How Bright Can Old Magnetars Be?

Magnetars should shine in X-rays for a long time, heated by the decay of their enormous magnetic fields. How bright an old magnetar stays, however, depends on two things we cannot see directly: the thin envelope at its surface, and where its electric currents flow. Our simulations show that magnetars whose currents live in the crust keep heating their surface and stay bright for a long time, while those whose fields thread the core fade quickly, becoming fainter than ordinary pulsars of the same age. Whether a middle-aged magnetar remains visible therefore depends on its hidden magnetic structure, which matters for how many magnetars we expect to find in our Galaxy.