In this artist's concept, a jet of particles moving at near the speed of light emerges from a massive star.
Enlarge / A jet of particles traveling at near the speed of light emerges from a massive star in this artist’s concept of THE BOAT.
NASA Goddard Space Flight Center Concept Imagery Lab
Scientists have been in a tizzy since several space detectors detected a powerful gamma-ray burst (GRB) in October 2022, a burst so energetic that astronomers nicknamed it the BOAT (Brightest Of All Time). Now, an international team of astronomers has analyzed an unusual spike in energy 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 . It’s the first high-confidence emission line ever observed in 50 years of studying GRBs, the authors say.
As we’ve reported before, gamma-ray bursts are very high-energy explosions in distant galaxies that last from a few milliseconds to several hours. There are two categories of gamma-ray bursts. Most (70%) are long bursts that last more than two seconds, often with a bright afterglow. They are usually associated with rapidly star-forming galaxies. Astronomers believe that 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 baby black hole would produce jets of highly energetic particles traveling at nearly the speed of light, powerful enough to pierce the remains of the progenitor star, emitting X-rays and gamma rays.
These gamma-ray bursts lasting less than two seconds (about 30%) are considered short bursts, usually emitted from regions where star formation is very weak. Astronomers believe that these gamma-ray bursts are the result of the merger of two neutron stars, or of a neutron star merging with a black hole, forming a “kilonova”. This hypothesis was confirmed in 2017 when the LIGO collaboration captured the gravitational wave signal of the merger of two neutron stars, accompanied by the powerful gamma-ray bursts associated with a kilonova.
Several papers were published last year, reporting analytical results of all the observational data. These results confirmed that GRB 221009A was indeed BOAT, appearing particularly bright because its narrow jet pointed directly at Earth. But the various analyses also yielded several surprising results that intrigued astronomers. Most remarkable is that a supernova should have occurred a few weeks after the initial explosion, but astronomers failed to detect one, perhaps because it was very faint and thick dust clouds in that part of the sky attenuated any incoming light.
Earlier this year, astronomers confirmed that BOAT came from a supernova, thanks to telltale signatures of key elements like calcium and oxygen that are expected in a supernova. However, they found no traces of the expected heavy elements like platinum and gold, raising the long-standing question of where these elements came from in the universe. BOAT may well be special in this respect; more data will tell us more.
“It gave me goosebumps”
Minutes after the BOAT eruption, Fermi’s gamma-ray burst monitor recorded an unusual spike in energy. Scientists now say it’s the first high-confidence emission line ever observed in 50 years of studying gamma-ray bursts.
According to the authors, the newly detected spectral emission line was likely caused by the collision of matter and antimatter, producing a pair of gamma rays blueshifted to higher energies because we are looking into the jet. Having spectral emission associated with a GRB is important because it can shed light on the specific chemicals involved in the interactions. Previous studies have reported possible evidence of absorption or emission lines in other GRBs, but these have generally turned out to be likely statistical noise.
That’s not the case with this latest detection, according to study co-author Om Sharan Salafia of the Brera Observatory in Milan, Italy. She adds that the chances of this turning out to be a statistical fluctuation “are less than a one in half a billion chance.” Her colleague and study co-author Maria Edvige Ravasio said that when she first saw the signal, “it gave me goosebumps.”
Why did it take astronomers so long to detect it? When BOAT first erupted in 2022, it overwhelmed most space-based gamma-ray detectors, including the Fermi Space Telescope, leaving them unable to measure the most intense part of the explosion. The emission line didn’t appear until a full five minutes after the explosion, when it had dimmed enough for Fermi to make a measurement. The spectral emission lasted about 40 seconds and reached a peak energy of about 12 MeV, compared to 2 or 3 MeB for visible light, the authors say.
Science, 2024. DOI: 10.1126/science.adj3638 (About DOIs).
.jpg)
.webp)
0 Comments