On October 8, 2022, an intense pulse of gamma rays swept through our solar system. The source, named GRB 221009A because of its date of discovery, turned out to be the brightest gamma-ray burst (GRB) ever recorded.

GRB 221009A lasted over 300 seconds. According to astronomers, such “long-lasting” GRBs are the birth cry of a black hole, formed when the core of a massive and rapidly spinning star collapses under its weight. Strong plasma jets are shot out of the developing black hole at near-light speeds, pierce the collapsing star and shine in gamma rays.

The enigma of GRB 221009A, the brightest burst ever observed, was what would happen after the first gamma-ray burst.

When the jets collide in the gas around the dying star, they produce a bright afterglow. This afterglow fades quite quickly, meaning we have to be quick and agile in catching the light before it disappears and takes its secrets with it.

Astronomers Edo Berger and Yvette Cendes of the Center for Astrophysics (CfA) quickly gathered data with the SMA as part of a campaign to use the best radio and millimeter telescopes in the world to analyze the afterglow of GRB 221009A.

Edo Berger, professor of astronomy at Harvard University and the CfA, said: “This burst, being so bright, presented a unique opportunity to examine the detailed behavior and evolution of an afterglow in unprecedented detail – we didn’t want to miss it!”

SMA project scientist and CfA researcher Garrett Keating said: “I have been studying these events for over twenty years, and this one was as exciting as the first GRB I ever observed. The rapid response capability allowed us to quickly direct the SMA to the location of GRB 221009A. The team was thrilled to see how bright this GRB’s afterglow was, which we were able to track for more than ten days as it faded.

Astronomers were stunned as they combined and analyzed data from the SMA and other observatories around the world. They found that the millimeter and radio wave measurements were significantly brighter than expected based on the visible and X-ray light.

CfA research associate Yvette Cendes said: “This is one of the most detailed datasets we’ve ever collected, and it’s clear that the millimeter and radio data just isn’t behaving as expected. A few GRBs in the past have shown a momentary excess of millimeters and radio emission believed to be the signature of a shock wave in the jet itself. Yet the excess emission in GRB 221009A behaves very differently than in these earlier cases.”

“It is likely that we have discovered a completely new mechanism to produce excess millimeter and radio waves.”

“One possibility is that the powerful jet produced by GRB 221009A is more complex than in most GRBs. It is possible that the visible and X-ray light is produced by one part of the jet, while the early millimeter and radio waves are produced by another part.

Berger said, “Fortunately, this afterglow is so bright that we will continue to study its radio emissions for months, if not years. With this much longer time span, we hope to decipher the mysterious origins of the early excess emissions.”

“Regardless of the exact details of this particular GRB, the ability to quickly respond to GRBs and similar events with millimeter-wave telescopes is an essential new capability for astronomers.”

“An important lesson from this GRB is that without fast-acting radio and millimeter telescopes, such as the SMA, we would miss out on potential discoveries about the most extreme explosions in the universe. We never know in advance when such events will occur, so we need to act as quickly as possible if we want to take advantage of these gifts from the cosmos.”

Magazine reference:

  1. Eric Burns (Louisiana State University). Focus on the ultra-bright gamma-ray burst GRB 221009A. The Astrophysical Journal Letters.