Understanding the cosmos: Bizarre flash of light traced back to kilonova a billion light years away

A flash of light in the sky that astounded scientists by lasting close to a minute has been traced back to an explosion a billion light years away. The discovery has broken several assumptions about powerful explosions in the universe.

The flash of light or a gamma-ray burst (GRB) was witnessed on December 11, 2021. The research has been documented in two different studies published in the journals Nature and Nature Astronomy.

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The source was found to be an astronomical event called a kilonova. These explosions take place during the merger of two neutron stars or a neutron star and a black hole. 

A neutron star is the collapsed core of a massive supergiant star, whereas a black hole is a place in space where gravity pulls so much that even light can not get out. 

Stellar black holes are made when the centre of a very big star falls in upon itself, or collapses, according to United States National Aeronautics and Space Administration. When this happens, it causes a supernova — an exploding star that blasts part of the star into space.

Both kilonova and supernova produce the most energetic form of light called GRBs, the former is known to generate shorter pulses lasting less than 2 seconds. This new event, however, generated a GRB that lasted roughly 50 seconds, puzzling scientists, including those from India.

GRBs can be divided into two classes: Long-duration (2 seconds to several minutes) and short-duration (a few milliseconds to 2 seconds) bursts. 

“This breaks the long-held traditional GRB paradigm that massive star collapses produce long GRBs and supernovae and neutron star mergers produce short GRBs and kilonovae,” Jillian Rastinejad, a PhD student at Northwestern University, told Down To Earth. She is the lead author of the Nature study.

The researchers observed this signal on December 11, 2021. As the source was relatively closer — only 1 billion light years away — they zoomed into it using multiple telescopes, including the 3.6 metre Devasthal Optical Telescope, sitting in Nainital district, Uttarakhand.

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The researchers studied the afterglow, long-lived radiation that follows GRBs to identify the source. They expected to trace the signal back to a supernova, which is known to be bright. Instead, they observed a faint object that faded quickly.

“Kilonovae are also known for red colours, which is a signature of the rare, heavy elements like gold produced in their ejecta,” Rastinejad explained. These clues suggested that the source could be a kilonovae.

The event produced very heavy elements about 1,000 times the mass of the Earth. “This supports the idea that kilonovae are the main factories of gold in the universe,” Matt Nicholl, an associate professor at the University of Birmingham, explained.

These findings show that there is yet another avenue for finding the elusive signatures of kilonovae, Brendan O’Connor, a PhD scholar at George Washington University, told DTE. He was not involved in the research. “This is great news for researchers who focus on those explosions,” he explained.

Researchers could not identify whether the GRBs came from the merger of two neutron stars or a neutron star colliding with a black hole. But they hope to observe more such events in the future with gravitational wave detectors. “They can tell us about the masses of the progenitors,” Rastinejad said.

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O’Connor thinks researchers may have missed detecting long-duration bursts from kilonova before.

These explosions are incredibly faint and can only be seen from nearby sources. However, most GRBs come from much larger distances, where detecting a kilonova is increasingly tricky, O’Conner explained.

“We would love to know how often mergers create long GRBs! Several long GRBs are detected per week, but this was the first time we identified one coming from a neutron star merger,” Rastinejad added.

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