Study of the UHECR Arrival Directions with the Pierre Auger Observatory

1. 52° Congresso SAIt 2008 Study of the UHECR Arrival Directions with the Pierre Auger Observatory Raffaella Bonino* for the Pierre Auger Collaboration (*) IFSI – INFN – Università di Torino

2. Outline • The origin of the highest energies cosmic rays (>1019 eV) is expected to be extra-galactic • What are these extra-galactic sources?  search for correlations • Somewhere downwards in the spectrum, the transition from galactic to extra-gal. must occur • Where?  study of large scale anisotropies (change in the large scale angular distribution) • The Galactic Center is one of the most interesting galactic target  look for localized excesses of CRs in the GC region at ~ 1018 eV Required tools: knowledge of the angular resolution of the Surface Detector  angular reconstruction and timing uncertainty model SAIt 2008 Raffaella Bonino

3. Auger Surface Detector SAIt 2008 Raffaella Bonino

4. SD arrival directionsarrival times and angular resolution • Especially for the analysis of small scale anisotropy a good angular resolutionand detector stability are required • The angular resolution is strictly dependent on theaccuracy in thearrival timemeasurementof the particles in the tanks The arrival direction is measured from the delays among the hit tanks SAIt 2008 Raffaella Bonino

5. “Start Time” • It should correspond to the arrival time of the shower front to the detectors • It’s identified with the arrival time of the first particle detected ⇒ the first bin above a fixed threshold in a 2 or 3-fold coincidence Start Time SAIt 2008 Raffaella Bonino

6. SD angular resolution Computed on an event by event basis: →  and derived from the fit of the arrival time of the first particle on the tank Based on: • Parabolic shower front model • Semi-empirical timing uncertainty model Angular resolution≡ angular radius that would contain 68% of the showers coming from a point-like source SAIt 2008 Raffaella Bonino

7. SD angular resolution SD-only Hybrid  Comparison with hybrid reconstruction (~0.6°) confirms the SD-only result SAIt 2008 Raffaella Bonino

8. Large scale anisotropy studies

9. Overview Objective: galactic: %-level modulation (models of gal. propagation) • CR’s originat ~1018eV extra-gal.:no structure except for a CMB-dipole (~0.6%) at higher energies: GZK cut-off → sources → anisotropy Difficulties: control of spurious modulations • sky exposure • instabilities due to atmospheric and instrumental effects • not constant acceptance • 3 complementary analysis in the EeV (=1018eV) range (5·105 events) SAIt 2008 Raffaella Bonino

10. Auger results • Results from the search for large-scale patterns: • no modulation in RA • 95% c.l. upper limit = 1.4% for 1< E < 3 EeV • Exposure-independent cross-checks confirm the lack of significant pattern • The AGASA 4% modulation is not confirmed (but the observed regions of the sky are different) SAIt 2008 Raffaella Bonino

11. The Galactic Centerregion

12. Why the Galactic Center? • GC contains a super-massive black hole → possible candidate to accelerate CR • It passes only 6° away from the AUGER zenith • Claims in the past from other experiments of large excesses in GC region: AGASA: excess = 22% E = 1018-1018.4 eV (,) = (280°,-17°) SUGAR: excess = 85% E = 1017.9-1018.5 eV (,) = (274°,-22°) H.E.S.S. detected a TeV -ray source close to Sagittarius A* SAIt 2008 Raffaella Bonino

13. Auger results as extended source • In our analysis GC treated both as point-like source • Data set divided into 2 energy bands: 0.1 < E < 1 EeV 1 < E < 10 EeV Conclusions: • No significant CRs flux excess in both energy ranges • Distribution of Li-Ma overdensity significances consistent with isotropic sky SAIt 2008 Raffaella Bonino

14. SAIt 2008 Raffaella Bonino

15. UHECR sources Hillas plot Sizes and magnetic field strengths of astronomical objects that are possible candidates as CR sources AGN Radio Galaxies BUT… GZK cut- off : only nearby sources (within ~ 100-200 Mpc) should contribute to the flux aboveEGZK = 6·1019 eV SAIt 2008 Raffaella Bonino

16. Ultra High Energy Cosmic Rays Particles with E ~ 1020 eV exist and have been detected • what are and where do they come from? Complementary studies: • Energy spectrum and composition • Origin→ study of anisotropy in arrival directions: LARGE SCALE: transition from galactic to extra-galactic origin = change in the large scale angular distribution because of different mechanisms of propagation SMALL SCALE: above 5·1019 eV cosmic rays are only slightly deflected (2°-3°) by magnetic fields → direct way to search for UHECR sources If sources are nearby and not uniformly distributed, an anisotropic arrival directions distribution is expected (“clustering”) SAIt 2008 Raffaella Bonino

17. SD angular resolution Computed on an event by event basis: →  and derived from the fit of the arrival time of the first particle on the tank Based on: • Parabolic shower front model • Semi-empirical timing uncertainty model [C.Bonifazi et al., astro-ph0705.1856] Space-angle uncertainty computed from and : Angular resolution ≡ angular radius that would contain 68% of the showers coming from a point-like source: SAIt 2008 Raffaella Bonino

18. All energies Δf fsol fantisid fsid 1st method -Fourier time analysis Spectral analysis on the « modified time » of the events : tmod=t+RA-LST • At sidereal frequency, recover the RA modulation • Frequency resolution Δf ~1/Tacquis • Compare the sidereal signal to: - the antisidereal signal - the average noise in other freq. Results: • no signal in sidereal frequency • solar modulation of 3.2% due to atmospheric effects • analysis repeated in 3 different energy ranges → no significant sid. modulation SAIt 2008 Raffaella Bonino

19. 2nd method - East-West method Characteristics: • Differential method • Allows to remove direction-independent phenomena (i.e. atmospheric and not-constant acceptance effects) where I(t) = physical CR intensity First harmonic analysis on (E-W)  amplitude of the sidereal modulation of I(t) SAIt 2008 Raffaella Bonino

20. Results(using the whole data set: Emedian ~ 6· 1017 eV) : • solar modulation: reduced from 4.2% to 0.8% (±0.4%) • sidereal modulation: (0.7±0.4)% corresponding to PRayleigh=24% • antisidereal modulation = (0.5±0.4)% • 95% c.l. upper limit = 1.4% 2nd method - East-West method (4.2 ± 0.4)% (0.8 ± 0.4)% Scan in energy 95% c.l. upper limits amplitudes SAIt 2008 Raffaella Bonino

21. 3rd method -Exposure-based study 1 < E < 3 EeV • Measure the residual modulation after dividing by the exposure • modulations compatible with the statistical + systematic errors • Upper limit derived from MC, taking into account the measured modulation Residual modulation Results: • A scan in energy has been done  no significant excess SAIt 2008 Raffaella Bonino

22. Large scale anisotropy 1st method 3rd method All energies fsol Scan in energy fsid fsid 2nd method Scan in energy 95% c.l. upper limits amplitudes SAIt 2008 Raffaella Bonino

23. Correlation of UHECR with nearby extra-gal. objects

24. What are AGN ? Active Galactic Nuclei: galaxies hosting central black holes that feed on gas and stars and may eject vast plasma jets into intergalactic space Different names → unified scheme SAIt 2008 Raffaella Bonino

25. Other possible UHECR sources BL – Lacs =subclass of blazars, active galaxies with beamed emission from a relativistic jet aligned toward our line of sight → potential sources of UHECRs AUGER results: • AUGER doesn’t support correlations reported by AGASA, Yakutsk and HiRes data • no excess from an extended search SAIt 2008 Raffaella Bonino