Massive star collapse confirmed as source of history's brightest gamma-ray burst

This groundbreaking discovery not only sheds light on the event itself but also opens up new avenues for understanding cosmic phenomena. The study detailing these findings is slated for publication in the prestigious journal Nature Astronomy on April 12.

Gamma-ray bursts are among the universe's most dramatic and violent events, often associated with the catastrophic end of massive stars. These bursts can release more energy in a few seconds than the sun will emit in its entire 10-billion-year lifespan. The GRB 221009A was no exception, offering a unique opportunity to study these cosmic giants.

Peter Blanchard, a postdoctoral fellow at Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), led the study. He explained the significance of the findings: "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," Blanchard said. "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. 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."

The detection of GRB 221009A was a feat in itself. When its light reached Earth on October 9, 2022, the burst was so powerful that it overwhelmed most of the world's dedicated gamma-ray detectors. Located approximately 2.4 billion light-years away in the constellation Sagitta, the burst's brightness and duration-a few hundred seconds-were off the charts.

Wen-fai Fong, an associate professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences and a member of CIERA, highlighted the burst's extraordinary nature: "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 Fong.

The intense brightness and energy of the event captured global attention among astronomers. "The event produced some of the highest-energy photons ever recorded by satellites designed to detect gamma rays," Blanchard noted. "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. It's so exciting to observe such a rare astronomical phenomenon as the B.O.A.T. and work to understand the physics behind this exceptional event."

Researchers utilized the JWST's Near Infrared Spectrograph to examine the aftermath of the GRB several months post-event, during the supernova's later stages. Initially, the sheer brilliance of the GRB masked any potential signs of a supernova. Blanchard compared the early detection challenges to facing oncoming headlights, which obscure visibility: "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."

Upon fading, the JWST revealed typical supernova signatures like calcium and oxygen. Interestingly, despite the GRB's extraordinary luminosity, the supernova appeared normal in brightness. "It's not any brighter than previous supernovae," said Blanchard. "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."

The absence of heavy elements in the supernova remains a puzzle. The rapid neutron capture process, or r-process, is known to produce heavy elements during cosmic events like neutron star mergers. However, the expected signatures were not found in this supernova, leading researchers to speculate on alternative mechanisms for heavy element formation.

Blanchard and his team are exploring the possibility that different types of massive star collapses might still contribute to the universe