NASA Goddard Space Flight Center Concept Imaging Laboratory
Scientists have been worried since several space detectors picked up a powerful gamma-ray burst (GRB) in October 2022 – a burst so energetic that astronomers named it BOAT (Brightest Of All Time). Now, an international team of astronomers has analyzed an unusual energy peak detected by NASA’s Fermi Gamma-ray Space Telescope and concluded that it is an emission spectrum, according to a new paper published in the journal Science. According to the authors, this is the first high-confidence emission line ever observed in 50 years of studying GRBs.
As previously reported, gamma-ray bursts are extremely high-energy explosions in distant galaxies lasting between milliseconds and several hours. There are two classes of gamma rays. Most (70 percent) are long bursts lasting more than two seconds, often with a bright afterglow. They are usually associated with galaxies with rapid star formation. Astronomers believe that the long bursts are associated with the death of massive stars that collapse to form a neutron star or black hole (or, alternatively, a newly formed magnetar). The infant black hole will produce jets of high-energy particles traveling near the speed of light, powerful enough to tear through the remains of the progenitor star, emitting X-rays and gamma rays.
These gamma-ray bursts, lasting less than two seconds (about 30 percent), are considered short bursts, usually emitted from regions with very little star formation. Astronomers believe that these gamma rays are the result of a merger between two neutron stars, or a neutron star merging with a black hole, forming a “kilonova.” This hypothesis was confirmed in 2017 when the LIGO collaboration detected the gravitational wave signal of two neutron stars merging, accompanied by the powerful gamma rays associated with the kilonova.
Several papers were published last year reporting the analytical results of all observational data. These findings confirmed that GRB 221009A is indeed the BOAT, appearing particularly bright because its narrow jet is pointed directly at Earth. But the various analyzes also yielded a few surprising results that puzzled astronomers. Most notably, a supernova should have occurred a few weeks after the initial burst, but astronomers didn’t detect one, perhaps because it was so faint and dense clouds of dust in that part of the sky obscured any incoming light.
Earlier this year, astronomers confirmed that BOAT came from a supernova, thanks to the telltale signatures of key elements like calcium and oxygen that one would expect to find with a supernova. However, they found no evidence of the expected heavy elements such as platinum and gold, addressing the long-standing question of the origin of such elements in the universe. BOAT may just be special in that regard; additional data will tell us more.
“I got goosebumps”
A few minutes after BOAT erupted, Fermi’s gamma-ray monitor registered an unusual energy spike. Scientists now say this feature is the first high-confidence emission line ever observed in 50 years of studying GRBs.
The newly discovered spectral emission line is likely caused by the collision of matter and antimatter, according to the authors, producing a pair of gamma rays that are blue-shifted to higher energies because we are looking into the jet. The presence of spectral emission associated with GRBs is important because it can shed light on the specific chemicals involved in the interactions. There have been previous studies reporting possible evidence of absorption or emission lines in other GRBs, but these have generally turned out to be likely to be statistical noise.
That’s not the case with this latest detection, according to co-author Om Sharan Salafia of the INAF-Brera Observatory in Milan, Italy, who adds that the odds of this turning out to be a statistical fluctuation “are less than one chance in half a billion.” His colleague and INAF co-author Maria Edvigue Ravasio said that when I first saw the signal, “I got goosebumps.”
Why did it take astronomers so long to discover it? When BOAT first erupted in 2022, it saturated most of the space’s gamma-ray detectors, including the Fermi Space Telescope, making them unable to measure the most intense part of that burst. The emission line did not appear until five minutes after the burst, when it was dim enough for Fermi to make a measurement. The spectral emission lasted about 40 seconds and reached a peak energy of about 12 MeV, compared with 2 or 3 MeB for visible light, according to the authors.
Science, 2024. DOI: 10.1126/science.adj3638 (About DOI).
