Scientists have discovered the biggest explosion ever observed in outer space, a blinding blast that is 10 times brighter than any known supernova and that has continued erupting for years since it was first detected in 2020, reports a new study. The radiant event is likely fueled by the consumption of a gigantic gas cloud by a supermassive black hole about eight billion light years from Earth. Though it is not as brilliant as a record-breaking gamma ray burst detected in October 2022, it has lasted much longer than that brief event, distinguishing it as by far the most energetic transient explosion ever spotted.
The universe is home to all manner of pyrotechnics, including the explosions of massive stars and the feeding frenzies of black holes. Supermassive black holes, which sit at the center of galaxies, frequently produce blazing light shows when stars or gas clouds fall into their gravitational grasp. Before these materials are gobbled up, they “accrete” into a disk around the black holes that often shine brighter than trillions of Suns. Now, astronomers led by Philip Wiseman, a research fellow at the University of Southampton, have unveiled the most dazzling explosion of this kind, which they identified it as “an extreme extension of the known scenarios of black hole accretion,” according to a study published on Thursday in the Monthly Notices of the Royal Astronomical Society.The event, known as AT2021lwx, was first detected three years ago by the Zwicky Transient Facility in California, but it flew under the radar for months until Wiseman and his colleagues calculated how far it was from Earth using clues imprinted into its light. The results revealed that it was eight billion light years away, which meant that the source had to be astonishingly bright to be seen across such a vast distance. “Suddenly, our eyes popped out because this thing can't be a supernova—it’s ten times too bright and you can't create this energy with it with an exploding star,” Wiseman told Motherboard in a call. “It looks like nothing else that we've ever seen.”
The unique nature of AT2021lwx prompted the researchers to obtain a number of follow-up observations of the source from sophisticated telescopes including NASA’s Neil Gehrels Swift Telescope, the European Southern Observatory’s New Technology Telescope in Chile, and the Gran Telescopio Canarias in Spain. The picture that emerged looked similar to tidal disruption events (TDEs), which occur when a star gets torn apart by a black hole, except it was three times brighter than any known TDE. The researchers considered whether a very large star might have created this kind of outburst, but this explanation didn’t match the data. Ultimately, the team proposed that the explosion was sparked by the collision of a huge hydrogen gas cloud with a supermassive black hole that is 100 million times more massive than the Sun. As the cloud accreted around the black hole, intense tidal forces sent shockwaves through the gas that lit them up and made them visible across billions of light years. “Maybe it's just a single giant cloud of gas that's effectively been dumped onto a black hole and then glows extremely brightly because so much material next to such a massive object gets heated up,” Wiseman said, adding that scenarios that involve “one supremely large and relatively compact cloud of gas just must be very rare.”To confirm their hypothesis, the researchers plan to both continue watching AT2021lwx and searching for similar transient outbursts. Given that these explosions can apparently last for years, they might open a revealing new window into the mysterious co-evolution of galaxies and supermassive black holes.“When [an explosion] lasts three, four, or five years, you get a lot more time to do detailed studies” that “really will help shed light on what actually happens when things are very close to black holes,” Wiseman said. “We haven't answered questions about how the centers of galaxies get molded by their black holes.” Fortunately, a host of sky surveys, including Zwicky, are accelerating the pace of discovery for this field, and even more powerful tools are expected to come online this decade. For instance, the Vera Rubin Observatory, which is currently being constructed in Chile, will be especially adept at spotting transients when it becomes operational next year.“It's only really in the last three years that these things have been found in the data, by Zwicky and some of the other sky surveys,” Wiseman said. “It's actually going to explode even more when the Vera Rubin Observatory comes online because that will go to much fainter magnitudes, which means we can see even further.” “Searching through that data to find similar things will be another challenge, but hopefully it throws up exciting results,” he concluded.