The Pierre Auger Observatory takes a big step forward in solving the mystery of the highest-energy cosmic rays

Scientists of the Pierre Auger Observatory have demonstrated that these particles do not arrive uniformly distributed across the sky. Instead their arrival directions turn out to be correlated with a set of Active Galactic Nuclei (AGNs), that is, supermassive black holes which are devouring large amounts of matter from their host galaxies spewing out as high energy radiation.

The result of the Auger Observatory suggests that these AGNs are also related with these ultra-high energy cosmic rays detected on Earth.These findings have recently been published in Science the last 9th of November (vol. 318).

Up to now Astronomy has been based in the detection of photons (light, radio waves, X and gamma rays,..) emitted by cosmic objects, since only neutral particles arrive to us without deflecting in the cosmic magnetic fields. However, atomic nuclei do have electric charge and therefore they suffer large deviations before arriving to Earth. As a consequence no relationship between cosmic rays and their sources had been found, apart of that of low energy cosmic rays from the Sun.

Theoretical predictions state that cosmic rays with energy over about 60 EeV (1 EeV = 1018 electron-volts) are stopped by the microwave radiation, remnant of the Big Bang. Nevertheless cosmic rays coming from nearby sources can keep their energy. The results of the Auger Observatory confirm these predictions since cosmic rays detected with energies over that value are correlated with AGNs located a few hundred light years from us, relatively close from a cosmological point of view.

The discovery of the Pierre Auger Observatory opens a new window to the nearby universe: the cosmic ray astronomy. In addition these particles have energies a million times larger that those presently achieved in large accelerators on Earth providing a fundamental tool for the study of the physical processes of very high energy.

Unfortunately these cosmic rays arrive to Earth very infrequently (around one per square kilometre and century) and therefore huge detectors are necessary. A cosmic ray of high energy entering the atmosphere generates a shower of particles which reach the ground. The Pierre Auger Observatory is provided with two types of detectors to register these particles: a surface detector consisting of 1600 particle counters spread on a 3000 km2 surface and a set of telescopes which detect the fluorescence light produced by the shower particles in the atmosphere. The combined information gathered by both detectors provides unprecedented accuracy on the arrival direction, energy and nature of the cosmic ray.

The Pierre Auger Observatory is an international consortium of 17 countries in which no one contributes more than 20% of the total cost (around USD$ 54 millions). The Observatory is named for the French physicist who for the first time detected a high energy cosmic ray at ground using a new technique which is the base of our present detectors.

The Observatory is located in Argentina near Malargüe in the province of Mendoza. It is leaded by prestigious scientists like J. Cronin from the University of Chicago (Nobel Prize in Physics) and Alan Watson from the University of Leeds. The present Observatory of the South Hemisphere will be complemented by a Northern Observatory in the Colorado State (USA).

It is worth noting that this discovery has been carried out even before completion of the South Observatory in Argentina. In the next years a much larger amount of data will be available covering the whole sky. We believe that the present finding will be followed by others which finally will lead us to solve the mystery of the physical processes which accelerates particles to such huge energies.

Spain is a member of the Pierre Auger collaboration since 2002 when the group of the Universidad de Santiago joined the collaboration. Presently, four Spanish Institutions are full members with a representative in the Collaboration Board of the Observatory: Universidad Complutense de Madrid, Universidad de Alcalá, Universidad de Granada and Universidad de Santiago de Compostela.

Source: Prof. Fernando Arqueros (UCM).