A particle with an energy of $10^{18}$ eV hitting Earth's atmosphere is like a tennis ball traveling at 200 km/h slamming into a wall. Astrophysicists have debated for decades where such projectiles come from. Gamma-ray bursts, brief and powerful explosions of gamma radiation, have long been suspected as accelerators. But calculations showed that if they accelerate particles, they should produce too many neutrinos and gamma-ray photons, which detectors do not see.
New work offers a way out. A gamma-ray burst ejects matter in a narrow relativistic jet, like a searchlight beaming at one point in the sky. Most jets are wide, like the light from a lantern illuminating a yard. But there are rare ones that are narrow, like a laser pointer. The authors considered a population of such narrow jets in the nearby universe (redshift z ≤ 0.36, that is, within about 4 billion light-years). These "laser pointers" aim more precisely and waste less energy, so they can accelerate particles more efficiently while producing less secondary radiation.
The model was fitted to data from the Pierre Auger Observatory: the spectrum and composition of particles. It turned out that at the highest energies, the flux is dominated by narrow jets limited to redshift z ≤ 0.36. The proximity of sources reduces particle losses during propagation: they do not have time to lose energy through interactions with the cosmic microwave background. The composition came out as 65% protons and 34% helium, which matches observations.
The predicted neutrino fluxes lie below the current limits from IceCube and Auger. The peak of the neutrino flux is within an order of magnitude of the estimate for the KM3-230213A event, a hint but not a contradiction. The cascade gamma-ray flux saturates the Fermi-LAT upper limit at energies around 1 TeV, but leaves room for contributions from other sources, such as blazars.
Future facilities such as AugerPrime and TA×4 will improve measurements of the spectrum, composition, and arrival directions of particles, allowing tests of correlations with nearby gamma-ray bursts. Next-generation neutrino detectors, IceCube-Gen2, GRAND, and PUEO, will either detect the predicted flux or impose stricter limits. Observations with LHAASO, CTA, and SWGO could resolve the cosmogenic component in gamma rays from individual bursts.
Thus, rare narrow searchlight beams illuminating the immediate neighborhood, rather than distant galaxies, turn out to be the key to unraveling the origin of the most energetic particles in the universe.
The gamma-ray burst GRB 221009A was so bright that it was called the 'Brightest Of All Time' (BOAT). Its jet had an opening angle of just 0.8 degrees, like a flashlight beam from a kilometer away illuminating a coin.
The idea that rare and powerful cosmic explosions could be sources of the most energetic particles resonates with the concept of 'gamma-ray bursts as weapons' in science fiction, for example in Dan Simmons' novel 'Hyperion', where such phenomena are linked to catastrophic events.