Astronomers have determined what caused the brightest cosmic explosion ever recorded.
Lasting a matter of minutes, the gamma-ray burst, named GRB 221009A, was observed by astronomers in October 2022. It has since been dubbed the "B.O.A.T.", the brightest of all time.
In a new study, a team of researchers led from Northwestern University used data from NASA's James Webb Space Telescope to confirm that the burst was caused by the collapse and subsequent explosion of a massive star some 2.4 billion light-years away.
While the confirmation has put one mystery to rest, the researchers found that the B.O.A.T. has caused another to deepen.
"When we confirmed that the gamma-ray burst was generated by the collapse of a massive star, that gave us the opportunity to test a hypothesis for how some of the heaviest elements in the universe are formed," Peter Blanchard, a Northwestern astrophysicist and the study's leader, said in a statement.
Elements such as gold and platinum are not normally produced by the nuclear reactions that fuel the hearts of stars—leaving a question as to where they are made.
Scientists have determined that heavy elements can be produced as a result of collisions between neutron stars, the ultradense collapsed cores of massive, "supergiant" stars.
However, heavy elements are far too common in the universe—and neutron star mergers too few and far between—for this to be the only source.
"It takes a very long time for binary neutron stars to merge," Blanchard said.
He added: "Two stars in a binary system first have to explode and leave behind neutron stars. Then it can take billions and billions of years for the two neutron stars to slowly get closer and closer, and finally merge."
On top of this, he continued, our observations of the distant—and therefore older—universe have shown that the cosmos was enriched with heavy metal before most binary stars would have had time to merge.
This, Blanchard said, is "pointing us to an alternative channel."
Astrophysicists have suggested that heavy elements might also be forged in the collapse of massive, rapidly spinning stars like the one believed to have generated the B.O.A.T.—however, the new study appears to have poured cold water on this hypothesis.
Blanchard said: "We did not see signatures of these heavy elements, suggesting that extremely energetic gamma-ray bursts like the B.O.A.T. do not produce these elements.
"That doesn't mean that all gamma-ray bursts do not produce them, but it's a key piece of information as we continue to understand where these heavy elements come from."
"Future observations with JWST will determine if the B.O.A.T.'s 'normal' cousins produce these elements," he added.
The B.O.A.T. was so powerful that Blanchard and his team waited six months before making their observations of the burst's aftermath.
"The gamma-ray burst was so bright that it obscured any potential supernova signature in the first weeks and months after the burst," Blanchard said.
He continued: "At these times, the so-called 'afterglow' of the gamma-ray burst was like the headlights of a car coming straight at you, preventing you from seeing the car itself.
"So we had to wait for it to fade significantly to give us a chance of seeing the supernova."
The team also cross-referenced its JWST data with observations made by the Atacama Large Millimeter/Submillimeter Array to carefully separate the afterglow from the light of the supernova itself.
While gold and platinum may not have been found in the supernova, the Near Infrared Spectrograph on the JWST showed the signatures of elements, such as calcium and oxygen, that are typical in the wake of such explosions.
The researchers also found that the supernova, unlike the associated gamma-ray burst, wasn't itself exceptionally luminous.
"It's not brighter than previous supernovae. It looks fairly normal in the context of other supernovae associated with less energetic gamma-ray bursts," Blanchard said.
He added: "You might expect that the same collapsing star producing a very energetic and bright gamma-ray burst would also produce a very energetic and bright supernova.
"But it turns out that's not the case. We have this extremely luminous gamma-ray burst, but a normal supernova."
What are gamma-ray bursts?
Gamma-ray bursts are brief flashes of high-energy light—typically lasting tens of milliseconds to several hours in duration—seen coming from distant galaxies.
According to NASA, they are "the most powerful class of explosions in the universe" and the brightest and most energetic events since the Big Bang.
They appear to be created when massive, dying stars explode to either form a neutron star or collapse into a black hole—and also from the merger of two neutron stars.
They shoot out along the star's axis of rotation in two different directions, not unlike the beams from a lighthouse on Earth.
A gamma-ray burst close enough to Earth and pointed in our direction could be deadly.
In fact, the European Space Agency says, "Some theories suggest that anything caught in the beam, out to a distance of around 200 light-years, will be vaporized."
The good news, however, is that astronomers do not believe that any stars within this distance of Earth are destined to go supernova—and even if they were, we would have to be tremendously unlucky to be in the line of fire.
That said, scientists believe Earth may have felt the effects of some 1,000 gamma-ray bursts in its lifetime, with a likely frequency of about one every 5 million years.
Exactly how the brightest burst ever seen could have been produced by an otherwise apparently normal supernovae is still to be determined.
One possibility, according to Tanmoy Laskar, a professor and an astrophysicist at the University of Utah and one of the paper's authors, is that it was a quirk of the shape and structure of the collapsing star's "relativistic jets."
These twin streams of material fly out of massive, dying, spinning stars along their axis of rotation at velocities approaching the speed of light—and the narrower they are, the more focused and intense the resulting beam of light.
"It's like focusing a flashlight's beam into a narrow column, as opposed to a broad beam that washes along a whole wall," Laskar said in a statement.
He added: "In fact, this was one of the narrowest jets seen for a gamma-ray burst so far, which gives us a hint as to why the afterglow appeared as bright as it did.
"There may be other factors responsible as well—a question that researchers will be studying for years to come."
One such factor, the researchers said, may be the birthplace of the supernova in question. The team's analysis of light from the galaxy from which the B.O.A.T. originated suggests it has both been undergoing intense star formation and has a low concentration of elements heavier than hydrogen and helium.
Whatever exactly caused the B.O.A.T., one thing is certain—it is, at least for now, a most singular event.
"As long as we have been able to detect gamma-ray bursts, there is no question that this gamma-ray burst is the brightest we have ever witnessed by a factor of 10 or more," Wen-fai Fong, a Northwestern astrophysicist and professor, said in a statement.
"The event produced some of the highest-energy photons ever recorded by satellites designed to detect gamma rays," Blanchard added.
"This was an event that Earth sees only once every 10,000 years. We are fortunate to live in a time when we have the technology to detect these bursts happening across the universe," he continued.
Blanchard concluded, "It's so exciting to observe such a rare astronomical phenomenon as the B.O.A.T. and work to understand the physics behind this exceptional event."
The full findings of the study were published in the journal Nature Astronomy.
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