March 2005E. Armengaud - Moriond Search methods for UHECR anisotropies within the Pierre Auger Observatory Eric Armengaud (APC/IAP - Paris) for the Auger Collaboration

March 2005 E. Armengaud - Moriond 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)

March 2005 E. Armengaud - Moriond Contents Angular reconstruction and resolution with the Surface Detector Exposure estimation methods Large-scale anisotropy search methods Source search methods

March 2005 E. Armengaud - Moriond 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 σ i : depend on  clock discretization error (25 ns binning)  distance to core (width of shower front increases with d)

March 2005 E. Armengaud - Moriond 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 45 o (Aires - SDSim)

March 2005 E. Armengaud - Moriond Exposure estimation methods Need background estimation to analyse event maps  Systematics can appear  Poor statistics at the highest energies 2 strategies : 1. Use our knowledge of detector acceptance 2. Exposure derivation from the events (scrambling) Preliminary raw event map (2004 subset : “T5 hexagons + Herald + >4tanks hit”) Equatorial coordinates – 3° smoothing

March 2005 E. Armengaud - Moriond Exposure derivation from the acceptance 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 Auger exposure (> 4 tank events)

March 2005 E. Armengaud - Moriond Exposure derivation : systematics Systematic effects, if correctly understood, can be taken into account in exposure computation Example : weather effects  a(T,P) ~ 1 + α(T-T o ) + β(P-P o )  [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 Low-energy data, Jan-Feb period, Gal. coordinates Day Night

March 2005 E. Armengaud - Moriond Scrambling method 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 Simulation : Histogram of pixel relative values Scrambling Acceptance

March 2005 E. Armengaud - Moriond Large-scale feature analysis methods

March 2005 E. Armengaud - Moriond 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

March 2005 E. Armengaud - Moriond From Rayleigh to dipole and C ℓ JCAP 0410 (2004) st harmonic analysis in R.A. : with and Dipole reconstruction (amplitude + orientation) – even with partial sky Higher C ℓ 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 C ℓ estimator:  Due to partial sky coverage, we need to invert a mode-mixing matrix M(ℓ,ℓ’) to recover the ‘true’ C ℓ :  Auger South exposure is large enough to do so.

March 2005 E. Armengaud - Moriond Angular power spectrum : example Small low-energy data sample (3 tanks only) Derived C ℓ :  Raw exposure computation  Weather systematics corrected Preliminary CℓCℓ Low-energy data, Jan-May period, Gal. Coordinates, exposure subtracted Multipole

March 2005 E. Armengaud - Moriond Small-scale feature analysis methods

March 2005 E. Armengaud - Moriond 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 1 EeV ~ distance to GC)

March 2005 E. Armengaud - Moriond 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 = 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

March 2005 E. Armengaud - Moriond 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)