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Cosmic ray anisotropy from Pierre Auger Observatory

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  1. Cosmic ray anisotropy from Pierre Auger Observatory J. R. T. de Mello Neto (for the Auger Collaboration) Universidade Federal do Rio de Janeiro V Nova Física no Espaço Campos do Jordão - SP

  2. Outline • Open questions • Coverage map • Previous anisotropy claims • Galactic center • Prescription results • Blind source search • Perspectives Ref: Auger contributions in the proceedings of ICRC 05 – Pune, India

  3. Cosmic flux vs. Energy Roughly a single power law Indication of Fermi shock acceleration mechanism? Spectrum extends beyond the energies that can be produced with shock acceleration in known shocks. • UHECR • one particle per century per km2 • many interesting questions S. Swordy

  4. Open questions • How cosmic rays are accelerated at ? • What are the sources? • How can they propagate along astronomical distances at such high energies? • Are they substantially deflected by magnetic fields? • Can we do cosmic ray astronomy? • What is the mass composition of cosmic rays?

  5. Anisotropy UHECR spatial distribution constrains models and sources : point-like for E > 10 EeV galactic magnetic diffusion for E < 10 EeV • If anisotropy/sources are seen • Start “charged particle astronomy” • probe magnetic fields • spectrometry • If no anisotropy/sources are seen • indication of top-down origin • re-think propagation • ???


  6. The Auger Observatory: Hybrid design • A large surface detector array combined with fluorescence detectors results in a unique and powerful design; • Simultaneous shower measurement allows for transfer of the nearly calorimetric energy calibration from the fluorescence detector to the event gathering power of the surface array. • A complementary set of mass sensitive shower parameters contributes to the identification of primary composition. • Different measurement techniques force understanding of systematic uncertainties in each.

  7. Hybrid events: 0.6° Surface detector: 2.2° for 3-fold events (E < 4 EeV) 1.7° for 4-fold events (3 < E < 10 EeV) 1.4° for 5 or more stations (E > 8 EeV) Angular resolution SD only

  8. Coverage map Check the proposition: what is observed is compatible with what is expected from an isotropic distribution; • isotropic background expectations (coverage map) • statistical estimator of the overdensity Need: Acurate determination of the coverage map is the real issue! • detector growing • weather effects • true large scale anisotropies

  9. Coverage determination • Two techniques for coverage map determination: • semi-analytical method • shuffling (two flavours) Acceptance almost independent of sidereal time and azimuth Four independent groups calculated the coverage.

  10. Shuffling method (2D) • MC based method : • Make N new realisations of the data arrival direction by resampling them : • 5 zenith angle bins • for each event : keep zenith, sample a new arrival time and a new azimuth from data in the same zenith angle bin • Average the N datasets in the window centered around the observed direction • This gives you the expected number of events in that direction

  11. Shuffling method (1D) • MC based method : • Make N new realisations of the data arrival direction by resampling them : • 5 zenith angle bins • for each event : keep zenith, sample a new day and a new UTC hours from data in the same zenith angle bin and draw phi uniformily • Average the N datasets in the window centered around the observed direction • This gives you the expected number of events in that direction

  12. Semi-analytical coverage map • Start with events zenith angle distribution • Fit it with some smooth functions : splines or polynomials times a Fermi-Dirac. • Convert the fitted zenith angle shape into a declination distribution (analytical) • Assume RA uniformity or use weather data to model RA variation • Integrate through the window • You have the expected number of events in any direction

  13. Semi-analytical method Isotropic simulation 10k events Coverage map in galactic coordinates Mollweide projection

  14. Li-Ma significance map

  15. Galactic center • Galactic Center is a “natural” site for cosmic ray acceleration • Supermassive black hole • Dense clusters of stars • Stellar remnants • SNR (?) Sgr A East • SUGAR excess is consistent with a point source, indicating neutral primaries • Neutrons would go undeflected, and neutron decay length at 1018 eV is comparable to the distance to the Galactic center (~8.5 kpc) Chandra

  16. Source at the Galactic center AGASA Significance (σ) 20o scales 1018 – 1018.4 eV 22% excess • Cuts are a posteriori • Chance probability is not well defined N. Hayashida et al., Astroparticle Phys. 10 (1999) 303

  17. Source at Galactic center SUGAR 5.5o cone 1018 – 1018.4 eV 85% excess J.A. Bellido et al., Astroparticle Phys. 15 (2001) 167

  18. Source at the Galactic Center Significance Coverage map 1.5o scale 3.7o scale (SUGAR like) 13.3o scale (AGASA like)

  19. Source at the Galactic center AGASA Original Cuts (1.0 – 2.5 EeV) top hat 20° 1155 / 1160.7 ratio = 1.00 ± 0.03 Enlarge energy range (0.8 – 3.2 EeV) top hat 20° 1896 / 1853.06 SUGAR (0.8 – 3.2 EeV) top hat 5° 144 / 150.9 ratio = 0.95 ± 0.08

  20. Point sources at the Galactic center SD only: Gaussian filtering 1.5 degree exp/obs 24.3/23.9 if S  CRthen for 0.8 EeV < E < 3.2 EeV S < 2.5    10-15 m-2 s-1@ 95 % Hybrid : Top hat window 1.0 degree Exp/obs 4/3.4 if S  CRthen for E > 0.1 EeV S < 1.2   10-13 m-2 s-1@ 95 %  uncertainty in CR flux Excess / Significance maps build using the individual pointing direction of the events.  Iron/proton detection efficiency ratio

  21. Galactic plane and Super Galactic plane Origin CR change galactic -> extra-galactic 1 – 10 EeV A) GP 1-5 EeV 5077 / 5083.3 B) SGP > 5 EeV 241 / 232.8 C) SGP > 10 EeV 68 /67.4

  22. Prescription results For each target: specify a priory probability levels and angular scales avoids uncertainties from “penalty factors” due to a posteriori probability estimation • Targets: • low energy: Galactic center and AGASA-SUGAR location • high energy: nearby violent extragalactic objects

  23. Blind search for point sources significance Li, Ma ApJ 272, 317-324 (1983) All distributions consistent with isotropy

  24. Conclusions • January 2004 - June 2005 • SD Array: • Unprecedented statistics in southern hemisphere (anisotropy) • Exposure 1750 km2 sr yr (1.07 total AGASA) • On time 94.3% • Gain one order of magnitude within the next two years (1500 physical events per day) • Hybrid: • Unprecedented core location and direction precision  excellent shower development and energy measurements • No previous claims of anisotropy were confirmed ! This is just the beginning! We have a lot of work ahead, including the Auger North Observatory! Thanks!

  25. 6 doublets 1 triplet above 4 x 1019 eV < 2.5 deg AGASA above 1019 eV Log E>19.4 Log E>19.6

  26. Agasa clustering • Agasa claimed high significance for their clustering • Analysis was done by tuning for maximum significance • No penalty factor or separate data set used Significant peak in the autocorrelation plots at zero degrees: implying presence of compact UHECR sources

  27. HiRes HiRes-I Monocular Data, E > 1019.5 eV No clustering seen so far! HiRes-I Monocular Data, E > 1018.5 eV Upper limit of 4 doublets (90% c.l.) in HiRes-I monocular dataset. HiRes Stereo

  28. GKZ suppression • Cosmic rays E = 1020 eV interact with 2.7 K photons • In the proton frame • Proton with less energy, eventually below the cutoff energy • EGZK= 5x 1019 eV Photon-pion production Photon dissociation Universe is opaque for E > EGZK !

  29. Detection techniques • Particles at ground level • large detector arrays (scintillators, water Cerenkov tanks, etc) • detects a small sample of secondary particles (lateral profile) • 100% duty cicle • aperture: area of array (independent of energy) • primary energy and mass compostion are model dependent • Fluorescence of N2 in the atmosphere • calorimetric energy measurement as function of atmospheric depth • only for E > 1017 eV • only for dark nights (14% duty cicle) • requires good knowledge of atmospheric conditions • aperture grows with energy, varies with atmosphere

  30. Pierre Auger South Observatory 3000 km2

  31. A surface array station Communications antenna GPS antenna Electronics enclosure Solar panels Battery box 3 photomultiplier tubes looking into the water collect light left by the particles Plastic tank with 12 tons of very pure water

  32. Station 102 Loma Amarilla Coihueco Los Morados Los Leones Surface detector Electronics temperature and VEM charge evolution over a week in April 2005 Trigger rates: T1: First level trigger T2: Second lever trigger ToT: Time over Threshold

  33. Surface detector Correlation of the trigger rate with temperature: T1 -0.04 ± 0.03 % per degree T2 0.08 ± 0.05 % per degree ToT 0.20 ± 0.50 % per degree SD array operates with stable trigger threshold even with 20 degrees daily temperature variations Surface detector array on-time in 2004: 94.3% Evolution of the physics event rate as a function of time. It is roughly related to the number of active stations by0.9 event per day per station

  34. The fluorescence detector Los Leones telescope

  35. The fluorescence telescope 30 deg x 30 deg x 30 km field of view per eye

  36. Atmospheric monitoring and FD detector calibration • Central Laser Facility (laser optically linked to adjacent surface detector tank) • Atmospheric monitoring • Calibration checks • Timing checks Atmospheric monitoring Absolute calibration Drum for uniform illumination of each fluorescence camera – part of end to end calibration . Lidar at each fluorescence eye for atmospheric profiling - “shooting the shower”

  37. Fluorescence detector Absolute calibration has been performed with a precision of 12%, with improvements planned to reduce this uncertainty to 8% The estimated systematic uncertainty in the reconstructed shower energy is 25%, with activity underway to reduce this significantly

  38. Hybrid detection Simultaneus detection in the sky and in the ground Golden events: independent triggers Sub-threshold events: FD promoted triggers

  39. Hybrid detector The hybrid analysis benefits from the calorimetry of the fluorescence technique and the uniformity of the surface detector aperture

  40. Construction progress In construction 1208 surface detector stations deployed 951 with eletronics and sending data Three fluorescence buildings complete each with 6 telescopes

  41. Spectrum: previous claims AGASA Continuation beyond the GZK limit? Extragalatic sources distributed uniformly M. Takeda et al., PRL 81 (1998) 1163

  42. Spectrum: previous claims HiRes HiRes mono spectrum consistent with GZK suppresion Fit to unbroken power law: Fit taking into account GZK suppression: HiRes Collab., arXiv:astro-ph/0501317

  43. Energy spectrum for Auger Observatory • Based on fluorescence and surface detector data • First model- and mass-independent energy spectrum • Power of the statistics and well-defined exposure of the surface detector • Hybrid data stablishes conection between ground parameter S and shower energy • Hybrid data confirm that SD event trigger is fully efficient above 3x1018 eV for θ<60o • Energy scale of the fluorescence detector (nearly calorimetric, model independent energy measurement)

  44. Constant intensity cut Constant intensity cut ↔ constant energy cut Cosmic rays are nearly isotropic: For a fixed I0 find S(1000) at each θ such that I(>S(1000)) = I0 The relative values of S(1000) give CIC(θ) Normalized so that CIC(38o) = 1; 38o is the median zenith angle Define the energy parameter S38= S(1000)/CIC(θ) for each shower : “the S(1000) it would have produced if it had arrived at 38o zenith angle”

  45. Energy spectrum for Auger Observatory Constant Intensity Cut Correlation FD-SD

  46. Energy spectrum for Auger Observatory Estimated Spectrum Percentage deviation from the best-fit power law Error bars Poisson statistics Systematic uncertainty: double arrows at two different energies

  47. Energy spectrum for Auger Observatory • No events above 1020 in spectrum • Two sigma upper limit is consistent with AGASA flux • With current level of statistics and systematics, no solid conclusion is possible

  48. Primary photon fraction upper limit Limited by statistics, Considerable increase in a near future. Obtain a bound at higher energy

  49. Primary photon fraction upper limit Further exploit surface detector observables

  50. The highest energy SD event (86 EeV) Properties of the 20 most energetic events High energy events