Combined energy spectrum of the Pierre Auger Observatory

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Combined energy spectrum of the Pierre Auger
Observatory

• V. M. Olmos Gilbaja
• Universidade de Santiago de Compostela
• Instituto Galego de Física de Altas Enerxías

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Differential flux
Energy reconstruction to count events in energy
bin Exposure at that energy
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Introduction

• Two independent techniques are used in the Pierre
  Auger Observatory
• Ground array of more than 1600 detectors
• 24 fluorescence telescopes
• Ground array ? high duty cycle ? sensitive to
  highest energies
• Fluorescence detector ? using events detected in
  coincidence with at least one surface detector ?
  extension of the spectrum down to 1018 eV

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Energy reconstruction with FD

• Determine the shower geometry
• Account for the Cherenkov contribution and the
  light scattering and attenuation
• Obtain the energy deposit from the absolute
  fluorescence yield
• Fit of a Gaisser - Hillas to measured energy
  deposit

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Energy reconstruction with SD

• Obtain the axis of the air shower from arrival
  time measurements
• Fit of a Lateral Distribution Function to
  measured signals
• Obtain signal at 1000 m S(1000)
• Account for the attenuation with zenith angle
  using a CIC method obtaining S38º
• Obtain the energy through a correlation of S38º
  with the fluorescence measurement of energy using
  hybrid events

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Exposure to hybrid events

• The calculation of the hybrid exposure requires
  the knowledge of the detector on-time
• Detection efficiency is influenced by external
  (lightnings) or internal (data acquisition
  failures) factors
• A Monte Carlo method which reproduces the data
  taking conditions including their time
  variability has been used to derive the exposure
  of the hybrid detection.

Corrected by 4 after validating the Monte Carlo
with air shower observations
systematic due to unknown primary composition
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Exposure to SD events

• Above saturation of trigger efficiency the
  acceptance becomes purely geometrical
• The exposure is obtained integrating over time
  the unitary acceptance per detector
• Monitoring information of active detectors
  provide information to determine the exposure
  with an uncertainty below 3
• Total exposure between Jan 2004 and Dec 2008 is
  12790 km2 sr yr

? 4.59 km2 sr between 0º and 60º
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Individual spectra

• Hybrid spectrum

• SD spectrum

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Energy Scale Systematics

• Absolute Fluorescence Yield 14
• Pressure dependence of Fluorescence Yield 1
• Humidity dependence of Fluorescence Yield 1
• Temperature dependence of Fluorescence Yield 5
• FD absolute calibration 11
• FD wavelength dependence response 3
• Rayleigh scattering in atmosphere 1
• Wavelength dependence of aerosol scattering 1
• FD reconstruction method 10
• Invisible energy 5
• Total 22

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Combining spectra

• Maximum likelihood method accounting for
  systematic and statistical uncertainties
• Flux scale parameters of kSD 1.01 and kFD
  0.99 show the good agreement between independent
  measurements
• Systematic uncertainty of the combined flux below
  4

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Combined spectrum of thePierre Auger Observatory
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Conclusions

• Two independent measurements of the cosmic ray
  energy spectrum with the Pierre Auger Observatory
  have been presented
• Both measurements have the same systematic
  uncertainty in the energy scale
• Combining both spectra enables the measurement of
  the ankle and the flux suppression at the highest
  energies
• Some comparisons with astrophysical models have
  been performed

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BACK UP SLIDES
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Event example
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Spectrum
?1 3.26 log(E/eV) 18.61 ?2
2.59 18.61 log(E/eV) 19.46 ?3 4.3
19.46 log(E/eV) log(E1/2/eV)
19.61 log(Wc/eV) 0.16