Some stars behave like lighthouses: they don't shine steadily but blink with short flares. Such stars are called magnetars, and their magnetic field is a quadrillion times stronger than Earth's. Since 2008, a special instrument in orbit has been watching them—Fermi GBM. It scans the entire sky at once and records every flare, like a watchman keeping a log.

Now scientists have opened this log and measured each flare: how long it lasted, what its energy was, and what "color" it had in gamma rays. It turns out that flares last about 200 milliseconds, shorter than a lightning strike. They also found that the flare's emission resembles light from two red-hot furnaces at once, not one. The temperature of such a "furnace" is about 14 million degrees, but for one magnetar it's higher, around 17 million degrees. This means its flares are harder, carrying more energy.

But the most surprising part is how the area of the glowing region changes as it heats up. For an ordinary furnace, the surface area grows by a simple rule, but for a magnetar it's different. This suggests that the flare region doesn't stay put; it quickly contracts or expands. Such details help explain why magnetars flare at all. And now that the log is open, anyone can test their ideas against this data.