Gamma-ray bursts occur when massive stars collapse or collide.
NASA Goddard Space Flight Center/ A. Simonett, Sonoma State University
The most powerful explosion astronomers have ever seen contains a mysterious signal they thought couldn’t exist. The signal provides the first detailed look inside a gamma-ray burst and suggests it involves the annihilation of matter and antimatter.
Gamma-ray bursts (GRBs) are the most powerful outbursts of radiation in the universe, produced by cosmic explosions and collisions. Physicists believe that the most energetic GRBs are produced when a star collapses to form a black hole. The black hole produces a jet of material traveling close to the speed of light, penetrating the collapsing star and emitting an explosion of radiation that can be observed on Earth. However, we still don’t know how this radiation is produced or what is contained in the jet.
Much of this mystery arises from the spectrum of light we can see: while the light observed from other objects in the universe contains characteristic spikes that tell us about the specific atoms or other matter that produced this burst of energy, the spectrum of light from a gamma ray burst is always smooth and featureless.
In the 1990s, researchers became excited about the possibility that some GRBs might show distinct lines, but careful analysis showed that these were statistical errors and concluded that GRB spectra could not possibly be spike-like.
now, Maria Ravasio Researchers from Radboud University in the Netherlands and their colleagues have discovered that GRB221009A, discovered in 2022 and dubbed the most luminous explosion since the Big Bang, actually has an energy peak of about 10 megaelectronvolts.
“When I first saw the lines, I thought we’d done something wrong,” Ravasio says. But after detailed statistical analysis and ruling out any instrument problems, Fermi Gamma Ray Space Telescope Ravasio and his colleagues concluded that the spectral spike was real: “When I realized it wasn’t a false alarm, I got goosebumps because I realized something big had happened.”
Nearly all GRBs exhibit a similar energy distribution, so astronomers analyze new GRB detections using the data analysis method that best suits this pattern. But Ravasio and her team instead used a method that allows for peaks, and found that this fit the data better. “That part of the GRB spectrum has been the same for years, and no one had looked at it,” Ravasio says. [GRB221009A] We can now look at that part of the spectrum better.”
This peak points to a specific physical process behind GRBs that is missing from the best models of GRBs.
To zero in on what this could be, Ravasio and his colleagues worked under the assumption that because the jet’s energy was so high, there were no intact atoms in it. This left one plausible explanation: the annihilation of an electron and its antimatter counterpart, a positron. Such an annihilation produces gamma rays with a distinct peak at 511 kiloelectron volts. “This already tells us the composition of the jet, which is something we haven’t understood since the first GRB,” Ravasio says.
The higher 10 MeV peak that the researchers observed was due to a shift in the energy spectrum caused by the high-speed jet producing the radiation, similar to how the siren of an approaching ambulance sounds higher-pitched.
This difference allowed them to calculate the speed of the jet that produced the burst, which was traveling at 99.99 percent of the speed of light.
The discovery of GRBs with their distinctive lines is “one of the biggest surprises in our field in more than a decade,” he said. Eric Burns At Louisiana State University.
Barnes, who helped analyze the original data that led to the discovery of GRB221009A, was presenting his results at a conference with his colleagues when he heard about Ravasio’s findings. “Nobody thought the paper was right,” Barnes says. “We read the title and all thought, ‘This is wrong. It can’t be right.'”
But the analysis conducted by Ravasio and his colleagues appears to be correct, he says. “It’s pretty surprising, because we were so sure that gamma-ray bursts don’t have lines, that we didn’t look for this, and so we missed this completely,” Burns says.
Other GRBs may have similar spectral peaks and be worth searching for, but the peak was only observed because it came from the most luminous GRB on record, Burns said.
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