Using the James Webb Space Telescope (JWST), scientists have finally solved the mysterious origins of the 'BOAT', possibly the largest cosmic explosion since the Big Bang.
The brightest gamma-ray burst of all time (hence the abbreviation The Brightest Of All Time), also called the BOOT, appears to have been launched by a supernova explosion that caused the death and collapse of a massive star located about 2.4 million light years. away. This is an event that probably also led to the birth of a black hole.
However, by solving this cosmic mystery, the team of astrophysicists has uncovered yet another celestial puzzle. That's because traces of heavy elements like gold and platinum, traces you would expect to linger around these types of supernovae, are nowhere to be found.
"This was an event that Earth experiences only once every 10,000 years," Peter Blanchard, team leader and scientist at Northwestern University, said in a statement. "The event produced some of the highest energy photons ever recorded by satellites designed to detect gamma rays."
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"When we confirmed that the GRB was generated by the collapse of a massive star, it gave us the opportunity to test a hypothesis about how some of the heaviest elements in the universe are formed," Blanchard added. "We saw no signatures of these heavy elements, indicating that extremely energetic GRBs like the BOAT do not produce these elements."
Scientists didn't miss the BOAT
The BOAT, officially named GRB 221009A, was first spotted on October 9, 2022 and immediately stood out from other GRBs due to its extreme nature. Astronomers saw it as an immensely bright flash of high-energy gamma rays, followed by a fading afterglow over many wavelengths of light.
The powerful GRB was first spotted by gamma-ray and X-ray telescopes, including NASA's Fermi Gammaray Space Telescope and the Neil Gehrels Swift Observatory. After the initial detection of the BOOT, astronomers rushed in awe to find the potential source. They pointed their telescopes at the constellation Sagitta, convinced that the answer must lie there.
"As long as we have been able to detect GRBs, there is no doubt that this GRB is the brightest we have ever seen by a factor of 10 or more," said one of the BOOT discoverers Wen-fai Fong, associate professor. of physics and astronomy and leader of the Fong Group at Northwestern, had said around the time of discovery.
However, Blanchard and colleagues did not rush to pursue the BOAT.
Instead, they wanted to watch the BOAT as it evolved, and follow it through its later stages. For example, they trained the JWST about six months after it was first spotted on the fading gamma-ray burst.
"The GRB was so bright that it obscured any potential supernova signature in the first weeks and months after the outburst," Blanchard said. "At such times, the so-called afterglow of the GRB was like the headlights of a car coming straight at you, so you couldn't see the car itself.
"So we had to wait until it faded significantly to give us a chance to see the supernova."
Using the JWST's Near Infrared Spectrograph (NIRSpec) instrument, Blanchard and colleagues observed the BOOT's infrared afterglow. This revealed the signature of elements such as calcium and oxygen, all of which are characteristic of supernovae.
What was surprising, however, was that even though BOAT is the most powerful cosmic outburst of its kind ever observed, the supernova that created it actually seemed pretty average for such an explosive star death.
"It's no brighter than previous supernovae. It looks fairly normal in the context of other supernovae associated with less energetic GRBs," Blanchard said. "You would expect that the same collapsing star that produces a very energetic and bright GRB would also produce a very energetic and bright supernova. But that turns out not to be the case. We have this extremely bright GRB, but a normal supernova."
The team is currently unsure how a 'normal' supernova could have created an explosion of energy as big as the BOAT.
Team member and assistant professor of physics at the University of Utah, Tammoy Laskar, thinks the extreme GRB could result from the shape and structure of near-light velocity jets launched by collapsing massive stars as they create black holes. If a massive star is spinning rapidly as it collapses, the rays it emits are at first narrow, then sharper and therefore brighter.
"It's like concentrating the beam of a flashlight into a narrow column, as opposed to a wide beam that washes over an entire wall," Laskar said. 'In fact, this was one of the narrowest jets yet observed in a gamma-ray burst, which gives us an idea why the afterglow appeared so bright.
"Other factors may also be responsible, a question that researchers will study in the coming years."
Furthermore, another aspect of this supernova that warrants a much deeper investigation is not something it has, but rather the things it appears to be missing.
The missing elements
The hearts of stars are like stellar furnaces that fuse light elements together to create heavier and heavier elements. This process forms elements up to and including iron, but after that even the heaviest stars have difficulty fusing heavier elements such as gold and platinum.
For years, scientists have suspected that these relatively heavier elements are created when massive, dense, dead stars called neutron stars collide, with the JWST recently playing a key role in helping to confirm such a theory.
Yet researchers also thought that the extreme environments that formed around supernovae that GRBs can launch could facilitate the "rapid capture" of neutrons, or the "r-process," which forges elements like gold. The reason for this suspicion is that neutron star collisions alone appear to be too rare to create the amounts of elements heavier than leading scientists see in the early universe.
'There's probably another source. It takes a very long time for binary neutron stars to merge. Two stars in a binary star system must first explode to leave neutron stars behind. Then it can take billions and billions of years for the two neutron stars to converge. stars slowly get closer and eventually merge," Blanchard said. 'But observations of very old stars indicate that parts of the universe were enriched in heavy metals before most binary neutron stars had time to merge.
"That points us to an alternative channel."
That alternate channel was theorized to be the collapse of a rapidly spinning and massive star, the exact type of event that scientists have now confirmed launched the BOOT.
Using the JWST, the team was able to peer into the deep layers of this supernova, where elements heavier than iron had to be forged. "The exploded material of the star is opaque at early times, so you can only see the outer layers," says Blanchard. 'But once it expands and cools, it becomes transparent. Then you can see the photons coming out of the inner layer of the supernova. Furthermore, different elements absorb and emit photons at different wavelengths depending on their atomic structure, giving each element a unique spectral signature."
That means looking at an object's spectrum can tell astronomers what elements are present.
"When examining the BOAT spectrum, we saw no signs of heavy elements, indicating that extreme events such as GRB 221009A are not primary sources," Blanchard said. "This is crucial information as we continue to try to determine where the heaviest elements are formed."
Blanchard also says that the failure to detect heavy elements around BOAT's supernova source does not mean scientists should give up on investigating the GRB channel for heavy element production just yet.
"That doesn't mean all GRBs don't produce them, but it's an important piece of information as we continue to understand where these heavy elements come from," he said. "Future observations with JWST will determine whether the 'normal' cousins of the BOOT produce these elements."
In addition to learning more about the BOAT and confirming its origins at the JWST, the team was also able to detect the signature of an intense burst of star formation in the event's host galaxy. This indicated that the star that died at BOOT's birth may have formed in a different environment than other supernova stars.
One aspect of this galaxy that could help further unlock the BOOT's secrets is the fact that it appears to have a low concentration of elements heavier than hydrogen or helium, which astronomers called "metals."
"This is another unique aspect of the BOOT that may help explain its properties," team member and Penn State University graduate student Yijia Li said in the statement.
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"We are fortunate to live in a time where we have the technology to detect these outbursts across the universe," Blanchard concluded. "It is so exciting to observe such a rare astronomical phenomenon as BOAT and try to understand the physics behind this exceptional event."
The research was published Friday (April 12) in the journal Nature Astronomy.
