Pierre Auger Observatory

A big detector for rare particles

The highest-energy cosmic rays are so rare that only a very large detector can catch them, about one particle per square kilometre per century above 10^20 electronvolts. Jim Cronin and Alan Watson proposed the observatory in 1992, and it was named after Pierre Auger, who had discovered extensive air showers in 1938. Malargüe in Mendoza Province, Argentina, offered flat, high, dry land, and the site is a plain at about 1,400 metres.

Phase I data taking began in January 2004, while the observatory was still being built, and the 3,000 square kilometre array was inaugurated on 14 November 2008.

Two ways of seeing a shower

When a cosmic ray hits the atmosphere, it creates a shower of billions of particles. The Auger array has about 1,660 water-Cherenkov tanks, each holding 12 tonnes of water viewed by photomultipliers, spaced 1.5 kilometres apart, and 27 fluorescence telescopes at four sites that watch the faint ultraviolet light of the shower on moonless nights. The combination of two independent methods lets each calibrate the other.

In 2017 Auger reported in Science that the arrival directions of cosmic rays above 8 × 10^18 electronvolts have a large-scale anisotropy: more arrive from one side of the sky, in a direction away from the Galactic Centre, evidence that they come from outside the Milky Way.

AugerPrime and open data

Construction of the AugerPrime upgrade, which adds scintillators and radio antennas to each tank to tell the kinds of particles apart, began in 2019. The collaboration of about 400 scientists from 17 countries makes part of its data open, with a public catalogue of the highest-energy events.

A detector the size of a county

The observatory spans an area larger than the county of Greater London, in the Pampa Amarilla, and uses local people for maintenance and tours. Its stations are built from standard parts, such as plastic tanks and solar panels, and are replaced when they fail. The detector's first result, in 2007, was the correlation of the highest-energy events with nearby active galaxies, which later data weakened, and the group published the revision. That is an example of an observatory doing what the best ones do: reporting a hint, testing it with more data and correcting the record when it did not hold.

Instruments and technologies

• Water-Cherenkov detectors Sealed tanks of 12 tonnes of ultrapure water with three photomultiplier tubes, powered by solar panels and linked by radio.
• Fluorescence telescopes Wide-angle telescopes with a UV filter and cameras of photomultipliers that record the track of a shower through the air.
• AugerPrime surface scintillators and radio antennas Added on top of each tank to measure the muon content of a shower.

The people

• James Cronin Physicist, Nobel Prize 1980; co-founder of the observatory.
• Alan Watson Physicist and co-founder.

Timeline

• 1992 Proposed by Jim Cronin and Alan Watson.
• 2004 Phase I data taking begins in January, during construction.
• 2008 3,000 km² array inaugurated on 14 November.
• 2017 Science paper reports a large-scale anisotropy above 8 × 10^18 eV.
• 2019 AugerPrime upgrade construction begins.

Key lessons for observation today

• Two methods calibrate each other The hybrid design uses the fluorescence telescopes to calibrate the tanks. Redundant independent methods improve accuracy.
• Make a wide area observable at low unit cost Each tank is simple, solar-powered and wireless. A large network needs simple, repeatable nodes.
• Share a catalogue openly A public event catalogue lets outsiders check the result. Open data in a collaboration builds trust.

Sources

• Pierre Auger Collaboration (2017), Observation of a large-scale anisotropy..., arXiv:1709.07321
• Fermilab News, Scientists inaugurate world's largest cosmic-ray observatory (2008)
• Pierre Auger Collaboration, Catalog of highest-energy cosmic rays, Phase I, arXiv:2211.16020
• Wikipedia, Pierre Auger Observatory (secondary)