Report card. Researchers have begun rating potential sites for the Cherenkov Telescope Array (artist’s conception, above), which will have observatories in the Northern and Southern hemispheres.


Namibia Gets Top Grade as Base for Major Gamma Ray Telescope

A patch of bushy land in southern Namibia has been singled out as the best candidate to host a major part of the world’s largest gamma ray telescope. Scientists meeting in Warsaw last week ranked the Namibian site as the best of five options for the southern array of the Cherenkov Telescope Array (CTA), which will be comprised of two observatories, one on each side of the equator. Meanwhile, four sites competing for the CTA’s northern array all earned equal ratings.

Cosmic gamma rays are believed to be produced by violent astrophysical events such as supernovas. They can't be observed directly from Earth because they are blocked by the atmosphere, but Cherenkov telescopes spot them by detecting the flash of light caused by their collision with atoms in the upper atmosphere.

The CTA, expected to cost $270 million and be fully operational in 2019, would be 10 times as powerful as current Cherenkov instruments. It will focus on resolving two mysteries: the origins of cosmic rays, and the nature of the dark matter that physicists believe constitutes 85% of all matter in the universe. The 120-telescope project will have two parts: a southern array with 100 instruments distributed over 10 square kilometers and a northern array with 20 instruments spread over 1 square kilometer. Six nations are bidding to host the arrays, with the United States, Mexico, and Spain competing for the northern site, and Argentina, Chile, and Namibia for the southern.

In Warsaw, scientists representing the 27-country CTA Consortium met to analyze and rank nine candidate sites. The group considered a wide range of issues, including data on weather, magnetic fields, and accessibility. The goal was “to quantify the sites [based] on their scientific potential and on … site risks or costs,” says Rene Ong, an astrophysicist at the University of California, Los Angeles, and a member of the U.S. site-bid team.

For the southern observatory, Namibia’s Aar site received the highest score. It sits on a privately owned farm more than 1600 meters above sea level in southern Namibia, some 120 kilometers west of the city of Luderitz. Namibia already hosts the world’s largest Cherenkov telescope array, known as the High Energy Stereoscopic System (H.E.S.S.).

A site in Chile and the H.E.S.S. site in Namibia finished second. Two sites in Argentina (El Leoncito and San Antonio de los Cobres) were third and fourth, respectively.

For the northern array, Spain’s Teide site was considered the front-runner prior to the meeting. But all four candidates, including two sites in Arizona and one at Mexico’s San Pedro Mártir observatory, ended up with equal scores.

Mexico’s candidate, however, may have to be eliminated. Temperature data suggest it gets too cold on some nights for CTA instruments, but scientists are now evaluating whether the instruments might be able to withstand colder temperatures.

The Warsaw meeting isn’t the last word on the issue. The rankings will be forwarded to a 15-nation funding panel that will make the final siting choice. A decision is expected by the end of December, with final approval for the project expected by the end of 2014. First science could start in 2017, with a goal of having the CTA fully operational 2 years later.