Design and status of the Pierre Auger Observatory

J. C. Arteaga Velázquez1 , Rebeca López2, R. Pelayo1 and Arnulfo Zepeda1

1Departamento de Física, Cinvestav, Apartado Postal 14-740, 07000, México, D.F., México .2Benemérita Universidad Autónoma de Puebla, 72570, Puebla, Puebla.

Abstract

The main goal of the Pierre Auger Observatory is to solve the mystery of the ultrahigh energy cosmic rays, particles of the highest energy (E 1018 eV) from which, their origin, composition and acceleration mechanism remain still unknown. In this way, the Auger Observatory will open a new window to the highest energetic phenomena in the universe and, in addition, it will contribute to a better knowledge of the fundamental laws governing the microcosmos. In this work, the design and the status of the observatory, as well as its detection techniques, are briefly described.

Introduction

What are the ultra high energy cosmic rays (UHECR)?

They are very energetic particles ( E 1018 eV) arriving to the Earth from the outer space.

They energy is several orders of magnitude greater than that provided on Earth by our most powerfull particle accelerators. Therefore, they keep valuable information from the most energetic regions in the Universe.

The mystery

The most intriguing aspect of the UHECR has to do with the production and propagation of those with energies above 1020 eV:

Their arrival to Earth implies that their sources are within a distance of 50 Mpc, which is the mean free path for protons with E > 5 X 1019 eV interacting with the 2.7 K Cosmic Background Radiation. However, no sufficiently energetic astrophysical environments have been found yet within the requisite distance.

Our knowledge about the UHECR can be summarized in the following chart:

Origin: Unknown. May be very high energy astrophysical objects (like AGN's, GRB's, clusters of galaxies, etc.) or decays and/or annihilation of very high mass relics created in the early universe ( such as magnetic monopoles, superheavy particles, etc.). No astronomical sources have been detected from the arrival directions of the measured cosmic rays with E > 1020 eV.

Energy spectrum: Not well known. The data from two different experiments (AGASA and HiRes), are in conflict between them. They report distinct results about this issue.

Composition: Unknown. Atomic nuclei? Photons? Neutrinos? It is strongly related with the nature of the source.

At present, the lack of statistics, which is the result of the low flux of UHECR above 1020eV (~ 1 particle/( km2 century) and the relative small size of the instruments employed in these studies, has prevent any conclusion about these issues.

The design of the Pierre Auger Observatory (PAO).

To shed light on the mystery of the UHECR, it is necessary to have a detector of kilometric size, such as the Pierre Auger Observatory (PAO). An international collaboration from about 13 countries, including Mexico, are participating now in this project.

The main features of the observatory are briefly described below:

Goal: to collect several UHECR events with unprecedent

statistics in order to know their spectrum, arrival directions and composition.

Location: to allow a full sky coverage, it will consist of two similar instruments located in both hemispheres. In the southern hemisphere, the PAO is being deployed in Malargüe, Mendoza Province, Argentina.

Size: To compensate the low flux of UHECR's, its total size will be of the order of 6000 km2.

Period of life: 20 years, for each observatory.

Detection techniques: It will be of hybrid type, that is, each observatory will use two complementary techniques based on the detection of the secondary air shower generated by the interaction of the UHECR with the atmosphere:

Detector Description Duty cycle

Detection technique

Surface Detector (SD)

Array of 1600 water Cherenkov detectors separated by 1.5 km from each other and spread over an area of 3000 km2.

100 % The SD will sample the lateral distribution of muons, electrons and photons at ground level.

Fluorescence detectors (FD)

Set of 4 FD´s detectors overlooking the SD array. Each FD composed by 6 telescopes. In a telescope 440 photomultipliers tubes will detect the air fluorescence light

10 %(only dark nights)

The FD will observe the longitudinal development of the air shower by detecting the fluorescence emission from the atmospheric N2

excited by the charged particles in the shower

Threshold energy: E 1018 eV. In particular, the detector will achieve its full efficiency at E 1019.5 eV.

Energy and angular resolution:. The FD and the SD will complement each other, allowing a better resolution than that allowed by one technique alone:Ecr 1019 eV

SD Hybrid

1020 eV

SD Hybrid

E/E

2.0o 0.35o

30% 20% 1.0o 0.2o

15% 10%

Research: Astronomy, astrophysics and particle physics with UHECR. The observatory will also have the capability to detect extraterrestrial neutrinos with high energy.

Status of the PAO.

The construction of the southern Pierre Auger Observatory began inMarch 1999 and its first stage was completed with the deployment,during 2001, of a prototype detector called the EngineeringArray (EA). The main characteristics of the EA are the following:

The Engineering Array (EA)

Description: The EA is subset detector of the PAO composed

by 40 water Cherenkov detectors, and two prototype fluorescence telescopes, the latter ones overlooking the Cherenkov detectors from Los Leones hill.

Size: The surface detectors cover an area of 46 km2.

Goal: This prototype was constructed to help the Pierre

Auger Collaboration to test several aspects of the observatory and improve them. For example:

• Performance of the initial design.• Deployment strategy. • Communication and data acquisition systems. • Components and subsystems working individually and

in integrated operation, etc.

Threshold energy: E 1018 eV.

The project is now in the pre-production phase, in which, 2 fluorescence buildings and 100 final design stations are expected to Be deployed and tested, checking, in this way, the performance of the final design. After this, the project will move into the full- production phase, under which the complete FD and SD will befinished. About the status of the observatory, we will make the followingcomments:

The EA has been observing UHECR events since July 2001 and now is an integral part of the PAO.

In hybrid mode, it has detected more than 70 events with energy 1018 eV since 2001.

Data collected has been very important for checking both the performance of the EA and the methods of data analysis.

At this stage, the statistics is not enough yet to make relevant physical conclusions about the UHECR.

At present pre-production parts of the detectors are arriving at the southern site and are being deployed. Up to now, 46 final design pre-production tanks have been added to the EA.

The two prototype telescopes of the EA, have been dismantled to allow installation of the final design telescopes (the full installation installation of 12 telescopes at Los Leones and Cohiueco buildings are expected to be finished by the end of 2003).

The focus now is production, transportation and deployment of quality components and the refinement of procedures for their installation in the southern site.

The southern site is expected to be completed by the year of 2005.

The full planned Southern PAO (left) and the EA (right).

Gamma Ray Bursts (GRB) are possible sources of UHECR (above GRB 971214).

The Active Galactic nuclei have favorable conditions for UHECR acceleration (above NGC 4261).