial for an accurate evaluation of the exposure. We discuss the relevance of monitoring data collected during operations, such as the status of the fluorescence detector, background light and atmospheric conditions, that are used in both simulation and reconstruction.