An avalanche on a sandpile starts with a single grain of sand. No one knows exactly where it will slip or how large the slide will be. But if you keep pouring sand, the pile will eventually reach a state where avalanches of all sizes, from tiny trickles to giant collapses, follow the same law. Physicists call this self-organized criticality: without external control, the system keeps itself at the edge, where any event can be the last straw.

In March 2023, the GECAM satellite caught the second-brightest gamma-ray burst ever observed — GRB 230307A. Ordinary detectors would have been overwhelmed by such a flux, but GECAM recorded the entire burst without gaps. In the light curve, astronomers identified hundreds of individual pulses, like separate avalanches in one large pile. Each pulse is a localized energy release. The scientists measured its peak brightness, duration, and the pause until the next pulse. It turned out that the distributions of these quantities follow power laws: there is no characteristic size, only a continuous spectrum from weak to strong events. The slopes of these power laws matched theoretical predictions for three-dimensional self-organized criticality.

Such behavior is typical of magnetic reconnection — a process in which magnetic field lines break and rearrange, releasing energy. This is how solar flares work. GRB 230307A behaved like a giant solar flare, but in a distant galaxy. This is strong evidence that the energy in a gamma-ray burst jet is carried by magnetic fields, not just the kinetic energy of colliding shells. The sandpile avalanche and the gamma-ray burst turned out to be relatives.