1. Energy Spectrum C. O. Escobar Pierre Auger Director’s Review December 2011 12/15/2011Fermilab Director's Review1

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. 12/15/2011Fermilab Director's Review2

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 12/15/2011Fermilab Director's Review3

4. SD Spectrum 12/15/2011Fermilab Director's Review4

5. SD Energy Calibration by the FD systematic uncertainty 7% (15%) at 10EeV (100 EeV) energy scale uncertainty: 22% (Fluorescence Yield 14%) 12/15/2011Fermilab Director's Review5

6. From S 1000 to S 38 For a given energy, the value of S 1000 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): 12/15/2011Fermilab Director's Review6

7. Energy Calibration continued Uses golden hybrid events: events for which both SD and FD provide independent energy estimators, S 38 and E FD, respectively. The relation between S 38 and E FD is well described by a single power-law function: E FD = A S 38 B with A= (1.68+/- 0.05)x10 17 eV B= 1.035+/- 0.009 12/15/2011Fermilab Director's Review7

8. SD Energy Resolution Obtained from the distribution of the ratio E SD /E FD (with E FD resolution at 7.6%) For E SD > 10 EeV  E /E SD = (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. 12/15/2011 Fermilab Director's Review 8

9. The Spectrum Combined: SD + Hybrid 12/15/2011Fermilab Director's Review9

10. The energy spectrum may tell us about the local source distribution From Taylor et al (astro-ph 1107.2055) Fe only 12/15/2011Fermilab Director's Review10

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. 12/15/2011Fermilab Director's Review11