An avalanche in a sandpile starts with a single grain. No one knows exactly where it will slip or how big the slide will be. But if you keep pouring sand, the pile itself reaches a state where avalanches of all sizes, from tiny trickles to massive collapses, follow the same law. Physicists call this self-organized criticality: without external control, the system keeps itself at the edge, where any event could 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 had special saturation protection and recorded the entire 35-second burst without a single gap. In the light curve, astronomers identified hundreds of individual pulses, like separate avalanches in one big 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 where magnetic field lines break and rearrange, releasing energy. This is exactly 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 the gamma-ray burst jet is carried by the magnetic field, not just by the kinetic energy of colliding shells.

The avalanche in a sandpile and the gamma-ray burst turned out to be relatives. Both are examples of systems that organize themselves into a critical state, where events of all scales follow one law. Now that a single burst has given us hundreds of pulses, we see this self-organization in action for the first time — not in the statistics of many objects, but in one cosmic explosion.