1. Auger at 10 18 eV Bruce Dawson University of Adelaide, Australia

2. Introduction Auger was primarily designed for energies beyond 10 19 eV but significant aperture at lower energies interesting physics around 10 18 eV existing observatory is making measurements, and some longer-term plans for enhancements to study in more detail sorry, no new results!

3. The Observatory Surface Array 1600 detector stations 1.5 km spacing 3000 km 2 Fluorescence Detectors 4 Telescope enclosures 6 Telescopes per enclosure 24 Telescopes total ~2/3 tanks in field, completed in early 2007. Routine data taking since Jan. 2004.

4. The Surface Array Detector Station Water Cherenkov detector, depth 1.2m, area 10 m 2

5. Fluorescence Detector (24 like this) 3.4 metre diameter segmented mirror Aperture stop and optical filter 440 pixel camera

6. Physics Motivation around 10 18 eV knee region increasingly well studied e.g. KASCADE results imply rigidity-dependent escape from galactic sources (SNRs?) or galaxy itself. at highest energies, no galactic anisotropy is evident, and search continues for a definite extragalactic signature (e.g. GZK suppression) region below and around 10 18 eV may be the transition between galactic and extragalactic sources. (from T. Gaisser in Hillas astro-ph/0607109)

7. Some possible models (Hillas astro-ph/0607109) Hatched region: possible galactic fall-off if KASCADE rigidity dependent turn-overs extend to the Si-Fe mass components. Many extragalactic scenarios: depends on source spectrum, source composition, evolution of sources, distribution of sources etc etc 3 possible extragalactic models: EGAL p J source =kE -2.4 exp(-E/E max ) (similar to Berezinsky et al.), EGAL H+He (Hillas JPhysG 2005), EGAL mixed (Allard et al. 2005). suppression e + e - on CMBR?

8. Some possible models (Hillas astro-ph/0607109) Transition region (galactic to extragalactic) depends on model. In some models (eg EGAL p or EGAL p+He) there would be rapid change in mass composition below and around 10 18 eV. e.g. EGAL p model: Extragalactic flux 50% at 10 17.3 eV, 80% at 10 17.7 eV

9. Some evidence for transitions, but energy of transition not clear

10. “Low” energy capabilities of Auger Fluorescence detector (FD), surface detector (SD) and hybrid apertures detector resolution (directions, energy…) physics topics

11. FD-only apertures for completed Auger. Trigger aperture. Significant aperture at lower energies. Mono aperture: Trigger Geometry fit Profile fit (X max viewed) Bellido et al. (Auger Collab) 29th ICRC HE15 (2005)

12. Two types of tank trigger: “threshold” or “time over threshold” (TOT) (100Hz, noisy) (1.6 Hz, very clean) “Threshold trigger”: 3-fold coincidence of signal above 1.75 V.E.M. (needed for fast signals from horizontal showers). TOT trigger is most relevant to low energy aperture: - requires 13 bins in a 120 bin (3us) window above threshold of 0.2 V.E.M. in 2 PMTs E ~ 5 EeV Each tank has 3 PMTs (summed here) 40 MHz (25ns) digitization. SD Trigger

13. TOT (pink) dominates over the threshold (blue) trigger Protons,  =25 deg log E= Protons, log E = 17.8 To calculate SD aperture- Lateral Trigger Probability (LTP) functions - for TOT trigger Allard et al. (Auger Collab) 29th ICRC HE14 (2005)

14. Allard et al. (Auger Collab) 29th ICRC HE14 (2005) 3 tanks with TOT = 3TOT (fully efficient at ~ 3 EeV) 4 tanks with TOT = 4TOT (fully efficient at ~ 7 EeV) mass-dependent aperture in the dip region - maybe we can exploit this real data from 0.4 - 0.9 EeV signal rise-time - potential for mass composition study here P. Ghia, 28th ICRC p337 (2003)

15. FD Aperture…SD Aperture…Hybrid aperture: Hybrid aperture is determined by FD since SD single-tank TOT trigger has low threshold. (Observatory trigger (T3) for hybrids is initiated by the FD and tank triggers are collected) (aperture as of October, 2004) Bellido et al. (Auger Collab) 29th ICRC HE15 (2005) single-tank TOT

16. ~3 EeV Dawson et al. (Auger Collab) 27th ICRC, 714 (2001) Hybrid Resolution (statistical errors only)

17. SD Angular resolution From time-variance model (using data from dual stations). Allows calculation of space angle error for each event. “Angular resolution” is the space angle containing 68% of the 3D probability distribution. Roughly, 3 stations: E < 4 EeV 4 stations: 3 < E < 10 EeV 5 or more: E > 8 EeV Bonifazi et al. (Auger Collab) 29th ICRC HE14 (2005)

18. From hybrid/SD comparisons on same showers. To get SD resolution subtract hybrid resolution (0.4 deg) in quadrature. Angular resolution =1.5 sigma Results approx. consistent with time variance method. Thus, SD angular resolution is better than 2.2 deg for 3 stations (E 8 EeV) SD Angular resolution from hybrid Bonifazi et al. (Auger Collab) 29th ICRC HE14 (2005)

19. Physics studies so far: search for anisotropies in arrival directions towards the Galactic Centre around 10 18 eV

20. AGASA experiment Hayashida et al. 1999 re-analysis of SUGAR experiment Bellido et al. 2001 GC Hints from earlier experiments …

21. no statistically significant excesses or deficits - with 4x more data than AGASA Letessier-Selvon et al. (Auger Collab) 29th ICRC HE14 (2005)

22. Energy spectrum - extension to lower energies anticipated (hybrid) but priority on the high energies! 1) M. Takeda et al. Astroparticle Physics 19, 447 (2003) 2) R.U. Abbasi et al. Phys Lett B (to be published) Auger energy calibration provided by fluorescence detectors Sommers et al. (Auger Collab) 29th ICRC HE14 (2005)

23. (Michael Unger, Karlsruhe) Auger telescopes view elevations up to 30 degrees. Care must be taken to avoid biases in X max studies, particularly for low energy (close-by) events. (solution - geometrical cuts to avoid FOV problem - core distance cuts to avoid SD bias at low E) Hybrid Mass Composition Studies First hybrid elongation rate (X max ) study in final stages. Expect measurements down to around 10 17.8 eV or lower

24. An example of a possible enhancement for Auger South - at one FD site, 3 extra telescopes A proposal by European Auger colleagues, under active study. Aim is to enhance useful low energy aperture.

25. 10 17.25 eV, 1.2km10 18 eV, 5km

26. Conclusions Auger was designed for E>10 19 eV, and this is the main focus of our endeavour But some lower-energy aperture came for free, and we are exploiting it some low energy enhancements possible, but only when construction is complete on the main observatory

28. (Hillas astro-ph/0607109)

29. FD aperture with energy