'Brightest of All Time' gamma-ray burst caused by exploding star, say scientists

Scientists at Northwestern University said Friday they had confirmed that the cause of the brightest gamma-ray burst ever observed (the circled infrared afterglow and host galaxy pictured 1-2 months afterward) was the destruction of a star, known as a supernova, 2.4 billion light years away. File photo courtesy NASA, ESA,CSA, STScl, A. Levan.

April 12 (UPI) -- Scientists at Northwestern University said Friday they had confirmed that the cause of the brightest gamma-ray burst ever observed in October 2022 was the destruction of a star, known as a supernova, 2.4 billion light years from Earth.

The team discovered that the phenomenon responsible for the historic burst of electromagnetic radiation -- dubbed the "brightest of all time" -- was the collapse and subsequent explosion of a massive star using NASA's James Webb Space Telescope, Northwestern said in a news release.

Scientists had been unable to uncover the cause until now because the once-in-10,000-years event was so bright, 10 times brighter than any previously seen gamma-ray burst.

But they were able to detect the supernova behind the burst by studying its aftermath with the JWST, about six months afterward, explained study lead Peter Blanchard.

"The GRB was so bright that it obscured any potential supernova signature in the first weeks and months after the burst. 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," he said.

Blanchard was able to observe the signature left by calcium and oxygen elements that occur within supernovas by viewing light at the infrared end of the light spectrum via JWST's Near Infrared Spectrograph and found to his surprise that it was nowhere as bright as expected given the GRB emanating from it.

"It's not any brighter than previous supernovae. It looks fairly normal in the context of other supernovae associated with less energetic GRBs," Blanchard said. "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 co-authors from the Harvard & Smithsonian Center for Astrophysics, Utah University, Penn State; University of California, the Netherlands' Radbound University, Space Telescope Science Institute, Arizona University/Steward Observatory, Columbia University, Flatiron Institute, Greifswald University and Guelph University were able to separate out supernova light from that of the bright afterglow before it by pairing JWST data with observations from the Atacama Large Millimeter/Submillimeter Array in Chile.

"Even several months after the burst was discovered, the afterglow was bright enough to contribute a lot of light in the JWST spectra," said Utah University Physics and Astronomy associate professor Tanmoy Laskar. "Combining data from the two telescopes helped us measure exactly how bright the afterglow was at the time of our JWST observations and allow us to carefully extract the spectrum of the supernova."

While the astrophysicists have yet to uncover how a "normal" supernova and a record-breaking GRB were produced by the same collapsed star, Laskar said it might be related to the shape and structure of the relativistic jets produced when massive, rapidly spinning stars collapse into black holes.

Ejected at rates close to the speed of light the narrower the jet, the more focused and brighter the beam of light it produces.

"It's like focusing a flashlight's beam into a narrow column, as opposed to a broad beam that washes across a whole wall," Laskar said. "In fact, this was one of the narrowest jets seen for a gamma-ray burst so far, which gives us a hint as to why the afterglow appeared as bright as it did. There may be other factors responsible as well, a question that researchers will be studying for years to come."

Modeling of the spectrum of B.O.A.T.'s host galaxy by Penn State graduate student Yijia Li found the lowest metallicity, a measure of the abundance of elements heavier than hydrogen and helium, of all previous GRB host galaxies.

"This is another unique aspect of the B.O.A.T. that may help explain its properties," Li said.

However, while the breakthrough resolved the main mystery it opened up a whole other set of paradoxes after the researchers were unable to detect evidence of the presence of platinum and gold, heavy elements they had expected to find despite an almost two-year-long search.

"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," said Northwestern's Blanchard.

"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. Future observations with JWST will determine if the B.O.A.T.'s 'normal' cousins produce these elements."

NASA confirmed in March last year that astronomers had concluded the "monstrous" October 2022 burst that lit up our galaxy was the brightest ever seen and a once-in-10,000-years explosion.

Astronomers had been studying the extragalactic burst for months after it triggered detectors on numerous spacecraft before calling it as the brightest GRB in human history.

"It is just an absolutely monstrous burst. It is extremely extraordinary; we've never seen anything remotely close to it," Eric Burns, assistant professor of physics and astronomy at Louisiana State University, told reporters at the 20th meeting of the American Astronomical Society's High Energy Astrophysics Division in Hawaii.

"The BOAT is a once-in-10,000-year event," Burns added. "So, there's a reasonable chance this is the brightest gamma-ray burst to hit Earth since human civilization began."