Brightest gamma-ray burst of all time aids search for origin of heavy elements

The brightest gamma-ray burst (GRB) ever recorded, resulting from the collapse and subsequent explosion of a massive star, was first detected by space observatories in October 2022. Now, an international research team that includes Penn State astronomers has used NASA's James Webb Space Telescope (JWST) to investigate the historic burst—GRB 221009A, dubbed the BOAT, or "brightest of all time"—to investigate whether the explosion, or supernova, could be a birthplace of heavy elements.

The research is described in a paper appearing today (April 12) in the journal Nature Astronomy.

"A major question in astronomy is where the heaviest elements in the universe originally came from," said Joel Leja, assistant professor of astronomy and astrophysics and co-hire of the Institute for Computational and Data Sciences at Penn State and a member of the research team. "They are too heavy to produce in the typical fusion in stars. One suggestion is that they are born in energetic winds powered by the collapse of massive stars. Investigating the supernova from GRB 221009A gave us a unique opportunity to explore this hypothesis."

The researchers speculated that evidence of heavy elements, such as platinum and gold, might reside within the newly uncovered supernova. The extensive search, however, did not find the signature that accompanies such elements.

"We did not see signatures of these heavy elements, suggesting that extremely energetic GRBs like the BOAT do not produce these elements," said Peter Blanchard, postdoctoral fellow at Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and leader of the study. "That doesn't mean that all GRBs 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 BOAT's 'normal' cousins produce these elements."

Birth of the BOAT and its aftermath

The initial, intense burst of gamma radiation from the BOAT triggered several detectors on space observatories, including Neil Gehrels Swift Observatory, whose Mission Operations Center is located at Penn State, on Oct. 9, 2022. The star that exploded is located approximately 2.05 billion light-years away from Earth in the direction of the constellation Sagitta, and the explosion lasted a few hundred seconds in duration. The burst's afterglow, which is created as X-rays, gamma rays and other particles are launched into space, was at least ten times brighter than that of previously observed gamma-ray bursts.

The exploding star also propelled debris and other matter into space in a supernova, and the researchers were able to use JWST to examine the supernova about six months after the initial burst.

"The GRB was so bright that it obscured any potential supernova signature in the first weeks and months after the burst," Blanchard said. "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."

The research team used JWST's Near Infrared Spectrograph to observe the object's light at infrared wavelengths. This revealed the characteristic signature of elements like calcium and oxygen typically found within a supernova. Surprisingly, the researchers said, it wasn't exceptionally bright—like the incredibly bright GRB that it accompanied.

"There had been some debate as to whether this supernova actually occurred because it was particularly hard to detect through the afterglow, but we also confirmed the presence of the supernova," Leja said. "It is a little counterintuitive that an explosion that produced such a luminous gamma-ray bust produced such an unremarkable supernova, and the team is still investigating why this might be the case."

Missing: Heavy elements

After confirming the presence of the supernova, the research team searched for evidence of heavy elements. While lighter elements such as hydrogen and helium were created during the Big Bang, and elements lighter than iron are produced in large amounts by fusion in the cores of stars, astrophysicists have an incomplete picture of all the mechanisms in the universe that can produce elements heavier than iron.

The primary mechanism for producing heavy elements, 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 are simply too many heavy elements in the universe and too few neutron star mergers.

"Some of oldest stars have heavy metals, which suggest that parts of the universe contained heavy metals before the earliest binary neutron star mergers," Leja said. "So there must be another source of heavy metals other than neutron star mergers."

Astrophysicists have hypothesized that heavy elements also might be produced by the collapse of a rapidly spinning, massive star—the exact type of star that generated the BOAT. Using the infrared spectrum obtained by the JWST, the team studied the inner layers of the supernova, where the heavy elements should be formed. Although the star's material is opaque immediately following its explosion, so only the outer layers are visible, the researchers said that the material becomes transparent as it expands and cools. This allows researchers to see the photons within the inner layer of the supernova.

"Different elements absorb and emit photons at different wavelengths, depending on their atomic structure, giving each element a unique spectral signature," Blanchard explained. "Therefore, looking at an object's spectrum can tell us what elements are present. Upon examining the BOAT'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."

The research team also investigated the spectrum of the BOAT's host galaxy, which might help explain why such an explosion that produced a record-breaking GRB also produced such a "normal" supernova. This analysis, led by Penn State graduate student Yijia Li, revealed that the 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 BOAT that may help explain its properties," Li said. "The energy released in the BOAT was completely off the charts, one of the most energetic events humans have ever seen. The fact that it also appears to be born out of nearly primordial gas may be an important clue to understanding its superlative properties."

The host galaxy's spectrum also shows signs of modest star formation as well as unique patterns of hydrogen emission that have not been observed in other GRB host galaxies. While star formation may explain some of the hydrogen emission, it is possible the BOAT impacts this as well. Thus, the research suggests that the environment in which the star that exploded was birthed could be different than other environments that resulted in gamma ray bursts.

"Extraordinary cosmic events like the BOAT offer unique opportunities to test hypotheses about the extreme physics of the cosmos," Leja said. "Not finding any evidence of heavy metal creation in this object is in some ways more exciting—these metals must be produced somewhere very energetic, and early, and quickly. If not events like the incredible BOAT, then where? What high-energy mysteries are we missing? Perhaps the JWST will catch something even more spectacular soon."

More information:
Peter K. Blanchard et al, JWST detection of a supernova associated with GRB 221009A without an r-process signature, Nature Astronomy (2024). DOI: 10.1038/s41550-024-02237-4

Provided by Pennsylvania State University