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It’s a dog-eat-dog cosmos. Not two weeks ago, on May 3, astronomers reported observing a star that was in the process of swallowing one of its own planets. Just two days earlier, another team had described a black hole tearing apart a star and consuming it in a process known as a tidal disruption event, or TDE.
Now an international group of astronomers is reporting that they are observing one of the most violent and energetic acts of cosmic cannibalism they have ever seen, possibly the biggest explosion yet in the history of the universe. Eight billion light-years from Earth, in the darkness beyond the constellation Vulpecula, a black hole perhaps a billion times larger than the sun appears in a great cloud of gas. A study of the phenomenon appears in the Monthly Notices of the Royal Astronomical Society.
The research began on April 13, 2021, when the Zwicky Transient Facility, a small telescope busy looking for exploding stars, or supernovae, saw a bright flash that didn’t match expectations. Most supernovae disappear after a few weeks; this one, known as AT2021lwx, continues – and has continued to explode for three years now.
In fact, the explosion was first detected a year earlier by the Asteroid Terrestrial-impact Last Alert System, or ATLAS, a network of robotic telescopes in Hawaii, South Africa and Chile. It was the real beginning of the cataclysm; as it continues, a worldwide network of telescopes and satellites has been monitoring it, measuring its emanations across the electromagnetic spectrum, from high-energy X-rays to infrared.
“Most supernova and tidal disruption events last only a few months before they disappear,” said Philip Wiseman, an astrophysicist at the University of Southampton and lead author of the new paper. “For that to be bright for two years is more unusual.”
What happened? “At first we thought this flare might be the result of a black hole consuming a passing star,” said Matt Nicholl of Queen’s University Belfast, who helped analyze the ongoing explosion. “But our model shows that a black hole would have to swallow up to 15 times the mass of the sun to stay bright for that long.”
Another idea is that there is an outburst from a quasar – energy gushing from the edge of a supermassive black hole at the heart of a galaxy. But there is no previous record of quasar activity at that location, nor is there any sign of a galaxy there.
Of the many unlikely explanations, the most likely, Dr. Wiseman and his colleagues concluded that a black hole as big as a billion suns is enjoying a long party in a huge cloud of gas. He has encouraged his colleagues to be vigilant in similar events.
“AT2021lwx is an extraordinary event that does not fit the class of common transients,” Dr. Wiseman said in an email. He added that, with a total radiation energy equal to 100 supernovae, “it is one of the brightest transients found.”
Jolt for jolt, that will put it in the colliding company black hole. “Colliding black holes release energy in gravitational waves of extreme luminosity – 10 billion times more ‘powerful’ than this explosion,” wrote Dr. Wiseman. “But the power only lasts for 20 milliseconds,” adding that this explosion has been going on for years.
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