space
Record bang: Largest explosion ever observed in the cosmos
The illustration, published by the University of Southampton, shows an artist’s rendering of gravitational mass gain as a black hole collects matter. photo
© John A. Paice/www.johnapaice.com/University of Southampton/dpa
A black hole devours a large molecular cloud. Eight billion years later, astronomers on Earth register the event. And can report a record.
About eight billion years ago, a large cloud of hydrogen plummeted into a supermassive black hole – triggering the most energetic explosion ever observed by astronomers. The burst of radiation was ten times stronger than any known supernova and lasted more than three years, write the scientists led by Philip Wiseman from the British University of Southampton in the “Monthly Notices of the Royal Astronomical Society”.
At first, the sky researchers could not make sense of the unusual cosmic event. Only the observation with many different instruments from the long-wave infrared range to high-energy X-rays helped them to find an explanation. The explosion could not be seen with the naked eye.
Explosions are not uncommon in the cosmos: from thermonuclear explosions on dying stars, to supernovae that rip apart entire stars, to the bursts of radiation that occur when supermassive black holes engulf entire stars. The range of such events is rich. But none of this matched the particularly energetic celestial event cataloged under the designation AT2021lwx.
First discovery
The explosion was first discovered in 2020 by the “Zwicky Transient Facility”, a special telescope at the Mount Palomar Observatory in the USA. With it, astronomers automatically look for transient events in the sky, such as stellar explosions. “So we came across it by accident,” Wiseman said, according to a statement from his university. The automatic telescope noticed the event and sounded the alarm.
At first, researchers thought it was a supernova, or a star falling into a black hole. Further observations showed that the explosion had taken place in a galaxy far, far away. It had taken the light eight billion years to reach Earth from there – so the explosion had taken place eight billion years ago, about six billion years after the Big Bang.
The long distance also means the blast was unusually energetic – and lasted for an unusually long time. “Typically, such explosions last a few months, then the radiation levels off,” Wiseman said. “For something to shine so brightly for more than two years is very unusual.”
The only objects in the cosmos with a brightness comparable to AT2021lwx are quasars – supermassive black holes at the centers of distant galaxies. They emit radiation because matter constantly falls into them from the outside and heats up in the process. “But such quasars flicker, their brightness fluctuates very strongly,” explained Mark Sullivan, a colleague of Wiseman’s at the University of Southampton.
In contrast, AT2021lwx first increased in brightness by a factor of 100 in about a hundred days and has since been decreasing very slowly. The researchers searched old data for further eruptions of the object – without success.
root cause analysis
To track down the cause of the explosion, Wiseman, Sullivan and their colleagues observed the celestial object for three years using a variety of instruments. With the data obtained in this way, one scenario finally emerged as the most likely explanation for the explosion: a large cloud of molecular hydrogen probably fell into a black hole with a mass about a billion times that of our sun. The cloud was not swallowed up in one fell swoop, but in parts – which triggered shock waves in the rest of the cloud and thus led to the strong radiation.
Astronomers hope to find many more similar events with the next generation of automated telescopes. “Because such explosions are obviously very rare,” says Wiseman. “But they are so energetic that they could play an important role in the evolution of the centers of galaxies.”