Transcript Arrival direction studies

52° Congresso SAIt 2008
Raffaella Bonino* for the Pierre Auger Collaboration
(*) IFSI – INFN – Università di Torino
Outline
The origin of the highest energies cosmic rays (>1019 eV) is expected to
be extra-galactic
 What are these extra-galactic sources?  search for correlations
Somewhere downwards in the spectrum, the transition from galactic to
extra-gal. must occur
 Where?  study of large scale anisotropies (change in the large scale
angular distribution)
The Galactic Center is one of the most interesting galactic target
 look for localized excesses of CRs in the GC region at ~ 1018 eV
Required tools: knowledge of the angular resolution of the Surface Detector
 angular reconstruction and timing uncertainty model
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Auger Surface Detector
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SD arrival directions
arrival times and angular resolution
Especially for the analysis of small scale anisotropy a good angular
resolution and detector stability are required
The angular resolution is strictly dependent on the accuracy in the arrival
time measurement of the particles in the tanks
The arrival direction is measured
from the delays among the hit tanks
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“Start Time”
It should correspond to the arrival time of the shower front to the detectors
It’s identified with the arrival time of the first particle detected
⇒ the first bin above a fixed threshold in a 2 or 3-fold coincidence
Start Time
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SD angular resolution
Computed on an event by event basis:
→  and  derived from the fit of the arrival
time of the first particle on the tank
Based on:
Parabolic shower front model
Semi-empirical timing uncertainty model
Angular resolution ≡ angular radius that would contain 68% of the showers
coming from a point-like source
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SD angular resolution
SD-only
Hybrid
 Comparison with hybrid reconstruction (~0.6°) confirms the
SD-only result
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Large scale anisotropy
studies
Overview
Objective:
galactic: %-level modulation (models of gal. propagation)
CR’s origin at ~1018eV
extra-gal.: no structure except for a CMB-dipole (~0.6%)
at higher energies: GZK cut-off → sources → anisotropy
Difficulties: control of spurious modulations
sky exposure
instabilities due to atmospheric and instrumental effects
not constant acceptance
3 complementary analysis in the EeV (=1018eV) range (5·105 events)
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Auger results
Results from the search for largescale patterns:
no modulation in RA
95% c.l. upper limit = 1.4%
for 1 < E < 3 EeV
Exposure-independent crosschecks confirm the lack of
significant pattern
The AGASA 4% modulation is
not confirmed (but the observed
regions of the sky are different)
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The Galactic Center
region
Why the Galactic Center?
GC contains a super-massive black hole → possible candidate to accelerate CR
It passes only 6° away from the AUGER zenith
Claims in the past from other experiments of large excesses in GC region:
AGASA:
SUGAR:
excess = 22%
E = 1018-1018.4 eV
(,) = (280°,-17°)
excess = 85%
E = 1017.9-1018.5 eV
(,) = (274°,-22°)
H.E.S.S.
detected a TeV -ray
source close to
Sagittarius A*
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Auger results
as extended source
In our analysis GC treated both
as point-like source
Data set divided into 2 energy bands:
0.1 < E < 1 EeV 
1 < E < 10 EeV 
Conclusions:
No significant CRs flux excess in both energy ranges
Distribution of Li-Ma overdensity significances consistent with isotropic sky
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UHECR sources
Hillas plot
Emax ( eV )  Z  B( G)  R( Kpc)
Sizes and magnetic field strengths
of astronomical objects that are
possible candidates as CR sources
AGN
Radio Galaxies
BUT…
GZK cut- off : only nearby sources (within ~ 100-200 Mpc) should
contribute to the flux above EGZK = 6·1019 eV
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Ultra High Energy Cosmic Rays
Particles with E ~ 1020 eV exist and have been detected
what are and where do they come from?
Complementary studies:
Energy spectrum and composition
Origin → study of anisotropy in arrival directions:
LARGE SCALE:
transition from galactic to extra-galactic origin = change in the large scale
angular distribution because of different mechanisms of propagation
SMALL SCALE:
above 5·1019 eV cosmic rays are only slightly deflected (2°-3°) by magnetic fields
→ direct way to search for UHECR sources
If sources are nearby and not uniformly distributed, an anisotropic arrival directions
distribution is expected (“clustering”)
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SD angular resolution
Computed on an event by event basis:
→  and  derived from the fit of the arrival
time of the first particle on the tank
Based on:
Parabolic shower front model
Semi-empirical timing uncertainty model
[C.Bonifazi et al., astro-ph0705.1856]
Space-angle uncertainty computed from  and : F   

 
   sin    

Angular resolution ≡ angular radius that would contain 68% of the showers coming
from a point-like source: AR  . F 
 
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
1st method - Fourier time analysis
Spectral analysis on the « modified time » of the events : tmod=t+RA-LST
At sidereal frequency, recover
the RA modulation
Frequency resolution Δf ~1/Tacquis
fsol
fantisid
Δf
All energies
fsid
Compare the sidereal signal to:
- the antisidereal signal
- the average noise in other freq.
Results:
no signal in sidereal frequency
solar modulation of 3.2% due to atmospheric effects
analysis repeated in 3 different energy ranges → no significant sid. modulation
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2nd method - East-West method
Characteristics:
Differential method
Allows to remove direction-independent phenomena
(i.e. atmospheric and not-constant acceptance effects)
I (t  t )
   (t )
E (t ) 

where I(t) = physical CR intensity
I (t  t )
   (t )
W (t ) 

I (t  t ) I (t  t )

 E (t )  W (t )   (t )  E (t )  W (t )


dI
E (t )  W (t )
 I ' (t ) 
dt
t
First harmonic analysis on (E-W)  amplitude of the sidereal modulation of I(t)
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2nd method - East-West method
Results (using the whole data set: Emedian ~ 6· 1017 eV) :
solar modulation: reduced from 4.2% to 0.8% (±0.4%)
(4.2 ± 0.4)%
(0.8 ± 0.4)%
Scan in energy
sidereal modulation:
(0.7±0.4)% corresponding to PRayleigh=24%
antisidereal modulation = (0.5±0.4)%
95% c.l. upper limits
amplitudes
95% c.l. upper limit = 1.4%
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3rd method - Exposure-based study
-
modulations compatible with the
statistical + systematic errors
Upper limit derived from MC, taking
into account the measured modulation
Residual modulation
1 < E < 3 EeV
Measure the residual modulation
after dividing by the exposure
Results:
Energy (EeV)
nb events (expected noise)
sid. ampl. –
(phase)
antisid.
ampl.
upper limits
95% c.l.
1<E<3
69641 (0.7%)
0.74% (330º)
0.6%
1.4%
3 < E < 10
7722 (2.0%)
1.2% (100º)
1.1%
3.2%
E > 10
1437 (4.7%)
3.5% (70º)
4.2%
8.6%
A scan in energy has been done  no significant excess
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Large scale anisotropy
1st method
3rd method
All energies
fsol
Scan in energy
fsid
fsid
2nd method
Scan in energy
95% c.l. upper limits
amplitudes
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Correlation of UHECR with
nearby extra-gal. objects
What are AGN ?
Active Galactic Nuclei:
galaxies hosting central black
holes that feed on gas and stars
and may eject vast plasma
jets into intergalactic space
Different names → unified scheme 
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Other possible UHECR sources
BL – Lacs = subclass of blazars, active galaxies with
beamed emission from a relativistic jet
aligned toward our line of sight
→ potential sources of UHECRs
AUGER results:
AUGER doesn’t support correlations reported by AGASA, Yakutsk and
HiRes data
no excess from an extended search
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