The extraordinary cause of the brightest gamma-ray burst of all time

In October 2022, a group of scientists, including researchers from Northwestern University, witnessed the brightest gamma-ray burst (GRB) ever seen, labeled GRB 221009A.

Now, a team led by Northwestern has confirmed that the cause of this extraordinary burst, nicknamed the B.O.A.T. ("brightest of all time"), is the collapse and explosion of a massive star. Using NASA’s James Webb Space Telescope (JWST), the team spotted the explosion, known as a supernova.

While this discovery addresses one mystery, it also deepens another. The researchers speculated that heavy elements like platinum and gold might be present in the supernova, but their extensive search didn’t find the expected signature. The origin of heavy elements in the universe remains a significant unanswered question in astronomy.

The findings of this research are published in the journal Nature Astronomy.

"When we confirmed that the GRB 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," said Peter Blanchard from Northwestern, who led the study. "We did not see signatures of these heavy elements, suggesting that extremely energetic GRBs like the B.O.A.T. do not produce these elements."

Blanchard, a postdoctoral fellow at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), worked with a team from various institutions including the Center for Astrophysics | Harvard & Smithsonian, University of Utah, Penn State, University of California, Berkeley, Radbound University in the Netherlands, Space Telescope Science Institute, University of Arizona/Steward Observatory, University of California, Santa Barbara, Columbia University, Flatiron Institute, University of Greifswald, and the University of Guelph.

When the light from GRB 221009A reached Earth on October 9, 2022, it was so intense that it overwhelmed most gamma-ray detectors worldwide. This powerful explosion occurred around 2.4 billion light-years away, in the direction of the constellation Sagitta, lasting several hundred seconds. Astronomers were in awe as they scrambled to observe this exceptionally bright event.

"As long as we have been able to detect GRBs, there is no question that this GRB is the brightest we have ever witnessed by a factor of 10 or more," said Wen-fai Fong, a physics and astronomy professor at Northwestern’s Weinberg College of Arts and Sciences and a member of CIERA.

"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."

A ‘normal’ supernova

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Instead of immediately observing the event, Blanchard and his team chose to study the GRB during its later stages. About six months after its detection, Blanchard used the JWST to examine its aftermath.

"The GRB was so bright that it obscured any potential supernova signature in the first weeks and months after the burst," Blanchard explained. "At these 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 for it to fade significantly to give us a chance of seeing the supernova."

Using the JWST’s Near Infrared Spectrograph, Blanchard observed the object’s light at infrared wavelengths and identified the characteristic signatures of elements like calcium and oxygen typically found within a supernova. Surprisingly, it wasn’t exceptionally bright, unlike the incredibly bright GRB that accompanied it.

"It’s not any brighter than previous supernovae," Blanchard noted. "It looks fairly normal in the context of other supernovae associated with less energetic GRBs. You might expect that the same collapsing star producing 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 luminous GRB, but a normal supernova."

Missing: Heavy elements

After confirming the presence of the supernova, Blanchard and his team searched for evidence of heavy elements within it. Astrophysicists have an incomplete understanding of the mechanisms in the universe that can produce elements heavier than iron.

The primary mechanism, the rapid neutron capture process, requires a high concentration of neutrons. So far, astrophysicists have only confirmed the production of heavy elements via this process in the merger of two neutron stars, a collision detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2017.

But scientists say there must be other ways to produce these elusive materials.

"There is likely another source," Blanchard suggested. "It takes a very long time for binary neutron stars to merge. Two stars in a binary system first have to explode to 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. But observations of very old stars indicate that parts of the universe were enriched with heavy metals before most binary neutron stars would have had time to merge. That’s pointing us to an alternative channel."

Astrophysicists have theorized that heavy elements might also be produced by the collapse of a rapidly spinning, massive star — the type of star that generated the B.O.A.T. Blanchard studied the inner layers of the supernova using the infrared spectrum obtained by the JWST, where the heavy elements should be formed.

"The exploded material of the star is opaque at early times, so you can only see the outer layers," Blanchard explained. "But once it expands and cools, it becomes transparent. Then you can see the photons coming from the inner layer of the supernova."

"Moreover, different elements absorb and emit photons at different wavelengths, depending on their atomic structure, giving each element a unique spectral signature," Blanchard added. "Therefore, looking at an object’s spectrum can tell us what elements are present. Upon examining the B.O.A.T.’s spectrum, we did not see any signature of heavy elements, suggesting extreme events like GRB 221009A are not primary sources.

This is crucial information as we continue to try to pin down where the heaviest elements are formed."

To distinguish the light of the supernova from the bright afterglow that preceded it, the researchers combined JWST data with observations from the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile.

"Even several months after the burst was discovered, the afterglow was bright enough to contribute a lot of light in the JWST spectra," said Tanmoy Laskar, an assistant professor of physics and astronomy at the University of Utah and a co-author on the study. "Combining data from the two telescopes helped us measure exactly how bright the afterglow was at the time of our JWST observations and allow us to carefully extract the spectrum of the supernova."

Although astrophysicists have yet to uncover how a "normal" supernova and a record-breaking GRB were produced by the same collapsed star, Laskar suggested it might be related to the shape and structure of the relativistic jets. When rapidly spinning, massive stars collapse into black holes, they produce jets of material that launch at rates close to the speed of light. If these jets are narrow, they produce a more focused — and brighter — beam of light.

"It’s like focusing a flashlight’s beam into a narrow column, as opposed to a broad beam that washes across a whole wall," Laskar explained. "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."

Additional clues may come from future studies of the galaxy in which the B.O.A.T. occurred. "In addition to a spectrum of the B.O.A.T. itself, we also obtained a spectrum of its 'host' galaxy," Blanchard said. "The spectrum shows signs of intense star formation, hinting that the birth environment of the original star may be different than previous events."

Team member Yijia Li, a graduate student at Penn State, modeled the spectrum of the galaxy, finding that the B.O.A.T.’s host galaxy has the lowest metallicity, a measure of the abundance of elements heavier than hydrogen and helium, of all previous GRB host galaxies. “This is another unique aspect of the B.O.A.T. that may help explain its properties,” Li said. 

The study, “JWST detection of a supernova associated with GRB 221009A without an r-process signature,” was supported by NASA (award number JWST-GO-2784) and the National Science Foundation (award numbers AST-2108676 and AST-2002577). This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope.

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