march 2005e. armengaud - moriond search methods for uhecr anisotropies within the pierre auger...
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March 2005 E. 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 σi : 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 – SimulationShowers injected at 45o
(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-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
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 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
March 2005 E. Armengaud - Moriond
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.
March 2005 E. Armengaud - Moriond
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
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 length @ 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 = 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
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)
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