Astronomers Explain What Caused the Largest Cosmic Explosion Ever Seen

It was only last October that telescopes spotted a gamma ray burst—caused by the collapse of a black hole—that was so powerful astronomers quickly dubbed it BOAT, for “Brightest of all Time.” That was a fair enough nickname for such a sensational emission—for a little while anyway. But BOAT has just been busted to second most powerful.

According to a new study published in the Monthly Notices of the Royal Astronomical Society there is a new champion out there: a cosmic explosion known as AT2021lwx. The explosion, located 8 billion light years from Earth, has been erupting for three years now, emitting two trillion times the light of our sun and 10 times the energy of the brightest supernova ever observed.

The very existence of such a formation—never before observed by astronomers—is further proof that there are whole new species of astronomical phenomena yet to be discovered. Where there is one AT2021lwx, there could be others—and still more objects not yet imagined, much less seen.

“AT2021lwx is an extraordinary event that does not fit into any common class of transient [or stellar eruptions],” the research team wrote. “Further follow-up and modeling of AT2021lwx is necessary to reveal more about the scenario that caused the flare.”

The eruption was initially spotted by telescopes at the Caltech-operated Zwicky Transient Facility in 2020, and at first, astronomers thought they might be witnessing a quasar, an eruption that occurs when gas and dust fall into a supermassive black hole. But quasars tend to fluctuate in energy and brightness, while AT2021lwx flicked on its high beams and has kept them burning at a steady luminosity ever since its discovery.

“With a quasar, we see the brightness flickering up and down over time,” said Professor Mark Sullivan, of the University of Southampton, a co-author of the paper, in a Royal Astronomical Society statement. “But looking back over a decade there was no detection of AT2021lwx, then it suddenly appeared as one of the most luminous things in the universe, which is unprecedented.”

The next best guess was a supernova, but the light from such stellar explosions typically lasts for months, not years. Further observations were conducted by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Hawaii, which typically scans the skies for dangerous near-Earth objects but can also make distant observations, joining the Zwicky facility in trying to puzzle out what the astronomers were seeing.

With a quasar and a supernova ruled out, the authors of the paper, led by astronomer Philip Wiseman at Southampton University, looked to what is known as a tidal disruption event. That’s when a star is pulled into the maw of a black hole and shredded in the process. But AT2021lwx had that beat too, shedding three times more light than any tidal disruption ever observed, and lasting much longer as well.

“We came upon this by chance, as it was flagged by our search algorithm when we were searching for a type of supernova,” said Wiseman, in the statement. “Most supernovae and tidal disruption events only last for a couple of months before fading away. For something to be bright for two plus years was immediately very unusual.”

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More telescopes still were brought online to study AT2021lwx, including NASA’s orbiting Neil Gehrels Swift Observatory, the New Technology Telescope in Chile, and the Gran Telescopio Canarias in La Palma, Spain. With those instruments conducting observations of their own, and with other alternatives ruled out, Wiseman and his colleagues have come to the conclusion that the brilliant, steady light of AT2021lwx is caused by a massive cloud of gas many thousands of times the size of our sun that was orbiting a black hole and was somehow disrupted—the astronomers don’t yet know how—causing the gas to fall into the hole. The entire formation, they have estimated, is 100 times the size of our solar system and is currently emitting 100 times more energy than the sun will in its entire 10 billion year lifetime. How long it will continue to burn is unclear, but its light is still streaming our way.

Wiseman’s team is not done studying AT2021lwx. The Vera Rubin Observatory’s Legacy Survey of Space and Time, in Chile, is set to come online in the next few years, and the astronomers will point that telescope AT2021lwx’s way too—and elsewhere as well.

“We are hoping to discover more events like this and learn more about them,” says Wiseman. “It could be that these events, although extremely rare, are so energetic that they are key parts of how the centers of galaxies change over time.” That fact touches close to home: Our own Milky Way has a supermassive black hole resting at its center.