D. Allard

Measurement of the depth of maximum of extensive air showers above 10(18) eV

We describe the measurement of the depth of maximum, Xmax, of the longitudinal development of air showers induced by cosmic rays. Almost four thousand events above 10^18 eV observed by the fluorescence detector of the Pierre Auger Observatory in coincidence with at least one surface detector station are selected for the analysis. The average shower maximum was found to evolve with energy at a rate of (106 +35/-21) g/cm^2/decade below 10^(18.24 +/- 0.05) eV and (24 +/- 3) g/cm^2/decade above this energy. The measured shower-to-shower fluctuations decrease from about 55 to 26 g/cm^2. The interpretation of these results in terms of the cosmic ray mass composition is briefly discussed.

Measurement of the energy spectrum of cosmic rays above 10(18) eV using the Pierre Auger Observatory

J. Abraham h, P. Abreu bl, M. Aglietta ay, E.J. Ahn ca, D. Allard aa, J. Allen cd, J. Alvarez-Muñiz bs, M. Ambrosio ar, L. Anchordoqui co, S. Andringa bl, T. Antičić v, A. Anzalone ax, C. Aramo ar, E. Arganda bp, K. Arisaka ci, F. Arqueros bp, H. Asorey b, P. Assis bl, J. Aublin ac, M. Ave ag,cj, G. Avila j, T. Bäcker am, D. Badagnani f, M. Balzer ah, K.B. Barber k, A.F. Barbosa l, S.L.C. Barroso r, B. Baughman cf, P. Bauleo by, J.J. Beatty cf, B.R. Becker cm, K.H. Becker af, A. Bellétoile ad, J.A. Bellido k, S. BenZvi cn, C. Berat ad, T. Bergmann ah, X. Bertou b, P.L. Biermann aj, P. Billoir ac, O. Blanch-Bigas ac, F. Blanco bp, M. Blanco bq, C. Bleve aq, H. Blümer ai,ag, M. Boháčová cj,x, D. Boncioli as, C. Bonifazi u,ac, R. Bonino ay, N. Borodai bj, J. Brack by, P. Brogueira bl, W.C. Brown bz, R. Bruijn bu, P. Buchholz am, A. Bueno br, R.E. Burton bw, N.G. Busca aa, K.S. Caballero-Mora ai, L. Caramete aj, R. Caruso at, A. Castellina ay, O. Catalano ax, G. Cataldi aq, L. Cazon bl,cj, R. Cester au, J. Chauvin ad, A. Chiavassa ay, J.A. Chinellato p, A. Chou ca,cd, J. Chudoba x, R.W. Clay k, E. Colombo c, M.R. Coluccia aq, R. Conceição bl, F. Contreras i, H. Cook bu, M.J. Cooper k, J. Coppens bf,bh, A. Cordier ab, U. Cotti bd, S. Coutu cg, C.E. Covault bw, A. Creusot bn, A. Criss cg, J. Cronin cj, A. Curutiu aj, S. Dagoret-Campagne ab, R. Dallier ae, K. Daumiller ag, B.R. Dawson k, R.M. de Almeida p, M. De Domenico at, C. De Donato be,ap, S.J. de Jong bf, G. De La Vega h, W.J.M. de Mello Junior p, J.R.T. de Mello Neto u, I. De Mitri aq, V. de Souza n, K.D. de Vries bg, G. Decerprit aa, L. del Peral bq, O. Deligny z, A. Della Selva ar, C. Delle Fratte as, H. Dembinski ak, C. Di Giulio as, J.C. Diaz cc, M.L. Díaz Castro m, P.N. Diep cp, C. Dobrigkeit p, J.C. D’Olivo be, P.N. Dong cp,z, A. Dorofeev by, J.C. dos Anjos l, M.T. Dova f, D. D’Urso ar, I. Dutan aj, M.A. DuVernois ck, J. Ebr x, R. Engel ag, M. Erdmann ak, C.O. Escobar p, A. Etchegoyen c, P. Facal San Luis cj,bs, H. Falcke bf,bi, G. Farrar cd, A.C. Fauth p, N. Fazzini ca, A. Ferrero c, B. Fick cc, A. Filevich c, A. Filipčič bm,bn, I. Fleck am, S. Fliescher ak, C.E. Fracchiolla by, E.D. Fraenkel bg, U. Fröhlich am, W. Fulgione ay, R.F. Gamarra c, S. Gambetta an, B. García h, D. García Gámez br, D. Garcia-Pinto bp, X. Garrido ag,ab, G. Gelmini ci, H. Gemmeke ah, P.L. Ghia z,ay, U. Giaccari aq, M. Giller bk, H. Glass ca, L.M. Goggin co, M.S. Gold cm, G. Golup b, F. Gomez Albarracin f, M. Gómez Berisso b, P. Gonçalves bl, D. Gonzalez ai, J.G. Gonzalez br,cb, D. Góra ai,bj, A. Gorgi ay, P. Gouffon o, S.R. Gozzini bu, E. Grashorn cf, S. Grebe bf, M. Grigat ak, A.F. Grillo az, Y. Guardincerri e, F. Guarino ar, G.P. Guedes q, J.D. Hague cm, V. Halenka y, P. Hansen f, D. Harari b, S. Harmsma bg,bh, J.L. Harton by, A. Haungs ag, T. Hebbeker ak, D. Heck ag, A.E. Herve k, C. Hojvat ca, V.C. Holmes k, P. Homola bj, J.R. Hörandel bf, A. Horneffer bf, M. Hrabovský y,x, T. Huege ag, M. Hussain bn, M. Iarlori ao, A. Insolia at, F. Ionita cj, A. Italiano at, S. Jiraskova bf, K. Kadija v, M. Kaducak ca, K.H. Kampert af, T. Karova x, P. Kasper ca, B. Kégl ab, B. Keilhauer ag, A. Keivani cb, J. Kelley bf, E. Kemp p, R.M. Kieckhafer cc, H.O. Klages ag, M. Kleifges ah, J. Kleinfeller ag, R. Knapik by, J. Knapp bu, D.-H. Koang ad, A. Krieger c, O. Krömer ah, D. Kruppke-Hansen af, F. Kuehn ca, D. Kuempel af, K. Kulbartz al, N. Kunka ah, A. Kusenko ci, G. La Rosa ax, C. Lachaud aa, B.L. Lago u, P. Lautridou ae, M.S.A.B. Leão t, D. Lebrun ad, P. Lebrun ca, J. Lee ci, M.A. Leigui de Oliveira t, A. Lemiere z, A. Letessier-Selvon ac, I. Lhenry-Yvon z, R. López bb, A. Lopez Agüera bs, K. Louedec ab, J. Lozano Bahilo br, A. Lucero ay, M. Ludwig ai, H. Lyberis z, M.C. Maccarone ax, C. Macolino ac,ao, S. Maldera ay, D. Mandat x, P. Mantsch ca, A.G. Mariazzi f,

Trigger and aperture of the surface detector array of the Pierre Auger Observatory

The 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 x 10(18) eV, for all zenith angles between 0 degrees and 60 degrees, 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

Cosmogenic neutrinos: parameter space and detectabilty from PeV to ZeV

While propagating from their source to the observer, ultrahigh energy cosmic rays interact with cosmological photon backgrounds and generate to the so-called ``cosmogenic neutrinos. Here we study the parameter space of the cosmogenic neutrino flux given recent cosmic ray data and updates on plausible source evolution models. The shape and normalization of the cosmogenic neutrino flux are very sensitive to some of the current unknowns of ultrahigh energy cosmic ray sources and composition. We investigate various chemical compositions and maximum proton acceleration energies Ep,max which are allowed by current observations. We consider different models of source evolution in redshift and three possible scenarios for the Galactic to extragalactic transition. We summarize the parameter space for cosmogenic neutrinos into three regions: an optimistic scenario that is currently being constrained by observations, a plausible range of models in which we base many of our rate estimates, and a pessimistic scenario that will postpone detection for decades to come. We present the implications of these three scenarios for the detection of cosmogenic neutrinos from PeV to ZeV (1014−21 eV) with the existing and upcoming instruments. In the plausible range of parameters, the narrow flux variability in the EeV energy region assures low but detectable rates for IceCube (0.06–0.2 neutrino per year) and the Pierre Auger Observatory (0.03–0.06 neutrino per year), and detection should happen in the next decade. If EeV neutrinos are detected, PeV information can help select between competing models of cosmic ray composition at the highest energy and the Galactic to extragalactic transition at ankle energies. With improved sensitivity, ZeV neutrino observatories, such as ANITA and JEM-EUSO could explore and place limits on the maximum acceleration energy.

Implications of the cosmic ray spectrum for the mass composition at the highest energies

The significant attenuation of the cosmic ray flux above ~5 × 1019 eV suggests that the observed high energy spectrum is shaped by the so-called GZK effect (GZK: Greisen–Zatsepin–Kuzmin). This interaction of ultra-high energy cosmic rays (UHECRs) with the ambient radiation fields also affects their composition. We review the effect of photodissociation interactions on different nuclear species and analyze the phenomenology of secondary-proton production as a function of energy. We show that, by itself, the UHECR spectrum does not constrain the composition of cosmic rays at their extragalactic sources. While the propagated composition (i.e., as observed at Earth) cannot contain significant amounts of intermediate mass nuclei (say between He and Si), whatever the source composition, and while it is vastly proton dominated when protons are able to reach energies above 1020 eV at the source, we show that the propagated composition can be dominated by Fe and sub-Fe nuclei at the highest energies, either if the sources are very strongly enriched in Fe nuclei (a rather improbable situation), or if the accelerated protons have a maximum energy of a few 1019 eV at the sources. We also show that in the latter cases, the expected flux above 3 × 1020 eV is very much reduced as compared to the case when protons dominate in this energy range, both at the sources and at Earth.

Search for First Harmonic Modulation in the Right Ascension Distribution of Cosmic Rays Detected at the Pierre Auger Observatory

We present the results of searches for dipolar-type anisotropies in different energy ranges above 2.5 x 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

A study of the effect of molecular and aerosol conditions in the atmosphere on air fluorescence measurements at the Pierre Auger Observatory

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 3000 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 2 to 8 g cm 2 in measurements of the shower maximum. 2010 Elsevier B.V. All rights reserved.

Extragalactic propagation of ultrahigh energy cosmic-rays ☆

Abstract In this paper we review the extragalactic propagation of ultrahigh energy cosmic-ray (UHECR). We present the different energy loss processes of protons and nuclei, and their expected influence on energy evolution of the UHECR spectrum and composition. We discuss the possible implications of the recent composition analyses provided by the Pierre Auger Observatory. The production of secondary cosmogenic neutrinos and photons and the constraints their observation would imply for the UHECRs origin are also addressed. Finally, we conclude by briefly discussing the relevance of a multi messenger approach for solving the mystery of UHECRs.

PROPAGATION OF ULTRAHIGH ENERGY NUCLEI IN CLUSTERS OF GALAXIES: RESULTING COMPOSITION AND SECONDARY EMISSIONS

We study the survival of ultrahigh energy nuclei injected in clusters of galaxies, as well as their secondary neutrino and photon emissions, using a complete numerical propagation method and a realistic modeling of the magnetic, baryonic, and photonic backgrounds. It is found that the survival of heavy nuclei highly depends on the injection position and on the profile of the magnetic field. Taking into account the limited lifetime of the central source could also lead in some cases to the detection of a cosmic-ray afterglow, temporally decorrelated from neutrino and gamma-ray emissions. We calculate that the diffusive neutrino flux around 1 PeV coming from clusters of galaxies may have a chance to be detected by current instruments. The observation of single sources in neutrinos and in gamma rays produced by ultrahigh energy cosmic rays will be more difficult. Signals coming from lower energy cosmic rays (E approx< 1 PeV), if they exist, might however be detected by Fermi, for reasonable sets of parameters.

Propagation of high-energy cosmic rays in extragalactic turbulent magnetic fields: resulting energy spectrum and composition

We extend previous studies of mixed-composition extragalactic cosmic-ray source models, investigating the influence of a nonnegligible extragalactic magnetic field on the propagated cosmic-ray spectrum and composition. We study the transport of charged particles in turbulent fields and the transition from a ballistic to a diffusive propagation regime. We introduce a method allowing a fast integration of the particle trajectories, which allows us to calculate extragalactic cosmic-ray spectra in the general case, without using either the diffusive or the rectilinear approximation. We find that the main features of the mixed-composition models – regarding the interpretation of the ankle and the non-monotonous evolution of the average cosmic-ray mass – remain essentially unchanged as long as the magnetic field intensity does not exceed a few nG.