A gamma-ray burst is like a shot from a cosmic cannon. First a brief flash of light, then a fading afterglow. But inside that flash, things aren't so simple: its color and brightness don't change at the same time.

Scientists took 20 single bursts caught by the Fermi satellite Fermi and broke them down over time. They looked at two things: when the burst becomes brightest and when its emission becomes hardest (that is, with the highest energy). It turns out there are three scenarios. In five bursts, hardness and brightness peaked together, as if someone switched on a lamp and immediately turned the brightness knob all the way up. In thirteen, hardness came before brightness, as if the lamp first switched to blue light and then got brighter. And in only two, it was the other way around: brightness before hardness.

This behavior is like three different springs. The first spring compresses and expands in sync with brightness — everything is synchronized. The second spring first becomes stiffer and then pushes harder. The third is rare: it's the opposite. Scientists don't yet know why, but this helps distinguish different radiation mechanisms in explosions.