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Regular-article-logo Tuesday, 08 July 2025

Ghost from a distant galaxy

An Indian astronomer sitting in Antarctica spots an elusive high-energy particle from a blackhole that produced a powerful outburst 10 billion years ago. The discovery may lead to a new era in astrophysics, says Prasun Chaudhuri

TT Bureau Published 09.05.16, 12:00 AM

An observatory buried in a mass of pristine ice near the South Pole. Another one orbiting the Earth, monitoring gamma rays, which are produced by the hottest and most energetic objects in the universe, such as supernova explosions, and regions around black holes. A ghostly particle that makes up the universe. And a passionate astronomer from Darjeeling surviving in minus 60°Celsius in the Antarctic winter of 2012.

This odd cast of characters features in a research paper published in the journal Nature Physics last month.

Nasa astrophysicist Roopesh Ojha. Pic courtesy: Lara Ojha and Tapio Pursimo

The man from Darjeeling is astrophysicist Roopesh Ojha with the Nasa Goddard Space Flight Center in the US. While on duty in IceCube - the observatory in Antarctica - he was the first to notice a dramatic outburst of energy from a faraway galaxy called PKS B 1424-418. "After a thorough study, we had discovered the first neutrino from beyond our own [Milky Way] galaxy," he told KnowHow. Experts believe this discovery could lead to a "new era in astrophysics".

Also Read: Living his dream in South Pole for 16 days

Scientists have been attempting to study the elusive neutrinos since they were predicted by the American theoretical physicist Wolfgang Pauli in 1931. Many physicists believe they hold the key to understanding parts of the universe that remain hidden from us, such as dark matter and dark energy.

Neutrinos are called ghost particles because they are so difficult to detect. Despite their sheer numbers, they rarely interact with other particles. They carry no electric charge and travel through the universe almost entirely unaffected by natural forces. Trillions of neutrinos pass through our bodies every second, without anybody detecting them. The only time they can be detected is when they collide head on with another particle.

Most neutrino detectors are vast underground water tanks with extremely sensitive sensors that can capture brief flashes of light emitted when a neutrino collides with a particle in the fluid.

IceCube, however, uses a kilometre-sized section of ice 2.5 kilometres beneath the surface. Embedded sensors capture the brief flashes of light emitted when a neutrino collides with a particle. "It is dark inside the ice, making it easy to detect the blue light produced when a neutrino collides with the nucleus of an ice molecule," says Ojha.

"Above the ground there is only a control building (see photo below). The actual telescope is below the ice so that if a neutrino happens to collide with an ice nucleus (an extremely rare event) it can be detected," he says.

Pic courtesy: National Science Foundation, the US

It's been a long wait for the scientists. Many of them, including Ojha, spent an entire winter at the South Pole in the hope of catching a glimpse of the ghost particle. "It is not possible to reach or leave the South Pole in winter", says Ojha.

It paid off. "One neutrino (christened Big Bird) was detected on December 4, 2012. Very powerful, extremely variable distant galaxies called blazars are thought to be the likely source of this high energy neutrino," says Ojha. Scientists at IceCube name detected neutrinos after characters from the children's TV show Sesame Street because, one, it's easier to remember names than letters and numbers and two, it is a tenuous link to their children who love the show.

"We combed through the field where Big Bird must have originated looking for astrophysical objects capable of producing high-energy particles and light," said co-author Felicia Krauss, a doctoral student at the University of Erlangen-Nuremberg in Germany. "There was a moment of wonder and awe when we realised that the most dramatic outburst we had ever seen in a blazar happened in just the right place at the right time."

Scientists at IceCube were first informed of the blazar by the Fermi Gamma-ray telescope orbiting the Earth looking for high-energy bursts in distant galaxies. "It alerted us that something unusual was happening in the blazar PKS 1424-418."

NASA Goddard scientist Roopesh Ojha explains the first plausible link between a blazar eruption and a neutrino from deep space.
Credit: NASA’s Goddard Space Flight Center

 

In a paper published on Monday, April 18, in Nature Physics, the team suggests the PKS B1424-418 outburst and Big Bird are linked. "Taking into account all of the observations, the blazar seems to have had means, motive and opportunity to fire off the Big Bird neutrino, which makes it our prime suspect," said lead author of the paper Matthias Kadler, a professor of astrophysics at the University of Wuerzburg in Germany.

Francis Halzen, the principal investigator of IceCube at the University of Wisconsin-Madison, and not involved in this study, thinks the result is an exciting hint of things to come. "IceCube is about to send out real-time alerts when it records a neutrino that can be localised to an area slightly larger than the apparent size of a full moon," he said. "We're slowly opening a neutrino window onto the cosmos."

It seems that nearly 10 billion years ago, the black hole at the centre of galaxy PKS B1424-418 produced the powerful outburst. Light from this blast began arriving on Earth only in 2012.

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