A flashlight doesn't cast a uniform circle of light; it has a bright spot in the center and a blurred halo at the edges. If you switch it off quickly, the edges are the last to fade, and by how the light dies, you can guess the shape of the reflector. That's roughly how a gamma-ray burst works: when a collapsing star ejects a narrow jet of plasma, we see only its core, while everything else is hidden. But once the source inside the jet runs dry, we start receiving radiation from its outskirts — fainter, delayed, and distorted by curvature. This is called the curvature effect. On the light curve, a plot of brightness over time, the transition shows up as a sharp break.
The authors based their work on a real burst, GRB 230307A. It's one of the rare cases where such a break was clearly captured at a late stage. They built a numerical model that calculates synchrotron emission (radiation from electrons spiraling in a magnetic field) from jets with different geometries. It turned out that neither a simple sphere nor a cylinder with sharp edges, the 'cylinder-pill' model, fits the observations. The data best matched a jet with a dense core. Its opening angle is just 0.0147 radians, roughly the size of a coin held at arm's length. Around the core, a 'wing' fades smoothly, described by a power law. In other words, the jet doesn't end with a cliff but tapers off gently, like a hillside rather than a step.
This approach turns the break into a diagnostic tool. Previously, the jet's shape was inferred mainly from the afterglow, which is visible for hours and days. Now we can probe the geometry of the prompt phase itself — the first tens of seconds when the central engine is active. For GRB 230307A, the wing model not only explained the break but also allowed estimating the core's parameters and the wing's slope. Thus, astronomers gain a ruler for measuring the invisible, and every new burst with a late break will add a stroke to the portrait of these cosmic cannons.
Gamma-ray bursts are so bright that at their peak they can outshine the entire rest of the Universe in gamma rays, but they last from milliseconds to a few minutes, then vanish without a trace.
In Carl Sagan's novel Contact, an alien signal is disguised as a gamma-ray burst, and real astronomers are just learning to read the geometry of a burst's source from such flashes, as if deciphering the handwriting of distant civilizations.