When scientists detected a gamma-ray burst known as GRB 221009A on October 9, 2022, they called it the Brightest of All Time, or BOAT. Most gamma-ray bursts occur when the core of a star more massive than our Sun collapses and becomes a black hole. These events regularly release as much energy in a few minutes as our Sun releases in its entire lifetime.
Subsequent studies showed that GRB 221009A was 70 times brighter and much more energetic than the previous record holder. Although scientists don’t yet understand why, they have received an exciting clue from NASA’s NuSTAR (Nuclear Spectroscopic Telescope Array) observatory.
In a study published in the journal Science Advances, scientists used observations of the NuSTAR event to show how the collapsing star ejected a jet of material that had a shape not previously seen among gamma-ray bursts, as well as other unique properties. It is possible that the source of these differences is the progenitor star, whose physical properties could influence the characteristics of the explosion. It’s also possible that a completely different mechanism launches the brightest jets into space.
This event was so much brighter and more energetic than any gamma-ray burst we’ve seen before that it didn’t even come close,” said Brendan O’Connor, lead author of the new study and an astronomer at George Washington University in Washington. “Then 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 caused this event; It’s gone. But now we have some data to tell us how it exploded.”
Jumbo Jet
Gamma rays are the most powerful form of light in the universe, but are invisible to the human eye. All known gamma-ray bursts originate from galaxies outside our Milky Way, but are bright enough to be seen billions of light-years away. Some blink into existence lasting less than two seconds, while so-called long gamma-ray bursts typically emit gamma rays for a minute or more. These objects can emit other wavelengths for weeks at a time.
GRB 221009A, a long gamma-ray burst, was so bright that it effectively blinded most gamma-ray instruments in space. US scientists were able to reconstruct this event using data from NASA’s Fermi Gamma-ray Space Telescope to determine its true brightness. (The ship was also detected by NASA’s Hubble and James Webb space telescopes, the agency’s Wind and Voyager 1 spacecraft, as well as ESA’s, or European Space Agency’s, Solar Orbiter.)
Similar to other gamma-ray bursts, GRB 221009A had a jet that erupted from the collapsing star as if it had been fired into space from a fire hose, with gamma rays emanating from the hot gas and particles in the core of the jet. But the GRB 221009A jet stood out in several ways. In almost every previously observed gamma-ray burst, the jet remained remarkably compact, with little or no scattered light or material outside the narrow beam. In contrast, in GRB 221009A, the jet had a narrow core with wider, sloping sides.
Some of the most energetic gamma-ray bursts showed similar properties, but the BOAT jet was unique in one important way: The energy of the material in GRB 221009A also varied, meaning that instead of all the material in the jet having the same energy—like a single bullet fired from the gun – the energy of the material varied with the distance from the nozzle core. This has never been seen before in a long gamma ray burst.
“The only way to create a different jet structure and change the energy is to change some of the properties of the star that exploded, such as its size, mass, density or magnetic field,” said Eleonora Troja, professor of physics at the University of Rome, who led the NuSTAR observations of this event. “That’s because the jet basically has to force itself out of the star. So, for example, the amount of drag it encounters would potentially affect the characteristics of the jet.”
Footprints in the snow
Astronomers can see the light from gamma-ray bursts, but the distance means they can’t distinguish images of the bursts directly. Scientists must interpret the light from these events to learn about the physical properties of distant objects. It’s like looking at footprints in the snow and inferring something about the physical characteristics of the person who left them.
In many cases, there may be more than one possible explanation for light from a cosmic event. More than one X-ray telescope observed GRB 221009A, including NASA’s Neil Gehrels Swift Observatory and the Neutron Star Interior Composition Explorer (NICER), as well as ESA’s XMM-Newton telescope.
NuSTAR data helped narrow down those possibilities. It shows that as the jet traveled into space, it collided with the interstellar medium, or the rarefied sea of atoms and particles that fills the space between stars. This collision created X-rays – particles of light slightly less energetic than gamma rays.
“There are several X-ray telescopes operating in space, each with a different power that can help astronomers better understand these cosmic objects,” said Daniel Stern, NuSTAR project scientist at NASA’s Jet Propulsion Laboratory in Southern California.