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Ambitious gamma-ray telescope takes shape

I wrote a shortened version of this piece for The Hindu on July 4, 2013. This is the longer version, with some more details thrown in.

Scientists and engineers from 27 countries including India are pitching for a next-generation gamma-ray telescope that could transform the future of high-energy astrophysics.

Called the Cherenkov Telescope Array (CTA), the proposed project is a large array of telescopes to complement existing observatories, the most potent of which are in orbit around Earth. By building it on land, scientists feel the CTA could be much more sophisticated than orbiting observatories, which are limited by logistical constraints.

Werner Hofmann, CTA spokesperson of the Max Planck Institute for Nuclear Physics, Germany, told Nature, a comparable orbiting telescope would have to be “the size of a football stadium”.

The CTA’s preliminary designs reveal that it boasts of greater angular resolution, and 10 times more sensitivity and energy-coverage, than existing telescopes. The collaboration will finalise the locations for setting up the CTA, which will consist of two networked arrays in the northern and southern hemispheres, by end-2013. Construction is slated for 2015 at a cost of $268 million.

One proposed northern hemisphere location is in Ladakh, Jammu and Kashmir.

Indian CTA collaboration

Dr. Pratik Majumdar, Saha Institute of Nuclear Physics (SINP), Kolkata, said via email, “A survey was undertaken in the late 1980s. Hanle, in Ladakh, was a good site fulfilling most of our needs: very clear and dark skies throughout the year, with a large number of photometric and spectroscopic nights at par with other similar places in the world, like La Palma in Canary Islands and Arizona desert, USA.”

However, it serves to note that the Indian government does not permit foreign nationals to visit Hanle. “I do think India needs to be more proactive about opening up to people from abroad, especially in science and technology, in order to benefit from international collaboration – unfortunately this is not happening,”said Dr. Subir Sarkar, Rudolf Peierls Centre for Theoretical Physics, Oxford University, via email. Dr. Sarkar is a member of the collaboration.

Each network will consist of four 23-metre telescopes to image weaker gamma-ray signals, and dozens of 12-metre and 2-4-metre telescopes to image the really strong ones. Altogether, they will cover an area of 10 sq. km on ground.

Scientists from SINP are also conducting simulations to better understand the performance of CTA.

Led by it, the Indian collaboration comprises Indian Institute of Astrophysics, Bhabha Atomic Research Centre, and Tata Institute of Fundamental Research (TIFR). They will be responsible for building the calibration system with the Max Planck Institute, and developing structural sub-systems of various telescopes to be fabricated in India.

Dr. B.S. Acharya, TIFR, believes the CTA can add great value to existing telescopes in India, especially the HAGAR gamma-ray telescope array in Hanle. “It is a natural extension of our work on ground-based gamma-ray astronomy in India, since 1969,” he said in an email to this Correspondent.

Larger, more powerful

While existing telescopes, like MAGIC (Canary Islands) and VERITAS (Arizona), and the orbiting Fermi-LAT and Swift, are efficient up to the 100-GeV energy mark, the CTA will be able to reach up to 100,000 GeV with the same efficiency.

Gamma rays originate from sources like dark matter annihilation, dying stars and supermassive black holes, whose physics has been barely understood. Such sources accelerate protons and electrons to huge energies and these interact with ambient matter, radiation and magnetic fields to generate gamma rays, which then travel through space.

When such a high-energy gamma-ray hits atoms in Earth’s upper atmosphere, a shower of particles are produced that cascade downward. Individual telescopes pick these up for analysis, but a network of telescopes spread over a large area would collect greater amounts, tracking them back better to their sources.

Here, CTA’s large collection area will come to play.

“No telescope based at one point on Earth can see the whole sky. The proposed CTA southern observatory will be able to study the centre of the galaxy, while the northern observatory will focus on extragalactic sources,” said Dr. Sarkar.

Gamma-ray astronomy has seen global interest since the early 1950s, when astronomers began to believe some cosmic phenomena ought to emit the radiation. After developing the telescopes in the 1960s to analyse it, some 150 sources have been mapped. The CTA is expected to chart a 1,000 more.

The HESS II gamma-ray telescope in the Khoma Highland, Namibia, is currently the world's largest telescope for gamma-ray astrophysics, possessing a 28-meter wide mirror.
The HESS II gamma-ray telescope in the Khoma Highland, Namibia, is currently the world’s largest telescope for gamma-ray astrophysics, possessing a 28-meter wide mirror.

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