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)


March 2005
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
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 – Simulation
Showers injected at 45o
(Aires - SDSim)
March 2005
E. Armengaud - Moriond
Exposure estimation methods


1.
2.
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)
March 2005
Preliminary raw event map
(2004 subset : “T5 hexagons + Herald + >4tanks hit”)
Equatorial coordinates – 3° smoothing
E. Armengaud - Moriond
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
March 2005
E. Armengaud - Moriond
Preliminary
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
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
March 2005
E. Armengaud - Moriond
Simulation :
Histogram of pixel relative values
Scrambling
Acceptance
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
March 2005
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
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
March 2005
E. Armengaud - Moriond
Angular power spectrum : example
Low-energy data, Jan-May period, Gal. Coordinates, exposure subtracted
Preliminary


Small low-energy data
sample (3 tanks only)
Derived Cℓ :
 Raw exposure
computation
 Weather systematics
corrected
Cℓ
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 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
Complementary methods (efficient tool to
understand details of detector behavior)
 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!
March 2005
E. Armengaud - Moriond