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Energy Spectrum

Energy Spectrum. C. O. Escobar Pierre Auger Director’s Review December 2011. Significance of the Energy Spectrum. Primary observation that provides constraints on models for the production and propagation of CR: Transition from galactic to extragalactic origin;

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Energy Spectrum

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  1. Energy Spectrum C. O. Escobar Pierre Auger Director’s Review December 2011 Fermilab Director's Review

  2. Significance of the Energy Spectrum • Primary observation that provides constraints on models for the production and propagation of CR: • Transition from galactic to extragalactic origin; • Suppression due to energy losses during propagation; • Features of the injection spectrum at the sources. Fermilab Director's Review

  3. How is the energy spectrum measured at Auger? • Two complimentary techniques: Surface Detector Array (SD) and Fluorescence Detector (FD) Types of spectra shown in this presentation: SD only for energies above 30 EeV; Hybrid spectrum (air showers measured by the FD and at least one triggered station of the SD), extending the spectrum down to 1 EeV Fermilab Director's Review

  4. SD Spectrum • Exposure: obtained by integrating the number of active detector stations of the surface array over time. It saturates above 30 EeV independently of the primary charge. Know to within 3% (see fig. below) • Event selection: SD station with the largest signal must be surrounded by live stations and the reconstructed zenith angle must be smaller than 60 degrees. • Energy estimator for the SD is the expected signal at 1000 m from the core (S1000) corrected for shower attenuation effects by the Constant Intensity Cut (CIC) method (S38). • Calibration of S38is based on hybrid events in which the energy is determined by the FD (more on this later on) Fermilab Director's Review

  5. SD Energy Calibration by the FD systematic uncertainty 7% (15%) at 10EeV (100 EeV) energy scale uncertainty: 22% (Fluorescence Yield 14%) Fermilab Director's Review

  6. From S1000 to S38 • For a given energy, the value of S1000 decreases with the zenith angle because of the attenuation of the shower and geometrical effects. We measure the attenuation curve by use of the Constant Intensity Cut method (CIC), an example of an attenuation curve is shown below (38 degrees is the median): Fermilab Director's Review

  7. Energy Calibration continued • Uses golden hybrid events: events for which both SD and FD provide independent energy estimators, S38 and EFD, respectively. • The relation between S38 and EFD is well described by a single power-law function: EFD = A S38B with A= (1.68+/- 0.05)x1017eV B= 1.035+/- 0.009 Fermilab Director's Review

  8. SD Energy Resolution • Obtained from the distribution of the ratio ESD/EFD(with EFD resolution at 7.6%) • For ESD > 10 EeVsE/ESD= (12+/- 1.0)% while at 30EeV it is 16% • FD energy scale uncertainty: 22% (dominated by fluorescence yield at 14%) • One final and important remark: The influence of the bin-to-bin migrations on the reconstruction of the flux due to the energy resolution is corrected by applying a forward folding algorithm. • Fermilab Auger group is working on this. Fermilab Director's Review

  9. The Spectrum • Combined: SD + Hybrid Fermilab Director's Review

  10. The energy spectrum may tell us about the local source distribution • From Taylor et al (astro-ph 1107.2055) Fe only Fermilab Director's Review

  11. Summary • The energy spectrum has been measured in two independent ways with a common systematic uncertainty in the energy scale (22%). • Two outstanding features are seen, the so-called ankle at around 4 EeV and a strong flux suppression above 40 EeV. Fermilab Director's Review

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