52° congresso sait 2008 raffaella bonino* for the pierre auger collaboration ( * ) ifsi – infn...

52° Congresso SAIt 200852° Congresso SAIt 2008

Raffaella BoninoRaffaella Bonino** for the Pierre Auger Collaboration for the Pierre Auger Collaboration((**)) IFSI – INFN – Università di Torino IFSI – INFN – Università di Torino

http://indico.in2p3.fr/conferenceDisplay.py?confId=420

08/05/2008 SAIt 2008 Raffaella Bonino

OutlineOutlineThe origin of the highest energies cosmic rays (>1019 eV) is expected to be extra-galacticextra-galactic What are these extra-galactic sources? search for correlations

Somewhere downwards in the spectrum, the transitiontransition from galactic to extra-gal. must occur Where? study of large scale anisotropies (change in the large scale angular distribution)

The Galactic CenterGalactic 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


Auger Surface DetectorAuger Surface Detector


SD arrival directionsSD arrival directionsarrival times and angular arrival times and angular

resolutionresolution 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


““Start TimeStart 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


SD angular resolutionSD angular resolutionComputed 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


SD angular resolutionSD angular resolution

Comparison with hybrid reconstruction (~0.6°) confirms the

SD-only result

SD-only Hybrid

Large scale anisotropy Large scale anisotropy studiesstudies


OverviewOverviewObjective: 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 modulationssky exposureinstabilities due to atmospheric and instrumental effectsnot constant acceptance

3 complementary analysis in the EeV (=1018eV) range (5·105 events)


Auger resultsAuger resultsResults from the search for large-scale patterns:

no modulation in RAno modulation in RA95% 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 of significant pattern

The AGASA 4% modulation is not confirmed (but the observed regions of the sky are different)

The Galactic CenterThe Galactic Centerregionregion


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*


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 rangesDistribution of Li-Ma overdensity significances consistent with isotropic sky


UHECR sourcesUHECR sources

Sizes and magnetic field strengths

of astronomical objects that are

possible candidates as CR sources

AGN

Radio Galaxies

)()()(max KpcRGBZeVE

Hillas plot

BUT…GZK cut- off : only nearby sources (within ~ 100-200 Mpc) should

contribute to the flux above EGZK = 6·1019 eV


Ultra High Energy Cosmic Ultra High Energy Cosmic RaysRays

Particles with E ~ 1020 eV exist and have been detectedwhat 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 scaleangular 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 sourcesIf sources are nearby and not uniformly distributed, an anisotropic arrival directionsdistribution is expected (“clustering”)


SD angular resolutionSD angular resolutionComputed 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 modelSemi-empirical timing uncertainty model [C.Bonifazi et al., astro-[C.Bonifazi et al., astro-ph0705.1856]ph0705.1856]

Space-angle uncertainty computed from and :

Angular resolution ≡ angular radius that would contain 68% of the showers comingfrom a point-like source:

sinF

FAR .


11stst method method - - Fourier time Fourier time analysisanalysis

Spectral analysis on the « modified time » of the events : tmod=t+RA-LST

fsid

fsol

fantisid

All energiesΔf

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.


Characteristics: Differential method Allows to remove direction-independent phenomena

(i.e. atmospheric and not-constant acceptance effects)

)()()()()()()(

)()()(

)()()(

tWtEttWtEttIttI

tttItW

tttItE

ttWtEtI

dtdI

)()()('

22ndnd method method - - East-West method East-West method

where I(t) = physical CR intensity

First harmonic analysis on (E-W) amplitude of the sidereal modulation of I(t)


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)%

22ndnd method method - - East-West method East-West method


33rdrd method method - - Exposure-based Exposure-based studystudy

Resid

ual m

odul

atio

n 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 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%

Measure the residual modulation after dividing by the exposure - modulations compatible with the

statistical + systematic errorsUpper limit derived from MC, taking into account the measured modulation

Results:


Large scale anisotropyLarge scale anisotropy1st method

2nd method

3rd methodAll energies

fsid

fsol

fsid

95% c.l. upper limits

amplitudes

Scan in energy

Scan in energy

Correlation of UHECR Correlation of UHECR with with

nearby extra-gal. nearby extra-gal. objectsobjects


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


Other possible UHECR Other possible UHECR sourcessources

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:

AUGER doesn’t support correlations reported by AGASA, YakutskAGASA, Yakutsk and HiResHiRes data no excess from an extended search

52° congresso sait 2008 raffaella bonino* for the pierre auger collaboration ( * ) ifsi – infn...

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