A particle with an energy of $10^{18}$ eV is like a tennis ball traveling at 200 km/h. Where do such projectiles come from? Gamma-ray bursts are powerful explosions that eject matter in a narrow beam, like a spotlight. Most such beams are wide, like the light from a flashlight. But there are rare ones, narrow like a laser pointer. They strike more precisely and waste less energy.
The authors considered narrow jets in the nearby Universe, within 4 billion light-years. Proximity matters: particles do not have time to lose energy along the way. The model matched data from the Pierre Auger Observatory: the particle composition came out as 65% protons and 34% helium, as observed. Predicted neutrino and gamma-ray fluxes do not exceed what detectors see. Previously, it was unclear how gamma-ray bursts could produce the required particles without excess radiation. Now it is clear: rare narrow jets nearby solve this problem.
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.