Arrival direction studies

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52 Congresso SAIt 2008

• Study of the UHECR
• Arrival Directions with the
• Pierre Auger Observatory

Raffaella Bonino for the Pierre Auger
Collaboration () IFSI INFN Università di
Torino
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Outline

• The origin of the highest energies cosmic rays
  (gt1019 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 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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Start Time

• It should correspond to the arrival time of the
  shower front to the detectors
• Its 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
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Overview

• Objective
• 
  galactic -level modulation (models of gal.
  propagation)
• CRs 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 (5105 events)

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Auger results

• Results from the search for large-scale patterns
• no modulation in RA
• 95 c.l. upper limit 1.4 for 1 lt E lt 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)

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The Galactic Centerregion
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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

SUGAR excess 85 E 1017.9-1018.5 eV (?,?)
(274,-22)
AGASA excess 22 E 1018-1018.4 eV (?,?)
(280,-17)
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 lt E lt 1 EeV ?
• 1 lt E lt 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

• 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 61019 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 51019 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
?? Angular resolution angular radius that
would contain 68 of the showers coming from a
point-like source
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1st method - Fourier time analysis
Spectral analysis on the  modified time  of the
events tmodtRA-LST

• At sidereal frequency, recover
• the RA modulation
• Frequency resolution ?f 1/Tacquis
• 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)

where I(t) physical CR intensity
First harmonic analysis on (E-W) ? amplitude of
the sidereal modulation of I(t)
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2nd method - East-West method
(4.2 0.4)
(0.8 0.4)
Scan in energy
95 c.l. upper limits
amplitudes
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3rd method - Exposure-based study
1 lt E lt 3 EeV

• Measure the residual modulation after
  dividing by the exposure
• modulations compatible with the statistical
  systematic errors
• Upper limit derived from MC, taking into account
  the measured modulation

Residual modulation
Results

• 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
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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 doesnt support correlations reported by
  AGASA, Yakutsk and HiRes data
• no excess from an extended search