FNWI --- IMAPP Department of Astrophysics
Radboud University > Faculty of Science > Department of Astrophysics

Pierre Auger Observatory

The Pierre Auger Observatory, the world's largest detector for cosmic rays. It has been completed in 2008 in the province of Mendoza in Argentina and has been sucessfully taken data with the highest statistics. The cosmic rays detected are the highest-energy particles known in the Universe. They reach energies of 10^20 eV, which is roughly the energy that sits in a flying golf-ball but it is carried by something that is smaller than a dust grain.

It is still unknown what astronomical object these particles orginate from and what fraction of an atom they are actually made of. So far these particles can only be detected by their reaction with the Earth's atmosphere, called extensive air shower, which makes detection of their nature more challenging.

As a group that combines expertise in Astronomy and Astroparticle Physics, the group in Nijmegen is able to combine the data from the Observatory in a multi-messenger approach with data from other astronomical observations. This gives the opportunity to learn more about the astronomical orginin of ultra-high energy cosmic rays and the mechanisms in the sources.

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Two techniques of detection are combined at the Pierre Auger Observatory, allowing a “hybrid” measurement of extensive air showers. The secondary particles of the interaction in the atmosphere that reach ground level are registered in water Cerenkov detectors. These detectors are spaced in a grid of 3000 km^2 with distances of 1.5 km to each other. They therefore enable to measure a so called footprint of the shower on the ground. Furthermore, telescopes overlooking the detector array detect the fluorescence light emitted from the air molecules that interact with the air showers. This results in an image of the shower developing and provides a lot in depth information about the nature of the cosmic rays. While the detector array can take data all the time, the telecopes are limited to good observing conditions, which limits the number of air showers that can be detected. As these ultra-high energy cosmic rays are very rare events, a high observation duty-cycle is essential.

Auger Engineering Radio Array


In parallel to the established techniques, a new technique is explored to register air showers: the detection of radio signals from air showers. The radiation is emitted by electrons and positrons in the air shower that interact with the Earth's magnetic field and emitt radiation. Observing this radiation might provide information about the shower development, which is crucial to understand the nature of the cosmic ray, and will have a longer observation window than the current fluorescence telescopes. This relatively new technique has never been explored in a large experiment. Testing it together with the established methods at the Pierre Auger Observatory will ensure that the results can be compared and optimised. The three detection methods are shown together in the image above. The radio antennas and water detectors are seen in the foreground, while the telescope is in the background on the hill overlooking the array.


In 2010 the first stage of the Auger Engineering Radio Array (AERA) was deployed in Argentina and has been measuring coincidences with the other detectors since April 2011. The antenna shown above is part of AERA. This detector is sensitive to radio signals from 30 to 80 MHz. In the first stage AERA consits of 21 stations with distance of 150 m to each other. It is planned that more than a 100 stations will follow.

Members of this insitute are strongly involved in hardware development for AERA as well as data analysis. With also taking an active role in LOFAR, the group in Nijmegen will be able to act as an interface between the two efforts in order to improve the possibilities of radio detection of cosmic rays.

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