Scientists find explanation for ‘impossible’ blast of light that hit Earth

Scientists believe they have found an explanation for an “impossible” blast of energy that hit Earth.

Last year, scientists reported that they had seen evidence that gamma-ray bursts could come out of mergers between neutron stars and another compact object, in the form of a neutron star or black hole. That was previously thought not to be possible.

Scientists had initially thought that the 50-second blast came when a massive star collapsed, but further work looking at the afterglow of the emission showed that it was in fact a “kilonova”, which happens when neutron stars merge with other compact objects. Previously, it was thought that only a supernova could make a long gamma-ray burst of that kind.

But researchers now think they have found the explanation behind the unprecedented, intense and seemingly impossible blast of light.

They simulated the evolution of a merger between a black hole and a neutron star. And they found that the black hole that was left behind after that merger can send out jets of material taken from the neutron star it had swallowed.

If the disc that surrounds the black hole has a weak enough magnetic field, then that jet can be bright and last for a long time.

The simulations showed that brightness and duration matched the mysterious gamma-ray burst that scientists reported last year, after spotting it the year before.

They had not necessarily been looking for an explanation for the gamma-ray burst, only to better understand how those jets work when they merge and what happens to them after. But they found that it matched observations of the “impossible” gamma-ray burst.

“So far, no one else has developed any numerical works or simulations that consistently follow a jet from the compact-object merger to the formation of the jet and its large-scale evolution,” said Ore Gottlieb, a researcher who helped lead the work. “The motivation for our work was to do this for the first time. And what we found just so happened to match observations of GRB211211A.” 

The simulation helps explain the gamma-ray burst, known as GRB211211A, and any others like it. But it could also help us better understand black holes and other extreme phenomena in the universe.

One of the many reasons that astronomers are interested in gamma-ray bursts is that they are “multi-messenger phenomena” – they send out both gravitational and electromagnetic waves, and scientists can observe both and compare them. But that makes such simulations difficult precisely because of the vastness of the space and time they involve.

To get around that, scientists split the scenario into two pieces, so that each could be handled by computers. They first simulated time before the merger, and then plugged that into a simulation of what happened after the merger.

“The daisy chaining of the two simulations allowed us to make the computation much less expensive,” Gottlieb said. “The physics is very complicated in the pre-merger stage because there are two objects. It gets much simpler after the pre-merger because there is only one black hole.”

That simulation showed that the objects initially merged to make an even more massive black hole, which then pulled the debris from the neutron star towards it. But some of that mass got stuck swirling around the black hole, in a disc – and when that matter then fell into the black hole, it was the power behind a jet that shot across the universe and hit the Earth in 2021.

The new research, ‘Large-scale evolution of seconds-long relativistic jets from black hole-neutron star mergers’, is published in The Astrophysical Journal today. The detection of the original gamma-ray burst was described in a Nature paper published in December last year.