Auger at 1018eV

Bruce DawsonUniversity of Adelaide,

Australia

Introduction

• Auger was primarily designed for energies beyond 1019eV

• but significant aperture at lower energies

• interesting physics around 1018eV• existing observatory is making measurements, and

some longer-term plans for enhancements to study in more detail

• sorry, no new results!

The Observatory

Surface Array 1600 detector stations 1.5 km spacing 3000 km2

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.

The Surface ArrayDetector Station

Water Cherenkov detector, depth 1.2m, area 10 m2

Fluorescence Detector (24 like this)

3.4 metre diameter segmented mirror Aperture stop

and optical filter440 pixel camera

Physics Motivation around 1018eV• 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 1018eV may be the transition between galactic and extragalactic sources.

(from T. Gaisser in Hillas astro-ph/0607109)

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 Jsource=kE-2.4 exp(-E/Emax) (similar to Berezinsky et al.), EGAL H+He (Hillas JPhysG 2005), EGAL mixed (Allard et al. 2005).

suppressione+e- on CMBR?

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 1018eV.

e.g. EGAL p model: Extragalactic flux 50% at 1017.3eV, 80% at 1017.7eV

Some evidence for transitions, but energy of transition not clear

“Low” energy capabilities of Auger

• Fluorescence detector (FD), surface detector (SD) and hybrid apertures

• detector resolution (directions, energy…)

• physics topics

FD-only apertures for completed Auger.

Trigger aperture.

Significant aperture at lower energies.

Mono aperture:

TriggerGeometry fit

Profile fit (Xmax viewed)

Bellido et al. (Auger Collab) 29th ICRC HE15 (2005)

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 tankhas 3 PMTs(summed here)

40 MHz (25ns)digitization.

SD Trigger

TOT (pink) dominates over thethreshold (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)

Allard et al. (Auger Collab) 29th ICRC HE14 (2005)

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)

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

~3 EeV

Dawson et al. (Auger Collab) 27th ICRC, 714 (2001)

Hybrid Resolution

(statistical errors only)

SD Angular resolution

From time-variancemodel (using data fromdual stations). Allowscalculation of space angleerror for each event.

“Angular resolution” isthe space angle containing68% of the 3D probabilitydistribution.

Roughly,3 stations: E < 4 EeV4 stations: 3 < E < 10 EeV5 or more: E > 8 EeV

Bonifazi et al. (Auger Collab) 29th ICRC HE14 (2005)

From hybrid/SDcomparisons onsame showers.

To get SD resolutionsubtract hybrid resolution (0.4 deg)in quadrature.

Angular resolution=1.5 sigma

Results approx.consistent with timevariance method.

Thus, SD angular resolution is better than 2.2 deg for 3 stations (E< 4 EeV)(and better than 1.7 deg for 4-folds (3-10 EeV), better than 1.4 deg for >8 EeV)

SD Angular resolution from hybridBonifazi et al. (Auger Collab) 29th ICRC HE14 (2005)

Physics studies so far: search for anisotropies in arrival directions towards the Galactic Centre around 1018eV

AGASA experimentHayashida et al. 1999

re-analysis of SUGAR experiment

Bellido et al. 2001 GC

Hints from earlier experiments …

no statistically significant excesses or deficits - with 4x more data than AGASA

Letessier-Selvon et al. (Auger Collab) 29th ICRC HE14 (2005)

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)

(Michael Unger, Karlsruhe)

Auger telescopes view elevations up to 30 degrees.Care must be taken to avoid biases in Xmax 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 elongationrate (Xmax) study infinal stages.

Expect measurementsdown to around 1017.8 eVor lower

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.

1017.25 eV, 1.2km 1018 eV, 5km

Conclusions

• Auger was designed for E>1019eV, 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

(Hillas astro-ph/0607109)

FD aperture with energy