On October 9, 2022, the brightest gamma-ray burst (GRB) ever recorded – dubbed GRB 221009A – swept through our solar system, lasting over 300 seconds, and overwhelming gamma-ray detectors on several orbiting satellites. To study this unprecedented event and better understand the origin of such extreme cosmic explosions, astronomers all over the world have used the most powerful telescopes in the world, such as the Submillimeter Array (SMA) in Hawaii.
According to the experts, such long-duration GRBs are the “birth cries” of black holes that are formed when the core of a massive and rapidly spinning star collapses under its own weight. Following such an event, the newborn black hole emits powerful jets of plasma at near the speed of light, piercing through the collapsing star and shining in gamma-rays. An important question astronomers struggled to answer was what happened after this initial burst of gamma-rays.
“As the jets slam into gas surrounding the dying star, they produce a bright ‘afterglow’ of light across the entire spectrum,” explained lead author Tanmoy Laskar, an assistant professor of Physics and Astronomy at the University of Utah. “The afterglow fades quite rapidly, which means we have to be quick and nimble in capturing the light before it disappears, taking its secrets with it.”
“This burst, being so bright, provided a unique opportunity to explore the detailed behavior and evolution of an afterglow with unprecedented detail,” added co-author Edo Berger, a professor of Astronomy at Harvard University.
The analysis of data from SMA and other telescopes all over the globe revealed that the millimeter and radio wave measurements were much brighter than those expected based on the visible and X-ray light. “A few GRBs in the past have shown a brief excess of millimeter and radio emission that is thought to be the signature of a shockwave in the jet itself, but in GRB 221009A the excess emission behaves quite differently than in these past cases,” said co-author Yvette Cendes, an astrophysicist at Harvard.
One possible explanation for this surprising phenomenon is that the powerful jet produced by GRB 221009A is more complex than in other GRBs, with the visible and X-ray light being produced by one portion of the jet, and the early millimeter and radio waves by a different component.
“Luckily, this afterglow is so bright that we will continue to study its radio emission for months and maybe years to come. With this much longer time span we hope to decipher the mysterious origin of the early excess emission,” Berger concluded.
The study is published in the Astrophysical Journal Letters.
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