With NuSTAR, scientists reveal new characteristics of the brightest gamma-ray burst ever detected

by Haygen Warren

On Oct. 9, 2022, NASA’s Fermi Gamma-ray Space Telescope, Neil Gehrels Swift Observatory, and Wind spacecraft detected the single brightest gamma-ray burst ever observed. The burst, named GRB 221009A, was 70 times brighter than the previous record holder and far more energetic than any other gamma-ray burst ever detected. However, scientists are still unsure how such an energetic burst could have been created.

Since its detection, GRB 221009A has been extensively studied by several teams of scientists across the world using data collected by various spacecraft, both on the ground and in space. With data collected by NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) X-ray observatory of GRB 221009A, a team of scientists is learning more about the gamma-ray burst’s origins and characteristics.

“This event was so much brighter and more energetic than any gamma-ray burst we’ve seen before, it’s not even close,” said lead author Brendan O’Connor of George Washington University.

Gamma-ray bursts are among the most energetic phenomena to occur in the universe. The bursts are produced when the core of a massive star — or any star with a mass that is greater than eight times the mass of our Sun — collapses inward, creating a black hole. The bursts of energy produced from the collapse are called gamma-ray bursts and often emit more energy in just a few minutes than our Sun will emit in its entire lifetime.

Every gamma-ray burst ever observed has occurred outside of our galaxy and is located billions of light-years away, yet can still be easily observed with telescopes and other spacecraft. What’s more, gamma-ray bursts can last for less than two seconds, while others last for up to a minute or more. Following the initial burst, their light will radiate at different wavelengths for several weeks at a time.

While normal gamma-ray bursts are bright, GRB 221009A was so incredibly bright that it blinded nearly all gamma-ray detection instruments currently in space — keeping them from fully observing the event. Using data from Fermi, scientists were able to reconstruct the event and determine the actual brightness of the burst.

As mentioned, several different observatories detected GRB 221009A, including NuSTAR. All of the observatories gathered similar data, but NuSTAR revealed something about GRB 221009A that was not seen by other observatories — a jet of material that is uniquely different than other gamma-ray burst jets. While O’Connor et al. are still unsure of how the jet gained its unique characteristics, they believe that the physical characteristics of the burst’s progenitor star could have had an effect on the jet’s appearance.

“When we analyzed the NuSTAR data, we realized that it also has this unique jet structure, and that was really exciting because we have no way of studying the star that produced this event; it’s gone now. But, we now have some data giving us clues about how it exploded,” O’Connor said.

In most gamma-ray bursts, the jets produced are not compact and often have lines of stray material moving away from them. GRB 221009A’s jet, though, was extraordinarily compact and had very little stray material/light outside of the jet. Additionally, GRB 221009A’s jet had a very narrow core with wide and sloping edges.

The infrared afterglow of GRB 221009A (circled in purple) as seen by the Hubble Space Telescope. (Credit: NASA/ESA/CSA/STScI/A. Levan (Rodan University)/Gladys Kober)

Many of the most energetic gamma-ray bursts ever detected have shared characteristics with GRB 221009A. However, one characteristic set GRB 221009A apart from the rest — the energy of the material within GRB 221009A’s jet varied as the distance from the jet’s core increased. This phenomenon has never been observed in a gamma-ray burst before, as the material within the jets of most gamma-ray bursts maintains the same energy throughout the entire jet.

As mentioned, O’Connor et al. believe that the physical characteristics of the progenitor star may have something to do with GRB 221009A’s unique characteristics.

“The only way to produce a different jet structure and vary the energy is to vary some property of the star that exploded, like its size, mass, density, or magnetic field. That’s because the jet has to basically force its way out of the star. So, for example, the amount of resistance it meets would potentially influence the features of the jet,” said Eleonora Troja of the University of Rome, who led NuSTAR’s observations of GRB 221009A.

The gamma-ray emmission of GRB 221009A compared to previous record holders. (Credit: Goddard Space Flight Center/Adam Goldstein (USRA))

While scientists are able to observe the light from gamma-ray bursts, their immense distance from Earth means that scientists are never able to resolve clear images of the events and the jets produced by the jets. Scientists are able to pull information about the physical characteristics of the bursts by analyzing and interpreting the light they see from the bursts.

Not all bright sources of light seen by telescopes are gamma-ray bursts, but by having telescopes like NuSTAR, Swift, and the Neutron Star Interior Composition Explorer collecting data on a bright event, scientists are able to narrow down the possibilities for how the light source was produced. In the case of GRB 221009A, NuSTAR’s data helped scientists determine that the light seen from GRB 221009A was in fact produced by a gamma-ray burst.

“There are multiple X-ray telescopes operating in space, each with different strengths that can help astronomers understand these cosmic objects better,” said NuSTAR project scientist Daniel Stern of NASA’s Jet Propulsion Laboratory.

O’Connor et al.’s results were published on June 7 in the journal Science Advances.

(Lead image: Artist’s depiction of a gamma-ray burst’s jet. Credit: NASA’s Goddard Space Flight Center)

Related Articles