SOUTHAMPTON (AP) — About eight billion years ago, a large cloud of hydrogen fell into a supermassive black hole — causing the most energetic explosion astronomers have ever observed. The radiation burst was 10 times more powerful than any known supernova and lasted more than three years, the scientists led by Philip Wiseman of Britain’s University of Southampton write in Monthly Notices of the Royal Astronomical Society.
At first, sky searchers could not understand the unusual cosmic event. Only observation with many different instruments from the long-wave infrared range to high-energy X-rays helped them find an explanation. The explosion cannot be seen with the naked eye.
Explosions are not uncommon in the universe: from thermonuclear explosions on dying stars, to supernovae that tear apart entire stars, to radiation explosions that occur when supermassive black holes swallow stars entirely. The scope of such events is rich. But none of this matched a particularly active celestial event classified under the designation AT2021lwx.
The first discovery
The explosion was first detected in 2020 by the Zwicky Transit Facility, a special telescope at Mount Palomar Observatory in the USA. With it, astronomers automatically look for transient events in the sky, such as stellar explosions. “We came across it by chance,” Wiseman said, according to a statement from his university. An automated telescope noticed the event and raised the alarm.
At first, the researchers thought it was a supernova, or a star falling into a black hole. Further observations showed that the explosion occurred in a galaxy far, far away. It took light eight billion years to reach Earth – so the explosion happened eight billion years ago, about six billion years after the Big Bang.
The long distance also means that the eruption was unusually energetic – and lasted for an unusually long time. “Typically, such eruptions last a few months, and then the radiation levels go down,” Wiseman said. “It is very unusual for something to shine so bright for more than two years.”
The only objects in the universe with a brightness comparable to AT2021lwx are quasars – supermassive black holes at the centers of distant galaxies. They emit radiation because matter is constantly falling into them from the outside and gets heated up in the process. “But quasars like these flicker, their brightness fluctuates wildly,” explains Mark Sullivan, a colleague of Weismann’s at the University of Southampton.
In contrast, the AT2021lwx first increased in brightness by a factor of 100 in about a hundred days and since then has been decreasing very slowly. Researchers searched ancient data for more volcanic eruptions of the body – to no avail.
Root cause analysis
To track down the cause of the outburst, Weisman, Sullivan, and their colleagues observed the celestial body for three years using a variety of instruments. With the data thus obtained, one scenario finally emerged as the most likely explanation for the explosion: a large cloud of molecular hydrogen might have fallen into a black hole a billion times the mass of our sun. The cloud was not swallowed up in one fell swoop, but in parts – creating shock waves in the rest of the cloud and thus giving rise to powerful radiation.
Astronomers hope to find many more similar events with the next generation of robotic telescopes. “Because, obviously, such eruptions are very rare,” Weissman says. “But it is so energetic that it could play an important role in the evolution of galactic centers.”
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