NASA's Fermi Finds Novel Feature in BOAT Gamma-Ray Burst

n October 2022, astronomers were stunned by what was quickly dubbed the BOAT — the brightest-of-all-time gamma-ray burst (GRB). Now an international science team reports that data from NASA’s Fermi Gamma-ray Space Telescope reveals a feature never seen before.

GRBs are the most powerful explosions in the cosmos and emit copious amounts of gamma rays, the highest-energy form of light. The most common type occurs when the core of a massive star exhausts its fuel, collapses, and forms a rapidly spinning black hole. Matter falling into the black hole powers oppositely directed particle jets that blast through the star’s outer layers at nearly the speed of light. We detect GRBs when one of these jets points almost directly toward Earth.

A few minutes after the BOAT erupted, Fermi’s Gamma-ray Burst Monitor recorded an unusual energy peak. Scientists now say this feature is the first high-confidence emission line ever seen in 50 years of studying GRBs.

When matter interacts with light, the energy can be absorbed and reemitted in characteristic ways. These interactions can brighten or dim particular colors (or energies), producing key features visible when the light is spread out, rainbow-like, in a spectrum. These features can reveal a wealth of information.

The science team says that the odds this feature is a fluke — just a noise fluctuation — are less than one chance in half a billion.

The BOAT, formally known as GRB 221009A, erupted Oct. 9, 2022, and promptly saturated most of the gamma-ray detectors in orbit, including those on Fermi. If part of the same population as previously detected GRBs, the BOAT was likely the brightest burst to appear in Earth’s skies in 10,000 years.

The putative emission line appears almost 5 minutes after the burst was detected and well after it had dimmed enough to end saturation effects for Fermi. The line persisted for at least 40 seconds, and the emission reached a peak energy of about 12 MeV (million electron volts). For comparison, the energy of visible light ranges from 2 to 3 electron volts.

The team thinks the most likely source for the emission line is the annihilation of electrons and their antimatter counterparts, positrons. When these particles collide, they produce a pair of gamma rays with an energy of 0.511 MeV. Because we’re looking into the jet, where matter is moving at near light speed, this emission becomes greatly blueshifted and pushed toward much higher energies.

If this interpretation is correct, to produce an emission line peaking at 12 MeV, the annihilating particles had to have been moving toward us at about 99.9% the speed of light.