Search methods for UHECR anisotropies within the Pierre Auger Observatory

1. Search methods for UHECR anisotropies within the Pierre Auger Observatory Eric Armengaud (APC/IAP - Paris) for the Auger Collaboration E. Armengaud - Moriond

2. The Pierre Auger Observatory • See talks by F. Arneodo and D. Newton • Hybrid detection of UHECR (fluorescence + surface detectors) • Highest statistics with SD-only events  Will be mostly used for anisotropy studies • Still under construction (~700/1600 SD, 2/4 FD) E. Armengaud - Moriond

3. Contents • Angular reconstruction and resolution with the Surface Detector • Exposure estimation methods • Large-scale anisotropy search methods • Source search methods E. Armengaud - Moriond

4. Angular reconstruction with SD • Iterative fit with the arrival times of particles from the shower: • Shower front ~ plane surface • Global fit with LDF core location estimation • If 4 tanks are hit : (variable) radius of curvature included • Full Chi2 example : • Weights σi : depend on • clock discretization error (25 ns binning) • distance to core (width of shower front increases with d) E. Armengaud - Moriond

5. Angular resolution with SD • Estimation from simulations: Angular resolution ~ 1° Resolution improves with θ Resolution improves with E • SD angular resolution can be derived from hybrid data Preliminary – Simulation Showers injected at 45o (Aires - SDSim) E. Armengaud - Moriond

6. Exposure estimation methods • Need background estimation to analyse event maps • Systematics can appear • Poor statistics at the highest energies • 2 strategies : • Use our knowledge of detector acceptance • Exposure derivation from the events (scrambling) Preliminary raw event map (2004 subset : “T5 hexagons + Herald + >4tanks hit”) Equatorial coordinates – 3° smoothing E. Armengaud - Moriond

7. Exposure derivation from the acceptance Auger exposure (> 4 tank events) • Exposure in any direction (α,δ) is derived from integration of array acceptance over its working period • Array growth and dead-times taken into account • Zenith angle distribution: • Analytically known when acceptance is saturated (high E) • Derived from simulations or empirically fitted from the data at lower energies Preliminary E. Armengaud - Moriond

8. Exposure derivation : systematics Low-energy data, Jan-Feb period, Gal. coordinates • Systematic effects, if correctly understood, can be taken into account in exposure computation • Example : weather effects • a(T,P) ~ 1 + α(T-To) + β(P-Po) • [effects of shower physics, electronics, calibration...] • T,P monitored at FD sites T captors on each SD station •  Correction to exposure for a given period is computed Day Night E. Armengaud - Moriond

9. Scrambling method Simulation : Histogram of pixel relative values Scrambling Acceptance • From a given event set, construct N >>1 Monte-Carlo sets which conserve the original • Zenith angle Θ distribution • Azimuth φ distribution • Solar time distribution • Exposure = average of MC event maps • Systematics, even uncontrolled, should be removed • Real large-scale anisotropy patterns also removed!! • Small statistical fluctuations remain E. Armengaud - Moriond

10. Large-scale feature analysis methods E. Armengaud - Moriond

11. Large-scale features : introduction • Deflections by galactic fields : R/kpc ~(E/EeV)/ (Z B/μG) Large-scale patterns are expected in various scenarios: • Low-energy, galactic sources • High-energy sources in nearby structures • Agasa detection at ~ 1 EeV : excess around GC • Auger South looks directly towards the GC Significance map (AGASA) at ~ 1EeV Possible ‘weather’ effect checked: - no signal in solar time harmonic analysis - signal in R.A. harmonic analysis E. Armengaud - Moriond

12. From Rayleigh to dipole and Cℓ • 1st harmonic analysis in R.A. : with and • Dipole reconstruction (amplitude + orientation) – even with partial sky • Higher Cℓ orders – even with partial sky : • We develop the fluctuations of event number on spherical harmonic basis • Assuming a stochastic and spectrally homogeneous field, we derive a Cℓ estimator: • Due to partial sky coverage, we need to invert a mode-mixing matrix M(ℓ,ℓ’) to recover the ‘true’ Cℓ : • Auger South exposure is large enough to do so. JCAP 0410 (2004) 008 E. Armengaud - Moriond

13. Angular power spectrum : example Low-energy data, Jan-May period, Gal. Coordinates, exposure subtracted • Small low-energy data sample (3 tanks only) • Derived Cℓ : • Raw exposure computation • Weather systematics corrected Preliminary Cℓ Multipole E. Armengaud - Moriond

14. Small-scale feature analysis methods E. Armengaud - Moriond

15. Source search : introduction • Clusters found by Agasa at the highest energies • Statistical significance still under debate; HiRes (stereo) does not confirm yet. • Motivations : • Directly pointing sources of UHECR • Important constraints on extragalactic magnetic fields • At lower E : neutrons from the GC (mean decay length @ 1 EeV ~ distance to GC) E. Armengaud - Moriond

16. Prescriptions on source searches • Requirement : protect the Collaboration from wrong claims. A finite data set will always show some “pattern” if a large number of trials are made • Method : a fixed excess probability P = 0.001 is distributed over a few a priori targets. • Current ‘targets’: • GC at low energy + Agasa/Sugar direction • 3 nearby objects (Cen A, NGC0253, NGC3256) • Targets can be changed in view of the data E. Armengaud - Moriond

17. Conclusions • Auger angular resolution: • ~ 1 degree • improved for hybrid data • Background estimation : • scrambling • analytical computation • Large scales : • Rayleigh analysis • Dipole reconstruction, angular power spectrum • Small scales : • Strict prescriptions to avoid wrong claims but blind source searches are also carried out to feed possible new prescriptions; • Autocorrelation analysis, triangle area distribution... • Analysis still going on: more events in the sky every day! Complementary methods (efficient tool to understand details of detector behavior) E. Armengaud - Moriond