The blast of radiation, known as gamma-rays, came in two bursts that hit Earth with more than 100 billion times the energy in visible light.
After decades of work, scientists have seen the most powerful ever blast from a gamma-ray burst, or GRB. Such blasts are the most intensely energetic phenomenon in the universe, suddenly appearing in the sky for only a brief period but releasing the same amount of energy in one second that the Sun will emit in its entire life.
GRBs are the most poweful explosions in the universe and are thought to come out of massive stars when they collapse at the end of their lives, or when binary star systems merge into much smaller objects. As they do, they throw out a flash of radiation that flies across the whole universe.
When observed, they begin with a bright flash that is then followed by an "afterglow". That afterglow throws out emissions across a range of different energies, including the very intense gigaelectronvolt gamma-rays – of which few have ever been seen.
Gamma-ray bursts were first spotted during the Cold War. During that period, the US and former Soviety Union had satellites in orbit in space, looking for gamma-rays being thrown out by the explosion of an atomic bomb.
Every so often, they would send an alert that they had spotted a huge increase in those gamma-rays. But there was no evidence of a nuclear explosion and the signals in fact appeared to be coming from space.
That is how gamma-ray bursts were first found. But they have stayed very mysterious in the years since, in part because the data was kept strictly classified and in part because they are so difficult to track down.
Now scientists hope to delve into that mystery after spotting the newly energetic blast, after trying for many years. Specialised telescopes were able to see the blast of photons, which reached about 100 billion times the level of visible light.
In an comment piece accompanying the release of three papers exploring the GRBs in the journal Nature, University of Nevada professor Bing Zhang said that the discovery was a "breakthrough" twice over. The results were both a "triumph" for the observatories that have been watching for such a result for more than a decade, he wrote, as well as a "triumph for GRB theories", which could now allow us to understand how such powerful explosions happen.
The breakthrough began when dedicated Nasa satellites spotted what appeared to be a gamma-ray burst, and sent a notification to other observatories that were looking for gamma-rays. That happened twice – in July 2018 and January 2019, allowing two teams of astronomers to see two GRB events from the ground for the first time.
"We were able to point to the region of origin so quickly that we could start observing only 57 seconds after the initial detection of the explosion," said Cosimo Nigro from the Deutsches Elektronen-Synchrotron, a research group that helped find the burst. Within a few minutes, DESY had seen about a thousand photons from the blast, which is known as GRB 180720B.
What they went on to see was the most intense blast of light ever picked up from a GRB, itself the most intensely energetic phenomenon known to humanity. It faded within half an hour.
"High-energy GRBs with energies in the region of tera-electron-volts (trillions of electron volts) were theoretically predicted. Astronomers have searched for such powerful bursts for 15 years," said Masahiro Teshima from the Institute for Cosmic Ray Research at the University of Tokyo.
"My international team and I are proud to announce the discovery of the first gamma-ray burst with observed energies up to 1 tera-electron-volt, by far the highest-energy photons ever detected from a GRB."
The GRB in question – known as 180720B – was very strong and lasted about 50 seconds. That is relatively long for such a burst, and suggests that it was thrown out by the death of a massive star, as its core collapses and forms a swirling black hole.
The burst is thought to be thrown out as the gas that srrounds the black hole forms into a disc around its edges, with gas jets that are ejected and create the flashes. They are created as particles are accelerated to speeds reaching the speed of light, creating gamma-rays as they interact wiht the matter and radiation that surround them.
The detection of the very-high-energy gamma radiation not only hows that there are extremely accelerated particles in GRBs, scientists say, but also is evidence that the particles still exist after the initial explosion. Before, scientists had presumed that the bursts could only be seen within the seconds and minutes following the explosion – but in the latest breakthrough, they were seen hours after the explosion.
David Berge, head of gamma-ray astronomy at DESY - a particle accelerator research centre in Germany, said: "For the first time, the two instruments have measured gamma radiation from gamma-ray bursts from the ground.
"These two groundbreaking observations have established gamma-ray bursts as sources for terrestrial gamma-ray telescopes.
"This has the potential to significantly advance our understanding of these violent phenomena."
Now astronomers hope to use the discovery to learn more about the context that give rise to such intense blasts. Astronomers hope that they will be able to spot as many as ten of the events per year when they switch on the Cherenkov Telescope Array, made up of over 100 individual telescopes that will look out for such gamma-rays – but that will not happen until 2023 at the earliest.
"Although long anticipated, the detection of TeV gamma rays from GRBs had been an extremely challenging endeavor. It was finally realized here with very high significance for the first time, after many years of technical improvements and dedicated efforts," said Professor Teshima.
"Continuing efforts with existing gamma-ray telescopes, as well as the new Cherenkov Telescope Array currently under construction, promise to bring forth new physical insight into the most luminous electromagnetic explosions in the universe."
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