Indian astronomers will soon be able to observe and monitor spectacular celestial events like the explosion of dying stars, the fall of matter into black holes and the collision of large cosmological objects better. This is because researchers at the Tata Institute of Fundamental Research (TIFR) and Bhabha Atomic Research Centre (BARC), both in Mumbai, are developing a new gamma-ray telescope.
The four-metre telescope will be able to operate in bright environments, such as during twilight hours and through moonlit nights, unlike traditional telescopes that can operate only in dark conditions. It will be the second such telescope to be available globally. The first one is in La Palma in the Canary Islands, and was set up jointly by Switzerland and Germany in 2011.
A view of upcoming MACE telescope next to the Hagar array at Hanle in Ladakh. The new 4 meter telescope will also be installed in this area.
Varsha Chitnis of the Department of High-Energy Physics at TIFR, said the imaging camera to be used in the telescope is different from conventional ones because it is set to use pixels made of Geiger-mode avalanche photo-diodes (GAPDs), while conventional telescope cameras depend on photomultiplier tubes (PMTs).
GAPDs – also known as single-photon avalanche diodes, SPADs – and PMTs are both types of devices used to detect photons. In a GAPD, an incoming photon knocks an electron out of a material via the photoelectric effect. A large voltage is applied across the material so that the knocked-out electron has high kinetic energy; as it moves around, it could liberate more electrons from the material. As a result, there is a relatively large current as a result of a single photon having impinged on the material. This is why detectors like GAPDs are well-suited to observing low-intensity radiation, including gamma-rays coming from celestial events.
“GAPDs need a much lower operation voltage, are more robust and have higher photon detection efficiency. They can be operated during strong moonlight and are ideal for a gamma-ray telescope,” Chitnis added.
The new telescope will be located near the seven-telescope High Altitude Gamma Ray (HAGAR) array at Hanle in Ladakh. This is the same village where the Indian Astronomical Observatory operated by Indian Institute of Astrophysics, Bengaluru, is also located.
The TIFR-BARC telescope will work in tandem with the Major Atmospheric Cherenkov Experiment (MACE), a 21-metre-wide gamma-ray telescope currently being installed in Hanle. MACE will operate in discovery mode, looking for candidate sources of gamma rays or very faint objects while the new telescope will keep an eye on blazars.
A blazar is a supermassive black hole that is ‘feeding’ on matter and emitting high-energy jets of radiation pointed in the direction of Earth. When it emits such a jet, it is said to be flaring. As such, blazars are among the most energetic objects known in the universe. In June this year, the IceCube neutrino detector at Earth’s south pole had reported that a neutrino it had picked up on in September had originated from a blazar 3.7 billion lightyears away.
Whenever the new telescope observes some flaring activity, a computer will alert MACE, which will then shift its focus to the active blazar as well.
Gamma rays provide the best way to study what is called the non-thermal universe (ed. note: parts of the universe that can be understood by observing radiation without having to measure the temperature of the objects emitting that radiation). Cosmic rays are an important part of the non-thermal universe. It is believed lower-energy cosmic rays are composed of protons accelerated by the remains of stars that have blown up, and that higher-energy cosmic rays are the result of protons accelerated in active galactic nuclei, including blazars. Gamma rays are also produced when charged particles are accelerated to such high energies through different processes.
Thus, the study of gamma-ray emissions from various celestial objects is expected to provide clues about the origin of cosmic rays as well as insights into the emission regions and processes in these sources.>