The Telescope Array observatory in Utah, US, is set for a $6.4m upgrade that will see some 400 detectors added to the facility. This will quadruple its collecting area from 730 km2 to 2500 km2. Increasing the size of the northern hemisphere's largest ultra-high-energy cosmic-ray detector will allow astronomers to learn more about the origins of the most energetic particles in the universe.

When a cosmic ray hits the Earth's atmosphere, it produces a cascade of secondary particles. The Telescope Array currently has 507 scintillation detectors, which generate light in response to incident radiation. By detecting the cascade of particles, astronomers can then obtain information on the direction and energy of the original ray.

Scintillating science

Many astrophysicists believe that ultra-high-energy cosmic rays are just protons, although some argue that they may include helium and nitrogen nuclei. Possible sources for the highest energy rays include active galactic nuclei, supernova remnants and colliding galaxies. The Telescope Array, which started collecting data in 2008, is able to observe cosmic rays with energies greater than 1 × 1018 eV.

The observatory involves institutions from Belgium, Japan, Russia, South Korea and the US. Japan has announced it will provide ¥450m ($4.6m) to fund the majority of the expansion, with researchers seeking to find the remaining $1.8m to complete the upgrade. One possible source could be the National Science Foundation in the US. The new detectors will be built over the next three years.

"These experiments are very large because the flux of cosmic rays at the highest energies is very low, about two per square kilometre per century," says Gordon Thomson, co-principal investigator for the Telescope Array and an astrophysicist at the University of Utah.

Cosmic hotspot

The Telescope Array has previously identified a possible "hotspot" of ultra-high-energy cosmic rays centred on the constellation Ursa Major. The observatory detects around 15 events per year, with a quarter in the hotspot – although this could be a statistical fluctuation. "If cosmic rays were [uniform], we would expect 0.9 events per year in the hotspot area of the sky, whereas we see about 3.5 events per year on average," says Thomson. To confirm and study the hotspot, researchers need to collect more data. "With the fourfold increase in data rate from a four-times-larger detector, we expect to answer interesting questions about the origin of cosmic rays," he adds.

The expansion, when complete, will make the Telescope Array similar in size to the Pierre Auger Observatory in Argentina, which is currently the world's largest cosmic-ray detector with a 3000 km2 collecting area. "Having observatories of a similar size in both the northern and southern hemispheres will enable full-sky surveys of ultra-high-energy cosmic rays," says Karl-Heinz Kampert, a particle physicist at the University of Wuppertal in Germany who is co-spokesperson for the Pierre Auger Observatory. "The Telescope Array suffers a lot from the relatively low rate of events set by the present area, so expanding the detector array is a natural step – this should strengthen indications of a hotspot seen in the northern sky."