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Abraham, J., P. Abreu, M. Aglietta, C. Aguirre, D. Allard, I. Allekotte, J. Allen, P. Allison, C. Alvarez, J. Alvarez-Muniz et al. "Correlation of the highest-energy cosmic rays with nearby extragalactic objects." SCIENCE 318, no. 5852 (2007): 938–943.
Abstract: Using data collected at the Pierre Auger Observatory during the past 3.7 years, we demonstrated a correlation between the arrival directions of cosmic rays with energy above 6 x 10(19) electron volts and the positions of active galactic nuclei (AGN) lying within similar to 75 megaparsecs. We rejected the hypothesis of an isotropic distribution of these cosmic rays with at least a 99% confidence level from a prescribed a priori test. The correlation we observed is compatible with the hypothesis that the highest-energy particles originate from nearby extragalactic sources whose flux has not been substantially reduced by interaction with the cosmic background radiation. AGN or objects having a similar spatial distribution are possible sources.
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Abraham, J., P. Abreu, M. Aglietta, C. Aguirre, D. Allard, I. Allekotte, J. Allen, P. Allison, J. Alvarez-Muniz, M. Ambrosio et al. "Upper limit on the cosmic-ray photon flux above 10(19) eV using the surface detector of the Pierre Auger Observatory." ASTROPARTICLE PHYSICS 29, no. 4 (2008): 243–256.
Abstract: A method is developed to search for air showers initiated by photons using data recorded by the surface detector of the Auger Observatory. The approach is based on observables sensitive to the longitudinal shower development, the signal risetime and the curvature of the shower front. Applying this method to the data, tipper limits on the flux of photons of 3.8 x 10(-3), 2.5 x 10(-3), and 2.2 x 10(-3) km(-2) sr(-1) yr(-1) above 10(19) eV, 2 x 10(19) eV, and 4 x 10(19) eV are derived, with corresponding limits on the fraction of photons being 2.0%, 5.1%, and 31% (all limits at 95% c.l.). These photon limits disfavor certain exotic models of sources of cosmic rays. The results also show that the approach adopted by the Auger Observatory to calibrate the shower energy is not strongly biased by a contamination from photons. (C) 2008 Elsevier B.V. All rights reserved.
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Abraham, J., P. Abreu, M. Aglietta, C. Aguirre, D. Allard, I. Allekotte, J. Allen, P. Allison, J. Alvarez-Muniz, M. Ambrosio et al. "Observation of the suppression of the flux of cosmic rays above 4x10(19) eV." PHYSICAL REVIEW LETTERS 101, no. 6 (2008).
Abstract: The energy spectrum of cosmic rays above 2.5 x 10(18) eV, derived from 20 000 events recorded at the Pierre Auger Observatory, is described. The spectral index gamma of the particle flux, J proportional to E-gamma, at energies between 4 x 10(18) eV and 4 x 10(19) eV is 2.69 +/- 0.02(stat) +/- 0.06(syst), steepening to 4.2 +/- 0.4(stat) +/- 0: 06 (syst) at higher energies. The hypothesis of a single power law is rejected with a significance greater than 6 standard deviations. The data are consistent with the prediction by Greisen and by Zatsepin and Kuz'min.
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Abraham, J., P. Abreu, M. Aglietta, C. Aguirre, D. Allard, I. Allekotte, J. Allen, P. Allison, J. Alvarez-Muniz, M. Ambrosio et al. "Upper limit on the diffuse flux of ultrahigh energy tau neutrinos from the Pierre Auger Observatory." PHYSICAL REVIEW LETTERS 100, no. 21 (2008).
Abstract: The surface detector array of the Pierre Auger Observatory is sensitive to Earth-skimming tau neutrinos that interact in Earth's crust. Tau leptons from nu(tau) charged-current interactions can emerge and decay in the atmosphere to produce a nearly horizontal shower with a significant electromagnetic component. The data collected between 1 January 2004 and 31 August 2007 are used to place an upper limit on the diffuse flux of nu(tau) at EeV energies. Assuming an E-nu(-2) differential energy spectrum the limit set at 90% C. L. is E(nu)(2)dN(nu tau)/dE(nu) < 1: 3 x 10(-7) GeV cm(-2) s(-1) sr(-1) in the energy range 2 x 10(17) eV< E-nu < 2 x 10(19) eV.
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Abraham, J., M. Aglietta, C. Aguirre, D. Allard, I. Allekotte, P. Allison, C. Alvarez, J. Alvarez-Muniz, M. Ambrosio, L. Anchordoqui et al. "Anisotropy studies around the galactic centre at EeV energies with the Auger Observatory." ASTROPARTICLE PHYSICS 27, no. 4 (2007): 244–253.
Abstract: Data from the Pierre Auger Observatory are analyzed to search for anisotropies near the direction of the Galactic Centre at EeV energies. The exposure of the surface array in this part of the sky is already significantly larger than that of the fore-runner experiments. Our results do not support previous findings of localized excesses in the AGASA and SUGAR data. We set an upper bound on a point-like flux of cosmic rays arriving from the Galactic Centre which excludes several scenarios predicting sources of EeV neutrons from Sagittarius A. Also the events detected simultaneously by the surface and fluorescence detectors (the 'hybrid' data set), which have better pointing accuracy but are less numerous than those of the surface array alone, do not show any significant localized excess from this direction. (c) 2006 Elsevier B.V. All rights reserved.
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Abraham, J., M. Aglietta, C. Aguirre, D. Allard, I. Allekotte, P. Allison, C. Alvarez, J. Alvarez-Muniz, M. Ambrosio, L. Anchordoqui et al. "An upper limit to the photon fraction in cosmic rays above 10(19) eV from the Pierre Auger Observatory." ASTROPARTICLE PHYSICS 27, no. 2-3 (2007): 155–168.
Abstract: An upper limit of 16% (at 95% c.l.) is derived for the photon fraction in cosmic rays with energies greater than 10(19) eV, based on observations of the depth of shower maximum performed with the hybrid detector of the Pierre Auger Observatory. This is the first such limit on photons obtained by observing the fluorescence light profile of air showers. This upper limit confirms and improves on previous results from the Haverah Park and AGASA surface arrays. Additional data recorded with the Auger surface detectors for a subset of the event sample support the conclusion that a photon origin of the observed events is not favored. (c) 2006 Elsevier B.V. All rights reserved.
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Abraham, J., M. Aglietta, I. C. Aguirre, M. Albrow, D. Allard, I. Allekotte, P. Allison, J. A. Muniz, M. G. do Amaral, M. Ambrosio et al. "Properties and performance of the prototype instrument for the Pierre Auger Observatory." NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT 523, no. 1-2 (2004): 50–95.
Abstract: Construction of the first stage of the Pierre Auger Observatory has begun. The aim of the Observatory is to collect unprecedented information about cosmic rays above 10(18) eV. The first phase of the project, the construction and operation of a prototype system, known as the engineering array, has now been completed. It has allowed all of the sub-systems that will be used in the full instrument to be tested under field conditions. In this paper, the properties and performance of these sub-systems are described and their success illustrated with descriptions of some of the events recorded thus far. (C) 2003 Elsevier B.V. All rights reserved.
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Allard, D., I. Allekotte, C. Alvarez, H. Asorey, H. Barros, X. Bertou, O. Burgoa, M. G. Berisso, O. Martinez, P. M. Loza et al. "Use of water-Cherenkov detectors to detect Gamma Ray Bursts at the Large Aperture GRB Observatory (LAGO)." NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT 595, no. 1 (2008): 70–72.
Abstract: The Large Aperture GRB Observatory (LAGO) project aims at the detection of high energy photons from Gamma Ray Bursts (GRB) using the single particle technique in ground-based water-Cherenkov detectors (WCD). To reach a reasonable sensitivity, high altitude mountain sites have been selected in Mexico (Sierra Negra, 4550 m a.s.l.), Bolivia (Chacaltaya, 5300 m a.s.l.) and Venezuela (Merida, 4765 m a.s.l.). We report on detector calibration and operation at high altitude, search for bursts in 4 months of preliminary data, as well as search for signal at ground level when satellites report a burst. (C) 2008 Elsevier B.V. All rights reserved.
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