Thanks to a fast reaction by observers and staff, near-simultaneous observations were made of GRB221009A from Gemini South in Chile. The image is a combination of 4 exposures with two instruments taken in the morning of Friday 14 October 2022. Credit: International Gemini Observatory/NOIRLab/NSF/AURA/B.O'Connor (UMD/GWU) & J. Rastinejad & W Fong (Northwestern Univ).
A 2022 gamma ray burst, the brightest ever detected, may provide a clue on the existence of axion-like particles, one of the possible explanations for dark matter, according to researchers from the Italian National Institute of Astrophysics (INAF).
On 9 October 2022, the flash of a gamma-ray burst saturated telescopes throughout the world. The event, called GRB 221009A, was the most luminous of its kind. Gamma-ray bursts are the most energetic explosions in the Universe, and GRB 221009A is believed to be the result of the collapse of a massive star.
The unexpected shape of the jet of material emitted by this event had already puzzled scientists. But the attention of the Italian group was drawn by the extremely high energy of its photons, estimated to be well above 10 TeV by the China-based LHAASO telescope that captured them.
“Photons with such an energy coming from a gamma ray burst at that redshift (that is, at that distance) should be completely absorbed by extragalactic background light (EBL),” says Giorgio Galanti, first author of the study published in Physical Review Letters1. EBL is the light emitted by all the stars throughout the evolution of the Universe. Its low-energy photons should, according to conventional physics, interact with the high-energy ones from the gamma ray burst and transform into an electron-positron couple.
But axion-like particles (ALPs) may solve the conundrum. These are very light particles whose existence is required by superstring theory, which aims at unifying all the fundamental forces and overcoming the Standard Model of conventional physics.
The photons shot out from GRBs go through multiple magnetic fields in their trip, that may transform them into ALPs, according to the superstring theory. Along the way, they can oscillate back into photons. Because ALPs are not absorbed by background light, the oscillation may grant the survival of several high-energy photons.
“The main caveat [on this interpretation] is that the observation of high-energy photons may still come from background radiation captured by chance during the observations of the gamma ray burst,” says Eleonora Troja, an astrophysicist at the University of Rome Tor Vergata, not involved in the study. “If we find high-energy photons in more than one gamma ray burst, the results of this paper would be reinforced,” she concludes.
