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A Window onto the Universe

In a remote plain near Malargüe in the Mendoza province of Argentina, local cow herds are sharing the pampa with a peculiar assortment of water tanks. These 1600 surface detector pools and a nearby array of telescopes belong to the international Pierre Auger Observatory,(1) the largest cosmic ray observatory in the world.

Cosmic rays are incredibly energetic particles (usually protons or heavy nuclei) that rain down on the earth from space at nearly the speed of light. When they strike the earth’s atmosphere, they produce air showers made of billions of secondary particles. Among these are some of the most energetic particles ever observed in nature, packing a hundred million times more energy than the particles produced in the world’s most powerful particle accelerator.
Since French physicist Pierre Auger spotted the first air shower in 1938, much progress has been made in understanding particles with low to moderate energies. However the few with extremely high energies (around 1020 eV), remain baffling. What are they made of? Where do they come from? What cosmic powerhouse is spitting them across the universe at such high speed? So far, no one knows. But recent data from the Auger Observatory is opening up new pathways toward the answer.
Last November, a collaboration of scientists from 17 countries announced that active galaxies outside of the Milky Way were likely sources for the mysterious rays.2 Their data reveals that the highest-energy particles do not reach earth from directions uniformly distributed across the sky. Instead, their arrival directions correlate with the positions of nearby Active Galactic Nuclei (AGN). AGNs are thought to be powered by massive black holes. Jets and shock waves surrounding these black holes could speed up the cosmic rays, thus explaining their high energies. The exact mechanism of how AGNs can accelerate particles, however, is still unknown.

capteur

© Pierre Auger Observatory

The Los Leones FD together with its closest SD tank.




Auger physicists derived their results by measuring each cosmic ray indirectly via the shower of particles it produces in the air. A “hybrid detector,” the Auger Observatory uses two independent methods to detect them. The first uses water placed in 1600 surface detector tanks spread across a section of the pampa roughly the size of Luxembourg. Each 12,000-liter tank is completely dark inside, but ignites when charged particles from a cosmic ray shower pass through it. At high speed, their electromagnetic shock waves produce Cherenkov light that can be measured by photomultiplier tubes mounted on the tanks. Slight differences in the detection times at different tank locations help scientists determine the trajectory of the incoming cosmic ray.
The second technique tracks air showers by observing the faint glow produced when particles collide with air molecules in the atmosphere. On moonless nights, finely tuned light sensors can measure this fluorescence and determine the cosmic ray’s direction and composition.
While the fluorescence detectors can measure cosmic ray showers in more detail than the water tank system, they can only do so on dark nights. By combining the two methods over a very large area, the observatory achieves unprecedented collecting power. In addition, comparing results from the different types of detectors helps scientists reconcile the two sets of data and produce more accurate results.
First proposed by Jim Cronin (Nobel Prize winner) and Alan Watson in 1992, the observatory was built by more than 370 physicists and engineers from more than 70 institutions around the world. It was officially inaugurated in November 2005. The 17 participating countries shared the €40 million cost. In France, CNRS was the primary financing organism and many French researchers are actively involved in the project,3 notably Pierre Billoir and Antoine Letessier-Selvon, from the LPNHE in Paris.4
With spectacular results obtained so shortly after the opening of the observatory, the subsequent phases of the vast international project are expected to quickly click into place. Next on the list, a sister site is to be built in Southern Colorado to widen even further the observatory’s coverage of the skies.
Lucille Hagège

Notes :

1. www.auger.org
2. Pierre Auger Collaboration, “Correlation of the highest-energy cosmic rays with nearby extragalactic objects.” Science. 318: 938-43. 2007.
3. Institut de Physique Nucléaire d'Orsay (CNRS / Université Paris Sud-11); Laboratoire Astroparticule et Cosmologie (CNRS / Université Paris 7 / CEA / Observatoire de Paris); Laboratoire de l'Accélérateur Linéaire (CNRS / Université Paris Sud-11); Laboratoire de Physique Nucléaire et des Hautes Énergies (CNRS / Universités Paris 6 et 7); Laboratoire de Physique Subatomique et de Cosmologie (CNRS / Université Grenoble 1 / Institut Polytechnique de Grenoble); Observatoire des Sciences de l'Univers de Besançon (CNRS/ Université de Besançon)
4. Laboratoire de physique nucléaire et des hautes énergies (CNRS / Universités Paris 6 and 7).

Contacts :

Antoine Letessier-Selvon
IN2P3, Ivry-sur-seine.
Antoine.letessier-selvon@in2p3.fr


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