In October 2022, surveys began to monitor the sky for explosions in space like a frog in a sock.
The reason? Something 2.4 billion light-years away spewed out the largest burst of gamma rays ever recorded. The event, GRB 221009A, reached a record magnitude of 18 teraelectron volts and was so powerful that it shook Earth’s outer atmosphere.
We later discovered that the event, nicknamed BOAT (for Brightest of All Time), was the birth of a black hole through the violent death of a massive star.
Now a new analysis of the evolving light has revealed the intricacies of that explosion and found that, for all its gamma-ray fury, the BOAT was actually surprisingly ordinary, something we didn’t expect.
“It is not brighter than previous supernovae,” says astrophysicist Peter Blanchard from Northwestern University in the US.
“In the context of other supernovae associated with less energetic gamma-ray bursts (GRBs), it looks pretty normal. One might expect that the same collapsing star that produces a very energetic and bright GRB would also produce a very energetic and bright supernova. But it turns out that’s not the case. We have this extremely bright GRB, but a normal supernova.”
Gamma ray bursts are the most powerful explosions that exist in the cosmos. As the name suggests, they are bursts of gamma rays – the most energetic light in the universe – that can erupt in 10 seconds with as much energy as the Sun emits in 10 billion years.
We know of at least two major events that can produce a GRB: the formation of a black hole when a massive star goes supernova, or the hypernova that accompanies the merger of two neutron stars.
The types of novae that produce gamma-ray bursts are also believed to be responsible for the production of heavy elements in the universe. The thing is: heavy elements simply didn’t exist until the stars created them.
Stars form largely from the hydrogen gas that is abundant in the universe, but they smash atoms within their cores to create heavier elements. For iron, this is the highlight because the fusion of iron atoms consumes more energy than it produces.
However, elements heavier than iron can arise in the violent turmoil of a giant cosmic explosion. And we saw it! After neutron star collisions, scientists have discovered elements that are too heavy to form through nuclear fusion.
But there is a lot we don’t know. If we can narrow down which explosions are most likely to produce these elements, we will have a new tool for understanding not only how the universe makes things, but also how frequently such explosions occur.
So Blanchard and his colleagues naturally wanted to take a look at GRB 221009A to see if the light it emitted had signatures of heavy elements.
But they had to wait. The explosion was so bright that it practically blinded our instruments.
“The GRB was so bright that it obscured any possible supernova signature in the first weeks and months after the eruption,” explains Blanchard.
“At those times, the so-called afterglow of the GRB was like the headlights of a car coming straight at you, preventing you from seeing the car itself. So we had to wait until it faded significantly to give us a chance to see the supernova.”
It wasn’t until about six months after we first saw the explosion that researchers were able to use the James Webb Space Telescope to study the light in the infrared wavelength range. This allowed them to determine that the supernova itself was relatively normal. The reason it was so bright was probably because the beam from the gamma ray burst was aimed directly at Earth.
The researchers then combined the JWST data with radio observations from the Atacama Large Millimeter/submillimeter Array to look for specific wavelength ranges that indicate the presence of heavy elements. Although they found things like calcium and oxygen, which are fairly common in supernovae, there was no evidence of heavy element production.
Now, the speed at which neutron stars merge is not enough to produce the amount of heavy matter we see in the universe. Huge explosions like GRB 221009A were expected to contribute, but the absence of heavy elements suggests we were wrong.
Therefore, we need to look at other potential sources to see if we can identify the culprit, the researchers say.
“We did not see any signatures of these heavy elements, suggesting that extremely high-energy GRBs like the BOAT do not produce these elements,” says Blanchard.
“That doesn’t mean that all GRBs don’t produce them, but it is important information as we continue to understand where these heavy elements come from. Future observations with JWST will determine whether the BOAT’s “normal” cousins produce these elements.”
The results were published in Natural astronomy.