52° Congresso SAIt 200852° Congresso SAIt 2008
Raffaella Bonino* for the Pierre Auger CollaborationRaffaella Bonino* for the Pierre Auger Collaboration
(*) IFSI – INFN – Università di Torino(*) IFSI – INFN – Università di Torino
08/05/2008
SAIt 2008 Raffaella Bonino
OutlineOutline
extra-galacticThe origin of the highest energies cosmic rays (>1019 eV) is expected tobe extra-galactic
What are these extra-galactic sources? search for correlations
transitionSomewhere downwards in the spectrum, the transition from galactic toextra-gal. must occur
Where? study of large scale anisotropies (change in the large scale
angular distribution)
Galactic CenterThe 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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SAIt 2008 Raffaella Bonino
Auger Surface DetectorAuger Surface Detector
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SAIt 2008 Raffaella Bonino
SD arrival directionsarrival times and angular resolutionSD arrival directionsarrival 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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SAIt 2008 Raffaella Bonino
“Start Time”“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
08/05/2008
SAIt 2008 Raffaella Bonino
SD angular resolutionSD angular resolution
Computed on an event by event basis:
→ and derived from the fit of the arrivaltime 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
08/05/2008
SAIt 2008 Raffaella Bonino
SD angular resolutionSD angular resolution
Comparison with hybrid reconstruction (~0.6°) confirms the
SD-only result
SD-only
Hybrid
Large scale anisotropystudiesLarge scale anisotropystudies
08/05/2008
SAIt 2008 Raffaella Bonino
OverviewOverview
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)
08/05/2008
SAIt 2008 Raffaella Bonino
Auger resultsAuger results
Results from the search for large-scale patterns:
no modulation in RAno modulation in RA
95% c.l. upper limit = 1.4%95% c.l. upper limit = 1.4%for 1 < E < 3 EeV
Exposure-independent cross-checks confirm the lack ofsignificant pattern
The AGASA 4% modulation isnot confirmed (but the observedregions of the sky are different)
The Galactic CenterregionThe Galactic Centerregion
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SAIt 2008 Raffaella Bonino
Why the Galactic Center?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:
SUGARSUGAR:
excess = 85%
E = 1017.9-1018.5 eV
(,) = (274°,-22°)
AGASAAGASA:
excess = 22%
E = 1018-1018.4 eV
(,) = (280°,-17°)
H.E.S.S.H.E.S.S.
detected a TeV -ray
source close to
Sagittarius A*
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SAIt 2008 Raffaella Bonino
Auger resultsAuger 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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SAIt 2008 Raffaella Bonino
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SAIt 2008 Raffaella Bonino
UHECR sourcesUHECR sources
Sizes and magnetic field strengths
of astronomical objects that are
possible candidates as CR sources
AGN
Radio Galaxies
Hillas plot
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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SAIt 2008 Raffaella Bonino
Ultra High Energy Cosmic RaysUltra High Energy Cosmic Rays
Particles with E ~ 1020 eV exist and have been detected
what are and where do they come from?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”)
08/05/2008
SAIt 2008 Raffaella Bonino
SD angular resolutionSD angular resolution
Computed on an event by event basis:
→ and derived from the fit of the arrivaltime of the first particle on the tank
Based on:
Parabolic shower front model
[C.Bonifazi et al., astro-ph0705.1856]Semi-empirical timing uncertainty model[C.Bonifazi et al., astro-ph0705.1856]
Space-angle uncertainty computed from and :
Angular resolution ≡ angular radius that would contain 68% of the showers coming
from a point-like source:
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SAIt 2008 Raffaella Bonino
1st method - Fourier time analysis1st method - Fourier time analysis
Spectral analysis on the « modified time » of the events : tmod=t+RA-LST
fsid
fsol
fantisid
All energies
Δf
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
At sidereal frequency, recover
the RA modulation
Frequency resolution Δf ~1/Tacquis
Compare the sidereal signal to:
- the antisidereal signal
- the average noise in other freq.
08/05/2008
SAIt 2008 Raffaella Bonino
Characteristics:
Differential method
Allows to remove direction-independent phenomena
(i.e. atmospheric and not-constant acceptance effects)
2nd method - East-West method2nd method - East-West method
where I(t) = physical CR intensity
First harmonic analysis on (E-W) amplitude of the sidereal modulation of I(t)
08/05/2008
SAIt 2008 Raffaella Bonino
Results (using the whole data set: Emedian ~ 6· 1017 eV) :
solar modulation: reduced from 4.2% to 0.8% (±0.4%)
sidereal modulation:
(0.7±0.4)% corresponding to PRayleigh=24%
antisidereal modulation = (0.5±0.4)%
95% c.l. upper limit = 1.4% 95% c.l. upper limit = 1.4%
amplitudes
95% c.l. upper limits
Scan in energy
(4.2 ± 0.4)%
(0.8 ± 0.4)%
2nd method - East-West method2nd method - East-West method
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SAIt 2008 Raffaella Bonino
3rd method - Exposure-based study3rd method - Exposure-based study
Residual modulation
1 < E < 3 EeV
A scan in energy has been done no significant excess
Energy (EeV)
nb events -(expected noise)
sid. ampl. –(phase)
antisid.ampl.
upper limits95% 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%
Measure the residual modulationafter dividing by the exposure
-modulations compatible with thestatistical + systematic errors
Upper limit derived from MC, takinginto account the measured modulation
Results:
08/05/2008
SAIt 2008 Raffaella Bonino
Large scale anisotropyLarge scale anisotropy
1st method
2nd method
3rd method
All energies
fsid
fsol
fsid
95% c.l. upper limits
amplitudes
Scan in energy
Scan in energy
Correlation of UHECR withnearby extra-gal. objectsCorrelation of UHECR withnearby extra-gal. objects
08/05/2008
SAIt 2008 Raffaella Bonino
What are AGN ?What are AGN ?
Active Galactic Nuclei: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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SAIt 2008 Raffaella Bonino
Other possible UHECR sourcesOther 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 results:
AGASA, YakutskHiResAUGER doesn’t support correlations reported by AGASA, Yakutsk andHiRes data
no excess from an extended search
