Gamma-Ray Burst Record Breaker Raises Questions on Element Formation Theories

An artist's depictation of GRB 221009A reveals the restricted relativistic jets originating from a central black hole, responsible for the GRB and the evolving leftover of the prime star ejected due to the supernova explosion. Peter Blanchard, a postdoctoral fellow at Northwestern University, with his team employed the James Webb Space Telescope to locate the supernova for the first time, supporting the notion that the collapse of a sizable star resulted in GRB 221009A. The event took place in a densely packed star-forming area of its host galaxy, shown by the backdrop nebula. (Image credit: Aaron M. Geller/Northwestern/CIERA/IT Research Computing and Data Services)

In 2022, specifically in October, a worldwide team of researchers that included Northwestern University astrophysicists, observed the most luminous gamma-ray burst (GRB) on record, namely GRB 221009A.

A team, led by Northwestern, has now substantiated the occurrence responsible for this historic outburst - known as the B.O.A.T (“brightest of all time”) - as the collapse and subsequent explosion of a heavy star. The James Webb Space Telescope (JWST) of NASA was employed by the team to spot the explosion or supernova.

Despite this discovery that solved one puzzle, a new mystery emerged.

The researchers theorized that proof of heavy elements like platinum and gold might be found in the newly discovered supernova. However, the meticulous search didn't reveal the signature concomitant with these elements. Hence, the source of heavy elements in the cosmos remains as one of the major unsettled questions in astronomy.

The research findings were published on April 12 in Nature Astronomy.

Northwestern’s Peter Blanchard, the study’s leader, stated, "We were unable to find traces of these heavy elements, inferring that extremely energetic GRBs like the B.O.A.T. might not form these elements. Yet, this doesn't imply that all GRBs don't form these elements. This discovery is crucial concerning the understanding of the origin of these heavy elements. Further observations with JWST will determine if the ‘normal’ cousins of the B.O.A.T. form these elements."

The authors of the study are associated with various institutions around the globe.

Ashley Villar of Harvard University and the Center for Astrophysics | Harvard & Smithsonian, the second author of the study, stated, "If correct, the B.O.A.T. should have been a treasure trove. It's quite astonishing that we didn’t see any evidence for these heavy elements."

On Oct. 9, 2022, the B.O.A.T.'s light reached Earth and was so intense that it saturated majority of the world’s gamma-ray detectors. This violent explosion took place approximately 2 billion light-years away from Earth, in the direction of the constellation Sagitta. The phenomenon's brightness was such that it left astronomers in awe.

The event resulted in some of the highest-energy photons ever registered by satellites designed for detecting gamma rays, stated Blanchard. "We are fortunate enough to witness such a spectacular astronomical occurrence as the B.O.A.T. and to comprehend the exceptional physics of this event," and added, "This kind of event only happens once every 10,000 years."

Blanchard, Villar, and their colleagues decided to examine the GRB during its later stages, rather than right after the event. Approximately six months post the initial detection of the GRB, Blanchard and Villar used the JWST to examine the aftermath of the GRB.

Blanchard stated, "The GRB was so intense that any possible signature of a supernova was masked during the initial weeks and months after the explosion. At these points, the afterglow of the GRB resembled the headlights of an oncoming car, blocking your view of the car itself. For us to observe the supernova, we had to wait for the glare to significantly decrease."

Villar commented, "We were lucky that the JWST had recently been launched and was able to carry out these observations. The B.O.A.T. was positioned behind the Milky Way, and its dust completely obstructed the blue light we would typically observe. The JWST can see through this dust, offering us a remarkable view in the infrared."

The team utilized the Near Infrared Spectrograph of the JWST to find the unique signature of elements such as calcium and oxygen often seen within a supernova. Unexpectedly, it was not exceptionally bright in comparison to the extraordinarily bright GRB it was accompanied by.

According to Blanchard, "The brightness was not extraordinary compared to previous supernovae. Considering other supernovae that are associated with less energetic GRBs, it appeared fairly normal. One might assume that the same collapsing star that produced an incredibly bright and energetic GRB would also produce a supernova of the same intensity. However, that was not the case. Despite having an exceptionally bright GRB, the supernova was ordinary."

Once the presence of the supernova was confirmed for the first time, Blanchard and his team began searching for indications of heavy elements within it. Currently, our understanding of all the universe's mechanisms that produce elements heavier than iron is incomplete.

The rapid neutron capture process, the primary mechanism for creating heavy elements, needs a high concentration of neutrons. Up to now, the manufacturing of heavy elements via this procedure has only been confirmed in the merger of two neutron stars, a collision detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2017. However, scientists believe that there must be other methods for creating these elusive materials due to the large number of heavy elements in the universe compared to the small number of neutron-star mergers.

Blanchard suggested, "There's probably another source out there. Binary neutron stars merging is a process that takes a very long time. Two stars in a binary system first need to explode to result in neutron stars. Then, the two neutron stars coming together can take billions of years. However, assessments of extremely old stars show that parts of the universe were enriched with heavy metals before most binary neutron stars would've had a chance to merge. This leads us to believe an alternate channel exists."

"We are fortunate to live in a time when we are technologically equipped to detect these bursts happening throughout the universe.” claimed Peter Blanchard, CIERA Postdoctoral Fellow.

Blanchard and his team used infrared spectrum obtained from the JWST to study the internal layers of the supernova, where the heavy elements should be formed. They hypothesized that heavy elements also could be produced by the collapse of a rapidly spinning, massive star, similar to the star that generated the B.O.A.T.

Blanchard described, "The exploded star material is opaque initially, limiting our view to the outer layers. Once it cools and expands, it becomes clear, and we can see the photons emanating from the inner layer of the supernova."

"Interestingly, different elements absorb and emit photons at varying wavelengths, based on their atomic structure, which gives each one a distinct spectral signature. Therefore, looking at an object's spectrum can inform us about the elements present in it. However, when examining the B.O.A.T.'s spectrum, we did not detect any heavy elements, suggesting events such as GRB 221009A are not primary sources. This discovery is critical as we continue our efforts to identify where the heaviest elements originate."

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

"Even a few months after the discovery of the burst, the afterglow was bright enough to significantly contribute to the JWST spectra," detailed Tanmoy Laskar, an assistant professor of physics and astronomy at the University of Utah and a co-author on the study. "Blending data from the two telescopes allowed us to accurately measure the brightness of the afterglow at the time of our JWST observations and enabled us to meticulously 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 said 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 said. “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 also 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 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 energy released in the B.O.A.T. 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 near-primordial gas may be an important clue to understanding its superlative properties.”

Reference: “JWST detection of a supernova associated with GRB 221009A without an r-process signature” by Peter K. Blanchard, V. Ashley Villar, Ryan Chornock, Tanmoy Laskar, Yijia Li, Joel Leja, Justin Pierel, Edo Berger, Raffaella Margutti, Kate D. Alexander, Jennifer Barnes, Yvette Cendes, Tarraneh Eftekhari, Daniel Kasen, Natalie LeBaron, Brian D. Metzger, James Muzerolle Page, Armin Rest, Huei Sears, Daniel M. Siegel and S. Karthik Yadavalli, 12 April 2024, Nature Astronomy. DOI: 10.1038/s41550-024-02237-4

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.