Search methods for UHECR anisotropies within the Pierre Auger Observatory

March 2005 E. Armengaud - Moriond

Search methods for UHECR anisotropies within the Pierre Auger Observatory

Eric Armengaud (APC/IAP - Paris)for the Auger Collaboration


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)


Contents

Angular reconstruction and resolution with the Surface Detector Exposure estimation methods

Large-scale anisotropy search methods Source search methods


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)


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 – SimulationShowers injected at 45o

(Aires - SDSim)


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 acceptance2. Exposure derivation from

the events (scrambling)

Preliminary raw event map (2004 subset : “T5 hexagons + Herald + >4tanks hit”) Equatorial coordinates – 3° smoothing


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)


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-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

Low-energy data, Jan-Feb period, Gal. coordinates

Day

Night


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


Large-scale feature analysis methods


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 ~ 1EeVPossible ‘weather’ effect checked: - no signal in solar time harmonic analysis - signal in R.A. harmonic analysis


From Rayleigh to dipole and Cℓ

JCAP 0410 (2004) 008

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.


Angular power spectrum : example

Small low-energy data sample (3 tanks only)

Derived Cℓ : Raw exposure

computation Weather systematics

corrected

Preliminary

Cℓ

Low-energy data, Jan-May period, Gal. Coordinates, exposure subtracted

Multipole


Small-scale feature analysis methods


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)


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


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)

Search methods for UHECR anisotropies within the Pierre Auger Observatory

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