It started as an inconspicuous flicker in the night sky. But closer observations show that astronomers have captured the largest cosmic explosion ever recorded, an event thought to be sparked by a massive cloud of gas swallowed by a supermassive black hole.

The outburst, which dates back 8 billion light-years away, is more than 10 times brighter than any known supernova, has lasted for more than three years so far, and is the most energetic outburst ever recorded.

“It went unnoticed for a year as it gradually brightened,” said Dr Philip Wiseman, an astronomer at the University of Southampton. Only when follow-up observations revealed how far away was it, astronomers became aware of the event Almost unimaginable scale.

“We estimate it to be a fireball 100 times the size of the solar system and about 2tn times brighter than the sun,” Wiseman said. “In three years, this event releases about 100 times as much energy as the sun releases in its 10 billion-year lifetime.”

Scientists believe the explosion, dubbed AT2021lwx, was the result of a massive cloud of gas — possibly thousands of times larger than our sun — falling into the inevitable opening of a supermassive black hole. The cloud of gas likely originated from the large dust “doughnut” that usually surrounds a black hole — though it’s unclear what knocked it out of orbit and into a cosmic sinkhole.

The AT2021lwx isn’t the brightest phenom ever. An even brighter gamma-ray burst, called GRB 221009A, was discovered last year, but this event lasted only a few minutes. In contrast, new events are still going strong, implying a much greater overall energy release.

The explosion was first detected in 2020 by the Zwicky Transient Facility in California, which investigates sudden increases in brightness in the night sky that could signal cosmic events such as supernovae or passing asteroids and comets. The event didn’t stand out initially, but when follow-up observations allowed its distance to be calculated, astronomers realized they had caught an extremely rare event.

“When I told our team these numbers, they were all in shock,” Wiseman said. “Once we understood how bright it was, we had to figure out a way to explain it.”

It was outside the reasonable range of a supernova (exploding star), so astronomers turned to another common occurrence that causes bright flashes in the night sky — so-called tidal disruption events. These events usually involve a star getting too close to a black hole and being shredded, part of it being swallowed and the rest being stretched into a spinning disk.

But simulations suggest that a star as massive as 15 times the mass of the Sun would be needed to explain AT2021lwx. “Encounters of such massive stars are very rare, so we think encounters with larger gas clouds are more likely,” Wiseman said.

Supermassive black holes are often surrounded by huge halos of gas and dust, and the authors speculate that some of this material may have been disrupted, possibly by collisions of galaxies, and sent inside. As matter spirals towards the black hole’s event horizon (its spherical outer boundary), it releases a flood of heat and light, illuminating part of the donut and heating it to 12-13,000C.

The findings are published in Monthly Notices of the Royal Astronomical Society.

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