Black hole: It was so violent in the cosmos

At first, the sky researchers could not understand 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 was invisible to the naked eye, write the scientists led by Philip Wiseman of the British University of Southampton in the “Monthly Notices of the Royal Astronomical Society”.

Supernovae can tear whole stars apart
Explosions in the cosmos are not uncommon: from thermonuclear explosions on dying stars, to supernovae (see image below) that tear entire stars apart, 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 highly energetic celestial event cataloged under the designation AT2021lwx.

The explosion was first detected in 2020 by the “Zwicky Transient Facility”, a special telescope at Mount Palomar Observatory in the US. With it, astronomers automatically search for transient events in the sky, such as stellar explosions. “So we came across this by accident,” Wiseman said, according to a statement from his university. The automatic telescope noticed the event and raised the alarm.

Milky Way far away
At first, researchers thought it was a supernova or a star falling into a black hole. Further observations showed that the explosion had occurred in a galaxy far, far away. The light had taken eight billion years to reach Earth from there – so the explosion had happened eight billion years ago, about six billion years after the Big Bang.

The long range also means that the explosion was unusually powerful – and lasted unusually long. “Usually, such explosions last a few months, after which the radiation decreases,” 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 in the centers of distant galaxies. They emit radiation because matter from outside constantly falls into them and heats up in the process. “But such quasars flicker, their brightness fluctuates very much,” explains Mark Sullivan, a colleague of Wiseman’s at the University of Southampton.

AT2021lwx, on the other hand, first increased in brightness by a factor of 100 in about a hundred days and has since declined very slowly. The researchers searched old data for further outbursts from the object – without success.

Cloud was swallowed to pieces
To track down the cause of the explosion, Wiseman, Sullivan and their colleagues observed the celestial body for three years using various 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 wasn’t swallowed up in one fell swoop, but in chunks – which sent shock waves through the rest of the cloud and led to the strong radiation.

Astronomers hope to find many more similar events with the next generation of automated telescopes. “Because such explosions are, of course, very rare,” says Wiseman. “But they are so energetic that they could play an important role in the evolution of the centers of galaxies.”