Observatory will permit studies of gamma rays with unprecedented precision

A consortium comprising scientists from 32 countries, including Brazil, plans to build the world's largest ground-based observatory for gamma-ray astronomy by 2022.

By Elton Alisson  |  Agência FAPESP – It will soon be possible to study gamma rays with unprecedented precision. A consortium comprising over 1,350 scientists and engineers from 32 countries, including Brazil, plans to build the Cherenkov Telescope Array (CTA), the world’s largest ground-based observatory for gamma-ray astronomy, by 2022. Gamma rays are the most energetic form of light and are produced in the hottest regions of the universe. After over a century of research, they are still little understood. The scientists will use the CTA to determine more about their sources and the roles they play in our galaxy and beyond.

Some of the researchers who belong to the CTA collaboration took part in the São Paulo School of Advanced Science on High-Energy and Plasma Astrophysics in the CTA Era (SPSAS-HighAstro) held on May 21-31 at the University of São Paulo’s Institute of Astronomy, Geophysics & Atmospheric Sciences (IAG-USP) in Brazil.

Supported by FAPESP through its SPSAS Program, the event was attended by 100 graduate students, half from São Paulo State and other parts of Brazil, and the other half from abroad. Here, the students received training in high-energy and plasma astrophysics to prepare them to use their skills with the CTA and other new gamma-ray detection instruments that are currently being built.

“The CTA is part of a new generation of gamma-ray detectors and can help identify more than a thousand new objects emitting the gamma radiation that reaches Earth, produced by cosmic rays [particles like protons, electrons and ions, traveling near the speed of light],” said Razmik Mirzoyan, a researcher at the Max Planck Institute for Physics in Munich, Germany, in a presentation during the event.

When gamma rays reach the Earth’s atmosphere, they interact with it and produce cascades of secondary subatomic particles known as air or particle showers. These ultrahigh energy particles can travel faster than light in air, creating a blue flash of “Cherenkov light,” similar to the sonic boom created by an aircraft exceeding the speed of sound.

Cherenkov light

Before the discovery of subatomic particles, English mathematical-physicist Oliver Heaviside (1850-1925) predicted what is now known as Cherenkov radiation by calculating how electrons move through a transparent medium at faster-than-light speeds. His contemporaries did not appreciate this discovery, however, and his work on the subject was forgotten, according to Mirzoyan.

“In the late nineteenth century, scientists believed space was filled with ‘ether’ [a substance of zero density thought to occupy so-called empty space, including the space between galaxies],” he said.

Almost 50 years after Heaviside began publicly predicting this kind of phenomenon in 1888, Russian physicist Pavel Cherenkov (1904-1990) discovered the effect experimentally, and it became known as Cherenkov light or Cherenkov radiation.

“In 1937, Cherenkov succeeded in measuring the anisotropy of this kind of emission,” Mirzoyan said. If the properties of a material are anisotropic, their values differ when measured along different axes. 

“He submitted an article on his findings to Nature, but it was turned down by the journal. Fortunately, Physical Review agreed to publish the article, in which Cherenkov mentioned the possibility of measuring negatively charged fast electrons.” 

In 1938, French physicist Pierre Auger (1899-1993), who had positioned particle detectors high in the Alps, noticed that two detectors located many meters apart both signaled the arrival of particles at exactly the same time. 

In 1948, while studying cosmic rays using a cloud chamber, which is a device used to detect subatomic particles, English physicist Patrick Blackett (1897-1974) presented research findings that mentioned the possibility of detecting Cherenkov radiation using the relativistic particles of the cascades that contribute to the light in the night sky. 

This was the start of a race to develop detectors capable of measuring Cherenkov light in the shapes of the particle showers produced by both cosmic rays and gamma rays, both coming from space. “Until then, Cherenkov light had been detected only in a liquid or solid medium,” Mirzoyan said.

Heightened sensitivity

According to the researchers who took part in the event, the current generation of gamma-ray detectors, comprising the High-Energy Stereoscopic System (HESS) in Namibia, the MAGIC (Major Atmospheric Gamma Imaging Cherenkov) Observatory on La Palma, a detector on one of the Canary Islands in Spain, and the Very Energetic Radiation Imaging Telescope Array System in Arizona (USA), began producing results in 2003 and has since increased the number of known sources of gamma radiation from ten to approximately 100.

The CTA will multiply this catalog tenfold by detecting more than a thousand new celestial objects since it will be ten times more sensitive and capable of unprecedentedly precise abilities to detect high-energy gamma rays.

This heightened sensitivity and precision in detecting gamma rays is made possible by the array’s huge data-collection area and its combination of three classes of Cherenkov telescopes distributed in the northern and southern hemispheres, covering energy ranges from 20 GeV to 300 TeV.

While the existing gamma-ray observatories have at most five Cherenkov telescopes operating at once, the CTA will consist of more than 100 ground-based telescopes of three different sizes, spread between two array sites in the northern and southern hemispheres to improve the chances of capturing extremely rare particle cascades.

The southern array will be built in Chile’s Atacama Desert near the Atacama Large Millimeter Array, owned by the European Southern Observatory. The northern array will be installed in the Canary Islands, near the MAGIC observatory.

As a result, the CTA’s collection area will be equivalent to 1 million square meters (m²), with a large field of view giving it access to an almost 360° view of the night sky.

Although a flash of Cherenkov light illuminates a large area (roughly 250 m in diameter), each particle shower lasts only a few billionths of a second. Moreover, the cascades are very rare: there may be one gamma-ray photon per m² per year from a bright source and one photon per m² per century from a faint source.

Each telescope will have a mount that allows it to move rapidly toward targets and will comprise a large segmented mirror to focus the Cherenkov light into a high-speed camera. Once the mirror reflects the light, the CTA camera captures and converts it into data for in-depth studies of its cosmic sources, such as the environs of black holes, supernova remnants, pulsar wind nebulae and active galactic nuclei.

“The first proposal to use a series of Cherenkov telescopes operating together to make observations was put forward in 1984,” Mirzoyan said.

Brazilian participation

The CTA project will cost some 400 million euros and is well on its way, according to its managers.

Brazil is collaborating with the project on different fronts. One is in the construction of the ASTRI Mini-Array, a precursor and initial seed of the CTA observatory, which is being built in partnership with Italy and South Africa. 

The mini-array will comprise nine Cherenkov telescopes with primary mirror diameters of 4.3 m and will be installed at the CTA’s southern site in Chile starting in 2018.

ASTRI will be more sensitive than HESS and will observe energies in excess of 100 TeV, equivalent to 100 trillion electron volts. An ASTRI prototype telescope recorded its first Cherenkov light in September 2014 at Serra la Nave in Catania, Italy. Its innovative modular camera, with a curved focal surface and silicon photomultipliers, was developed in partnership with the Brazilian engineers at IAG-USP. 

A group led by Professor Elisabete Maria de Gouveia Dal Pino at IAG-USP is responsible for the construction of three of the nine ASTRI telescopes under the aegis of a Thematic Project supported by FAPESP.

Another group of researchers at the University of São Paulo in São Carlos is developing the camera mount for the CTA’s medium-size telescopes in partnership with the CTA’s German team as part of another Thematic Project supported by FAPESP.

A third group of researchers, this time at the Brazilian Center for Research in Physics (CBPF), is developing the structural components for the CTA’s large-size telescopes.

“The ASTRI prototype telescope is almost ready. Testing is in its final stage, and the construction of the mechanical parts and the structures for the nine telescopes will begin soon,” Dal Pino told Agência FAPESP.